Regional Screening Levels (RSLs) - Frequent Questions

 

To download the most recent Regional Screening Level tables, please go to the Generic Tables page. For assistance/questions please use the RSL Contact Us page.

This page presents many questions asked by site users and the applicable responses. Please search this page for answers to your questions prior to contacting technical support staff. Researching the questions and answers posted here will greatly reduce the time it takes for you to solve many problems that arise from calculating and using this SL site.

To simplify the process of finding a relevant FQ, the following categories are provided. Simply click the category and you will be taken to list of relevant questions.

Background/history of RSLs

General Use Questions

Exposure Questions

General Toxicity Value Issues

Chemical-specific Issues

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Background/history of RSLs

1. What are SLs?
2. Why are SLs used?
3. How do SLs differ from cleanup standards?
4. How often do you update the SL Table?
5. Can I get a copy of a previous SL table?
6. Does my region or state recommend the use of the tables where THQ=1.0 or THQ=0.1? What table do I use and when do I use it?

General Use Questions

7. How can I get the calculator results or the other web pages to print on one page?
8. Where can I find out about WATER9, CHEMDAT8, and CHEM9?
9. Do the fish tissue and/or soil SLs apply to wet-weight or dry-weight data?
10. Why do some of the numbers on the SL Table exceed a million parts per million (1E+06 mg/kg)? That's not possible!
11. What is the preferred citation for information taken from this website?
12. Are the tap water RSLs based on total (unfiltered) or dissolved (filtered) concentrations?
13. Why is rounding in the RSL tables and calculator different from SSSG Appendix A?
14. If I have NAPL present in my soil is the soil screening level (SSL) protective of groundwater model used in the RSLs still valid?
15. Will changing the groundwater-soil system temperature automatically change the volatility status of my chemicals?
16. How do I handle analytical detection limits or nondetected concentrations higher than the RSL?

Exposure Questions

17. The exposure variables table in the SL background document lists the averaging time for non-carcinogens as "ED*365." What does that mean?
18. What populations and what exposures are considered in each type of RSL?
19. Do the RSLs factor inhalation from vapor intrusion?
20. Is T in the unlimited source, chronic, VF_s equation equal to the exposure duration (ED)?
21. How are the residential exposure durations (EDs), used in the RSL equations, determined for carcinogenic (age-adjusted), noncarcinogenic (child and adult), and mutagenic (multiple age bins), exposures?
22. How do the RSLs incorporate bioavailability into the soil ingestion equations?

General Toxicity Value Issues

23. Where else can I go for toxicity studies (values) not on this site?
24. I can't find the chemical that I am interested in. Why isn't it in your database? Are there other places where I should look to find the information that I need?
25. Can the oral RfDs in the SL Table be applied to dermal exposure?
26. Why isn't oral/inhalation route-to-route extrapolation used to generate toxicity factors on the Screening Table?
27. Previous Regional Tables used Inhalation Reference Doses (RfDi) and Slope Factors (SFI). Why does the new table use RfCs and IURs?
28. What are the sources of toxicity values and chemical-specific parameters used to calculate RSLs?
29. How do I convert (mg/m3)-1 to (µg/m3)-1?
30. How do I convert ATSDR inhalation MRLs in parts per million (ppm) to mg/m³ and other general conversions?
31. Why do Tap Water RSLs differ from IRIS drinking water concentrations when both are based on a target cancer risk of 1E-06?
32. Why do air RSLs differ from IRIS inhalation unit risk values when both are based on a target cancer risk of 1E-06?
33. How are appendix 'toxicity screening values' used in RSL calculation and designated in the RSL tables?
34. Various forms of a chemical, such as salts, acid forms, and ion forms, are commonly listed in the Regional Screening Levels (RSLs). How are the toxicity values, chemical-specific parameters, and analytical methods chosen for these different forms

Chemical-specific Issues (sorted alphabetically by chemical)

35. [Asbestos] Where can I find asbestos screening levels?
36. [Benzene] The slope factors for benzene are actually ranges, yet the SL table shows only a single number. Which number was chosen and why?
37. [Cadmium] The cadmium RSLs are labeled "Cadmium (Diet)" and "Cadmium (Water)." What do I use for soil and where do the toxicity values come from?
38. [Cadmium] Why doesn’t cadmium use toxicity values from IRIS?
39. [Chlordane] Is the CAS number for Chlordane really for Technical Chlordane and what should I use for screening?
40. [Chromium] What are the sources of chromium toxicity values and how was the IUR derived?
41. [Copper] How was the copper RfD derived?
42. [DCE, trans 1,2-] Why was the RfC for trans-1,2 dichloroethylene from the PPRTV removed from the RSLs?
43. [2,4/2,6-dinitrotoluene] 2,4/2,6-dinitrotoluene mixture has a cancer slope factor, why don't the individual isomers use the same slope factor?
44. [Haloacetic Acids] How do I apply the haloacetic acid MCLS?
45. [Lead] What is the inorganic lead residential soil SL?
46. [Manganese] For manganese, IRIS shows an oral RfD of 0.14 mg/kg-day, but the SL Table uses 0.024 mg/kg-day. Why?
47. [Mercury] Why is there no oral RfD for mercury? How should I handle mercury?
48. [PAHs] Where did the SFOs for carcinogenic PAHs come from?
49. [Perchlorate] Why is the tap water screening level for Perchlorate of 11 μg/L different from the preliminary remedial goal (PRG) of 15 μg/L calculated by the Office of Solid Waste and Emergency Response in its January 8, 2009, guidance (PDF)?
50. [PCBs] Since an earlier FAQ said that route to route extrapolations were not used by the RSLs to develop toxicity values, how were the inhalation unit risks derived for Polychlorinated biphenyls (PCBs)?
51. [TCDD] Why was the TCDD (Dioxin) oral slope factor of 130,000 (mg/kg-day)-1 (or 1.3E-04 (pg/kg-day)-1)chosen?
52. [TCE] What toxicity values are used for TCE?
53. [TPH] How are total petroleum hydrocarbon (TPHs) considered in calculating RSLs?
54. [Trihalomethanes] How do I apply the trihalomethane MCLs?
55. [vinyl chloride] IRIS presents 2 types of toxicity values for vinyl chloride yet the SL table shows only a single number. Which number was chosen and why?
56. [White Phosphorus] Why is the White Phosphorus CAS the generic phosphorus CAS of 7723-14-0 when it should be 12185-10-3?
57. [Xylene] Where do the RfDs and RfCs for the xylene congeners come from?

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1. What are SLs?

   The screening levels (SLs) presented on this site are developed using risk assessment guidance from the EPA Superfund program and can be used for Superfund sites. They are risk-based concentrations derived from standardized equations combining exposure information assumptions with EPA toxicity data. SLs are considered by the Agency to be protective for humans (including sensitive groups) over a lifetime; however, SLs are not always applicable to a particular site and do not address non-human health endpoints, such as ecological impacts. The SLs contained in the SL table are generic; they are calculated without site-specific information. They may be re-calculated using site-specific data.

2. Why are SLs used?

   They are used for site "screening" and as initial cleanup goals, if applicable. SLs are not de facto cleanup standards and should not be applied as such. The SL's role in site "screening" is to help identify areas, contaminants, and conditions that require further federal attention at a particular site. Generally, at sites where contaminant concentrations fall below SLs, no further action or study is warranted under the Superfund program, so long as the exposure assumptions at a site match those taken into account by the SL calculations. Chemical concentrations above the SL would not automatically designate a site as "dirty" or trigger a response action; however, exceeding a SL suggests that further evaluation of the potential risks by site contaminants is appropriate. SLs are also useful tools for identifying initial cleanup goals at a site. In this role, SLs provide long-term targets to use during the analysis of different remedial alternatives. By developing SLs early in the decision-making process, design staff may be able to streamline the consideration of remedial alternatives.

3. How do SLs differ from cleanup standards?

   SLs are generic screening values, not de facto cleanup standards. Once the Baseline Risk Assessment (BLRA) is completed, site-specific risk-based remediation goals can be derived using the BLRA results. The selection of final cleanup goals may also include (Applicable or Relevant and Appropriate Requirements (ARARs) and to be considered guidance (TBCs), as well as site-specific risk-based goals.

   In the Superfund program, this evaluation is carried out as part of the nine criteria for remedy selection outlined in the National Oil and Hazardous Substances Pollution Contingency Plan (NCP). Once the nine-criteria analysis is completed, the SL may be retained as is or modified (based on site-specific information) prior to becoming established as a cleanup standard. This site-specific cleanup level is then documented in the Record of Decision.

4. How often do you update the SL Table?

   It is anticipated that the SLs will be updated approximately semiannually in the Fall and Spring. Please take note of the "What's New" page to identify when toxicity values are updated. To be notified when the RSLs are updated, please sign up here.

5. Can I get a copy of a previous SL table?

   We do not distribute outdated copies of the RSL tables. Each new version of the table supersedes all previous versions. If you wish to maintain previous versions of the RSLs for a long-term project, you can download the entire table and save multiple versions with a time-stamp. If you need to obtain historical versions of tables (e.g., for litigation, research, etc.), you may submit a FOIA request (see FOIA). FOIA requests may be directed to the Office of Land and Emergency Management in EPA Headquarters (POC: Wanda McLendon). Please be advised that prior to Fall 2008, there were 3 different regional versions (Regions 3, 6, and 9), and you would have to specify a regional version.

6. Does my region or state recommend the use of the tables where THQ=1.0 or THQ=0.1? What table do I use and when do I use it?

   Generally, if you are screening only one contaminant, the THQ=1.0 table can be used. Generally, if you are screening multiple chemicals it is preferred, to use the THQ=0.1 tables.

   The rationale for using THQ=0.1 for screening is that when multiple contaminants of concern are present at a site or one or more are present in multiple exposure media, the total hazard index could exceed 1.0 if each were screened at the HQ of 1.0. If you are unclear as to which set of tables (THQ=1.0 or THQ=0.1) to use at a site, consult an EPA risk assessor in your region or the environmental protection agency for your state.

7. How can I get the calculator results or the other web pages to print on one page?

   Historically this was an issue for the RSL calculator. Now calculator results can be accessed in PDF or Spreadsheet files. At the top of the RSL calculator results page, output links can be found.

8. Where can I find out about WATER9, CHEMDAT8, and CHEM9?

   These programs help estimate various chemical-specific parameters such as diffusivity in air and water. WATER9 is an analytical model for estimating compound-specific air emissions from wastewater collection & treatment systems. CHEMDAT8 is a Lotus 1-2-3 spreadsheet that includes analytical models for estimating VOC emissions from treatment, storage and disposal facility (TSDF) processes. CHEM9 is a compound properties processor that is based upon an EPA compound database of over 1000 compounds. It provides the capability to estimate compound properties that are not available in the database, including the compound volatility and the theoretical recovery (fraction measured (Fm)) for EPA test methods 25D and 305.

9. Do the fish tissue and/or soil SLs apply to wet-weight or dry-weight data?

   The fish SLs represent the concentration that can be consumed at the rate indicated in the Technical Background Document. Therefore, wet or dry weight is not an inherent assumption of the SL numbers. Rather, users of the Table should consider whether their population of interest is more likely to consume the fish using a preparation method that is better simulated by a wet or dry weight. (For example, consumption of raw or fried fish would be more likely represented by wet weight, whereas consumption of smoked or dried fish might be better represented by dry weight.) In other words, the use of a site-specific sample as wet or dry weight should be governed by its representativeness for the population of interest.

   The RSL Calculator does not provide default fish SLs. If the fish scenario is selected, the calculator will automatically switch to site-specific mode. On the next page the user is required to enter a site-specific fish consumption rate. The previous default fish intake rate of 54,000 mg/day from Standard Default Exposure Factors has been removed. Intake rates can be found in 2011 Exposure Factors Handbook. Please consult your Regional risk assessor when determining appropriate fish consumption rates.

   The soil SLs are based on dry weight because the soil intake rates are based on dry weight.  Most soil data  is typically reported as dry weight. As always, please consult your Regional risk assessor when applying the RSLs to site-specific data.

10. Why do some of the numbers on the SL Table exceed a million parts per million (1E+06 mg/kg)? That's not possible!

    For certain low-toxicity chemicals, the SLs exceed possible concentrations at the target risks. Many years ago, these SLs were rounded to the highest possible concentration, or 1.0E+06 ppm. This type of truncation has been discontinued so that Table users can adjust the SLs to a different target risk whenever necessary. For example, when screening chemicals at a target HQ of 0.1, noncarcinogenic SLs may simply be divided by 10. Such scaling is not possible when SLs are rounded. Users who are interested in truncation can also consult the Soil Screening Guidance for a discussion of "Csat," the saturation concentration, which reflects physical limits on soil concentrations.

    SLs may also exceed a non-risk based 'ceiling limit' concentration of 1.0E+05 mg/kg ('max') for relatively less toxic inorganic and semivolatile contaminants. The ceiling limit of 1.0E+05 mg/kg is equivalent to a chemical representing 10% by weight of the soil sample. At this contaminant concentration (and higher), the assumptions for soil contact may be violated (for example, soil adherence and wind-borne dispersion assumptions) due to the presence of the foreign substance itself.

    The calculator, if operated in site-specific mode, will give the option to apply the C_sat substitution rule as well as the option to apply the theoretical ceiling limit.

11. What is the preferred citation for information taken from this website?

    United States Environmental Protection Agency. Regional Screening Levels for Chemical Contaminants at Superfund Sites. (Insert date accessed).

12. Are the tap water RSLs based on total (unfiltered) or dissolved (filtered) concentrations?

    The tap water RSLs do not address total vs dissolved components in the drinking water; this is a sampling issue. The tap water RSLs are for the concentration in the water at the tap regardless of how the water gets there or is sampled. The decision about whether to use total or dissolved data in a risk assessment is a site-specific one; consult your regional risk assessor.

13. Why is rounding in the RSL tables and calculator different from SSSG Appendix A?

    The Supplemental Guidance for Developing Soil Screening Levels for Superfund Sites in Appendix A presents SSLs above 10 rounded to 2 digits, and below 10 rounded to 1 digit. The RSL tables round to 2 digits for results above and below 10, and the calculator results display 3 digits. The rationale for providing "extra" digits is to assist users in checking the math. When individual exposure route results are rounded, many times it is impossible to reproduce the total across multiple routes. Enough digits are provided in RSL tables and calculator results for the user to apply their own rounding protocol.

14. If I have NAPL present in my soil is the soil screening level (SSL) protective of groundwater model used in the RSLs still valid?

    The RSL tables and the default calculator settings do not substitute C_sat for risk-based calculations. If the risk-based concentration exceeds C_sat, the resulting SSL concentration may be overly protective. This is because the dissolved, absorbed and vapor concentrations cease to rise linearly as soil concentration increases above the C_sat level (pure product or nonaqueous phase liquid (NAPL) is present). The SSL model used in the RSL calculator is not a four phase model. If a NAPL is present at your site more sophisticated models may be necessary. The calculator, if operated in site-specific mode, will give the option to apply the C_sat substitution rule.

15. Will changing the groundwater-soil system temperature automatically change the volatility status of my chemicals?

    No. When changing the groundwater-soil system temperature, the criteria that determines whether a chemical is volatile or not may also change. The Henry’s Law constants and vapor pressures that are used in RSL calculations are based on standard laboratory conditions of 25°C. When the groundwater-soil system temperature is changed, the Henry’s Law constants and vapor pressures will change accordingly. A volatile chemical at 25°C may not be volatile at a lower temperature. Conversely, a nonvolatile may become volatile at temperatures above 25°C. When the groundwater-soil system temperature is changed the RSL calculator will provide the user with recalculated Henry's Law constants and vapor pressures. These values can be compared to the volatility status requirements presented in section 4.9.4 of the user's guide. The RSL calculator does not automatically change the volatility status of a chemical to reflect the user's change of the groundwater-soil system temperature. In site-specific user-provided mode of the calculator, the user may change the volatility status of a chemical if deemed appropriate by the user’s regional risk assessor. Changing the volatility status can impact the inclusion of dermal exposure to soil, inhalation of volatiles during household use of water, and the soil to groundwater Method 1 calculations. See section 4.9.4 and 4.8.3 of the user's guide for more information.

16. How do I handle analytical method detection limits or nondetected concentrations higher than the RSL?

    The RSLs are calculated purely on a risk basis and presented without consideration of theoretical ceiling limits, C_sat substitutions, detection limits, or analytical methods. The intention of the supporting text for the RSLs is to clearly define the RSL calculation process and not to provide instruction on every aspect of the risk assessment process. Before analytical data and appropriate screening levels are identified for a site, the quality assurance project plan (QAPP) will outline the procedures to ensure that the media collected and analyzed meet project requirements for data screening and risk calculation. Consult your regional risk assessor for assistance on determining acceptable data requirements. For more information on analytical methods, consult Hazardous Waste Test Methods / SW-846, Analytical Methods and Procedures for Pesticides, and Clean Water Act Analytical Methods.

17. The exposure variables table in the SL background document lists the averaging time for non-carcinogens as "ED*365." What does that mean?

    ED is exposure duration, in years, and * is the computer-ese symbol for multiplication. Multiplying ED by 365 simply converts the duration to days. In fact, the ED term is included in both the numerator and denominator of the SL algorithms for non-cancer risk, canceling it altogether. See RAGS for more information.

18. What populations and what exposures are considered in each type of RSL?

    The following table lists the land uses addressed, media addressed and the age of the receptor utilized in the RSL table.

    		Exposure Routes (Cancer)			Exposure Routes (Noncancer)
    Land use	Media	Oral	Dermal	Inhalation	Oral	Dermal	Inhalation
    Resident	Soil	Adult + Child	Adult + Child	Both	Child	Child	Both
    	Tap Water	Adult + Child	Adult + Child	Both	Child	Child	Both
    	Air	NA	NA	Both	NA	NA	Both
    Composite Worker	Soil	Adult	Adult	Adult	Adult	Adult	Adult
    	Air	NA	NA	Adult	NA	NA	Adult
    Soil to Groundwater	Soil	Adult + Child	Adult + Child	Both	Both	Child	Child

    NA = Not Applicable
    
    The following table lists the land uses addressed, media addressed, and the age of the receptor utilized in the RSL calculator.  

    		Exposure Routes (Cancer)			Exposure Routes (Noncancer)
    Land use	Media	Oral	Dermal	Inhalation	Oral	Dermal	Inhalation
    Resident	Soil	Adult + Child	Adult + Child	Both	Both	Both	Both
    	Tap Water	Adult + Child	Adult + Child	Both	Both	Both	Both
    	Air	NA	NA	Both	NA	NA	Both
    Recreator	Soil/Sediment	Adult + Child	Adult + Child	Both	Both	Both	Both
    	Surface Water	Adult + Child	Adult + Child	NA	Both	Both	NA
    Outdoor Worker	Soil	Adult	Adult	Adult	Adult	Adult	Adult
    	Air	NA	NA	Adult	NA	NA	Adult
    Indoor Worker	Soil	Adult	NA	Adult	Adult	NA	Adult
    	Air	NA	NA	Adult	NA	NA	Adult
    Composite Worker	Soil	Adult	Adult	Adult	Adult	Adult	Adult
    	Air	NA	NA	Adult	NA	NA	Adult
    Construction Worker	Soil	Adult	Adult	Adult	Adult	Adult	Adult
    	Air	NA	NA	NA	NA	NA	NA
    Fish	Fish	Adult	NA	NA	Adult	NA	NA
    Soil to Groundwater	Soil	Adult + Child	Adult + Child	Both	Both	Both	Both

    NA = Not Applicable

19. Do the RSLs factor inhalation from vapor intrusion?

    Air RSLs represent screening levels for indoor or outdoor air. The residential and industrial air RSL values can be used to screen chemicals that are detected in the air (e.g., indoor and outdoor) from a variety of sources. There are no RSLs specific to the vapor intrusion pathway, i.e., for subsurface sources that may contribute to indoor air contamination. EPA's recommended vapor intrusion screening levels can be found online at: https://www.epa.gov/vaporintrusion/vapor-intrusion-screening-levels-visls.

    For guidance on vapor intrusion assessment, see EPA's Vapor Intrusion Site. The OSWER Technical Guide For Assessing And Mitigating The Vapor Intrusion Pathway From Subsurface Vapor Sources To Indoor Air (OSWER Publication 9200.2-154; June 2015) can be found there among other resources and information.

20. Is T in the unlimited source, chronic, VF_s equation equal to the exposure duration (ED)?

    No, the chronic volatilization factor (VF_s) equation requires a relatively long T (exposure interval in seconds) due to the use of an approximation in deriving the average flux. Equations B1 and B2 in Jury et al. 1990 (Evaluation of Volatilization by Organic Chemicals Residing Below the Soil Surface. WATER RESOURCES RESEARCH, VOL. 26, NO. 1, PAGES 13-20, JANUARY 1990.) support this statement. In the current VF equation, VF is inversely proportional to the flux time. Plotting VFs where T=ED results in the following curve for all volatiles. The shape of this curve helps support the conclusion the VF model requires a long T.

    

21. How are the residential exposure durations (EDs) used in the RSL equations determined for carcinogenic (ag-adjusted), noncarcinogenic (child and adult), and mutagenic (multiple age bins), exposures?

    Residential exposure duration (ED_res) is set at 26 years, according to an OSWER directive based on the 2011 version of the Exposure Factor’s Handbook. When evaluating carcinogenic exposure, intakes are age-adjusted to account for exposure as a child and an adult within the 26 years. The OSWER directive sets child exposure at 6 years (ED_res-c). Therefore, ED_res – ED_res-c = 20 years of adult exposure (ED_res-a). For this exposure, child intakes are used with ED_res-c and adult intakes are used for ED_res-a.

    For noncarcinogenic residential exposures, the child and adult receptors are evaluated separately. The child exposure duration (ED_res-c) is limited to 6 years, and the adult exposure is assumed to be the entire 26 years (ED_res).

    The calculations for mutagenic exposures include four age bins: 2 years (ED_0-2), 4 years (ED_2-6), 10 years (ED_6-16), and 10 years (ED_16-26). The RSLs assume standard child intake rates through age 6 and standard adult intake rates for exposure from age 6 through 26. The RSL calculator allows for the entry of unique intakes for each of the four age-bins if desired.

22. How do the RSLs incorporate bioavailability into the soil ingestion equations?

    Absolute bioavailability can be thought of as the absorption fraction (PDF) (20 pp, 133 K). Relative bioavailability accounts for differences in the bioavailability of a contaminant between the medium of exposure (e.g., soil) and the media associated with the toxicity value (e.g., the arsenic RfD and SFO are derived from drinking water studies). The 60% oral RBA for arsenic in soil is empirically-based. It represents an upper-bound estimate from numerous studies where the oral RBA of soil-borne arsenic in samples collected from across the U.S. was experimentally determined against the water-soluble form. This RBA does not apply to dermal exposures to arsenic in soil for which the absorbed dose is calculated using a dermal absorption fraction (ABSd) of 0.03 (Exhibit 3-4 of USEPA, 2004). Absolute bioavailability can be thought of as the absorption fraction (PDF). Relative bioavailability accounts for differences in the bioavailability of a contaminant between the medium of exposure (e.g., soil) and the media associated with the toxicity value (e.g., the arsenic RfD and SFO are derived from drinking water studies). The 60% oral default RBA for arsenic in soil is empirically-based. It represents an upper-bound estimate from numerous studies where the oral RBA of soil-borne arsenic in samples collected from across the U.S. was experimentally determined against the water-soluble form. This RBA does not apply to dermal exposures to arsenic in soil for which the absorbed dose is calculated using a dermal absorption fraction (ABSd) of 0.03 (Exhibit 3-4 of USEPA, 2004).

    Arsenic screening levels for ingestion of soil are now calculated with the default relative bioavailability factor (RBA) of 0.6. The RBA can be adjusted using the calculator in site-specific/user-provided mode the same way toxicity values can be changed. The RBA for soil ingestion is shown in the calculator output. The 2012 document, Compilation and Review of Data on Relative Bioavailability of Arsenic in Soil (PDF) (58 pp, 474 K) provides supporting information.

    In 2017, the EPA has released a standard operating procedure for an in vitro bioaccessibility assay for arsenic in soil.  The in vitro method for predicting oral RBA of arsenic in soil (EPA SW846 Method 1340) has been validated, and it is now recommended that the in vitro method be used to estimate site-specific RBA, when site-specific RBA is needed. This method can provide a more comprehensive characterization of RBA variability at the site. The default value represents the 95th percentile of many arsenic soil samples, and it is expected that the site-specific RBA will be less than 0.6 at most sites, which means that the default should be protective for screening. Site-specific RBAs derived with the in vitro method should be verified with your Regional Risk Assessor. Guidance related to these topics can be found in the Soil Bioavailability at Superfund Sites: Guidance.

23. Where else can I go for toxicity studies (values) not on this site?

    The EPA toxicity value hierarchy is explained in the User's Guide of this website. For chemicals not listed in the hierarchy, toxicity information may be obtained by contacting the U.S. EPA Superfund Health Risk Technical Support Center at (513) 569-7300 or the Agency for Toxic Substances and Disease Registry (ATSDR) Information Center at 1-888-422-8737. Consult with your regional risk assessor when considering toxicity values not listed on these tables. The EPA's CompTox Chemicals Dashboard provides public access to chemical data. It is a widely used resource for chemistry, toxicity, and exposure information for hundreds of thousands of chemicals. For occupational exposure standards, try NIOSH, WHO, or OSHA. For information on nerve agents, contact DENIX.

24. I can't find the chemical that I am interested in. Why isn't it in your database? Are there other places where I should look to find the information that I need?

    The Generic Tables are not completely alphabetical. Some chemicals are listed together under a broader chemical group.

    If you are trying to locate various PAHs or PCBs, they are listed in the table under Polynuclear Aromatic Hydrocarbons and Polychlorinated Biphenyls, respectively. Also, dioxin congeners may be compared with the SL for congener 2,3,7,8-TCDD, once the appropriate Toxicity Equivalence Factors have been applied.

    Chemical groups are in bold type in the tables and chemicals in those groups are indented. Your chemical may be listed in one of the following chemical groups:

    • Cyanides
    • Dioxins
    • Furans
    • Lead Compounds
    • Mercury Compounds
    • Per- and Polyfluoroalkyl Substances (PFAS)
    • Perchlorates
    • Phosphates, Inorganic
    • Phthalates
    • Polychlorinated Biphenyls (PCBs)
    • Polynuclear Aromatic Hydrocarbons (PAHs)

    If you still cannot find the chemical in the database, it means that we have no EPA toxicity value for it. The SL table only includes chemical species for which we have toxicity values or MCLs.

    Consult with your regional risk assessor when searching for toxicity values not listed on these tables.

    There are many other useful toxicological/risk assessment sites on the internet. In many cases, data may be available but will require a literature search.

    The calculator allows the user to calculate SLs for a chemical not in our database. Select "Test Chemical" in the pick list and one can enter chemical-specific information for any chemical not already listed.

    RSL Chemical Name	Synonym
    Ethyl Chloride	Chloroethane
    Picramic Acid	2-Amino-4,6-dinitrophenol
    Stirofos	Tetrachlorovinphos
    Tertyl	Trinitrophenylmethylnitramine
    o-cresol	2-methylphenol
    m-cresol	3-methylphenol
    p-cresol	4-methylphenol
    Methylene Chloride	Dichloromethane
    Hexahydro-1,3,5-trinitro-1,3,5-triazine	RDX
    Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine	HMX
    Hexachlorocyclohexane, Gamma	Lindane

25. Can the oral RfDs in the SL Table be applied to dermal exposure?

    Not directly. Oral RfDs are usually based on administered dose and therefore tacitly include a GI absorption factor. Thus, any use of oral RfDs (or SFOs) in dermal risk calculations should involve removing this absorption factor. Consult the Risk Assessment Guidance for Superfund, Part A, Appendix A, for further details on how to do this. (See also RAGS Part E.)

    Note that the SL table displays the GIABS used in dermal SL calculations.

26. Why isn't oral/inhalation route-to-route extrapolation used to generate toxicity factors on the Screening Table?

    Previous versions of regional screening tables did contain some route-to-route extrapolation, because of the scarcity of inhalation toxicity factors. However, this was not optimal due to the uncertainty associated with making such adjustments (e.g., point-of-entry, first-pass, and route-specific effects may not be adequately considered by simple extrapolations). With the increasing availability of Tier III toxicity values, generic route-to-route extrapolation has been discontinued. Chemical-specific route-to-route extrapolation may be used by Tier I, II, or III sources after thorough consideration of the chemical-specific issues.

27. Previous Regional Tables used Inhalation Reference Doses (RfDi) and Slope Factors (SFI). Why does the new table use RfCs and IURs?

    In the past, some regional tables converted RfCs to RfDs and IURs to SFIs for inhalation. This was initially done because risk equations once relied upon RfDs and SFIs in units of mg/kg/day and 1/mg/kg/day, respectively. However, as the inhalation guidance has evolved, RfCs and IURs, in units of mg/m³ and μg/m³ respectively, have become the recommended toxicity factors. Please see Methods for Derivation of Inhalation Reference Concentrations (RfCs) and Application of Inhalation Dosimetry for more information. Also please see the FAQ concerning route-to-route extrapolation.

28. What are the sources of toxicity values and chemical-specific parameters used to calculate RSLs?

    Details of the toxicity value hierarchy can be found in Section 2.3 of the RSL User's Guide. Details of the chemical-specific parameter hierarchy can be found in Section 2.4 of the RSL User's Guide. Below is a brief description of the toxicity hierarchy.

    In 2003, EPA's Superfund program revised its hierarchy of human health toxicity values, providing three tiers of toxicity values in a memo (PDF). Three tier 3 sources were identified in that guidance, but it was acknowledged that additional tier 3 sources may exist. The 2003 guidance did not attempt to rank or put the identified tier 3 sources into a hierarchy of their own. However, when developing the screening tables and calculator presented on this website, EPA needed to establish a hierarchy among the tier 3 sources. The toxicity values used as “defaults” in these tables and calculator are consistent with the 2003 guidance. 

    Users of these screening tables and calculator wishing to consider using other toxicity values, including toxicity values from additional sources, may find the discussions and seven preferences on selecting toxicity values in the attached Environmental Council of States paper useful for this purpose (ECOS website), (ECOS paper).

    When using toxicity values, users are encouraged to carefully review the basis for the value and to document the basis of toxicity values used on a CERCLA site.

    Please contact a Superfund risk assessor in your Region for help with chemicals that lack toxicity values in the sources outlined above.

29. How do I convert (mg/m³)^-1 to (µg/m³)^-1?

    Vanadium Pentoxide has an inhalation unit risk (IUR) of 8.3 (mg/m³)^-1 presented in a PPRTV however, the RSL equations and database require IURs to be in (µg/m³)^-1. For the conversion to be correct, the superscript of -1 must be addressed. The IUR could be presented as 8.3 (m³/mg) without the superscript. From this point, multiply by 1mg/1000µg and the mg will cancel leaving 8.3E-03 (m³/µg). Now flip the units to give 8.3E-03 (µg/m³)^-1.

30. How do I convert ATSDR inhalation MRLs in parts per million (ppm) to mg/m³ and other general conversions?

    ATSDR lists the chronic inhalation MRL for formaldehyde as 0.008 ppm. To convert to a RfC in units of mg/m³, multiply the ppm value by the molecular weight in grams/mole then divide by 24.45. For example: (0.008 ppm * 30.026 g/mole)/24.45 = 0.0098244 mg/m³ which is rounded to 9.82E-03 mg/m³ in the RSL tables. The number 24.45 in the equation above is the volume (liters) of a mole (gram molecular weight) of a gas or vapor when the pressure is at 1 atmosphere (760 torr or 760 mm Hg) and at 25°C.

    The EPA hosts an Indoor Air Unit Conversion calculator that converts between common units of indoor air measurements including: µg/m³, mg/m³, µg/l, mg/l, ppb, and ppm. Use standard pressure and temperature values if unknown.

31. Why do Tap Water RSLs differ from IRIS drinking water concentrations when both are based on a target cancer risk of 1E-06?

    There are three reasons: 1) IRIS calculations include only exposure to tap water due to ingestion, while the RSLs also include dermal and inhalation exposures. 2) For ingestion and dermal exposure routes the RSL cancer equations age-adjust the intake rates between the child and the adult based on body weight and exposure duration while IRIS only considers the adult intake. 3) The RSL equations also time-adjust the lifetime intake of 70 years over a 26 year exposure period at 350/365 days a year while IRIS does not time adjust for exposure duration or days per year.

32. Why do air RSLs differ from IRIS air concentrations derived from the same inhalation unit risk values when both are based on a target cancer risk of 1E-06?

    The RSL equation time-adjusts the lifetime of 70 years over a 26 year exposure period at 350/365 days a year while IRIS does not time adjust for exposure duration or days per year.

    Example calculation:

    Take Azobenzene for instance. The IRIS IUR is 3.1E-05 (µg/m3)-1 and the IRIS air concentration at TR=1E-06 is 3E-02 (µg/m3) (rounded from 3.22E-02) while the RSL is 9.1E-02 (µg/m3). The difference in the values is the ratio of 365 days per year to 350 days per year (which is 1.04) and the ratio of 70 years to 26 years (which is 2.69). 3.2E-02 (µg/m3)-1 x 1.04 x 2.69 = 9.1E-02 (µg/m3).
    
    The IRIS equation is TR/IUR or 0.000001/0.000031 = 0.032(µg/m3) ; rounded to 0.03 (µg/m3). The RSL equation is (TR*AT*LT)/(EF*ED*ET*IUR) or (0.000001*365*70)/(350*26*(24/24)*0.000031) = 0.091 (µg/m3).

33. How are appendix 'toxicity screening values' used in RSL calculation and designated in the RSL tables?

    The Superfund Health Risk Technical Support Center (NCEA-Cincinnati) derives 'toxicity screening values' in circumstances where data are "insufficient to support derivation" of a Provisional Peer-Reviewed Toxicity Value (PPRTV) under current guidelines, yet it is determined that "information is available for this chemical, which may be of limited use to risk assessors." In such cases, NCEA-Cincinnati summarizes available information in an appendix to a 'PPRTV Derivation Support Document.' All derivation support documents prepared by the Superfund Health Risk Technical Support Center receive internal and external scientific peer review to ensure their appropriateness within the limitations detailed in the respective document. Neither PPRTVs nor appendix toxicity screening values typically receive the multi-program review provided for IRIS values. 

    PPRTVs are developed according to a Standard Operating Procedure and are derived after a review of the relevant scientific literature and generally using the same methods, sources of data, and Agency guidance for value derivation by the U.S. EPA IRIS Program. PPRTVs comprise the second tier of the OLEM (formerly OSWER) recommended sources of human health toxicity values. The RSLs for some chemicals are based upon appendix toxicity screening values. To alert users when these toxicity values are used, the key presents an "X" (for Appendix) rather than a "P" (for PPRTV). The reason for this distinction is because appendix toxicity screening values are distinct from PPRTVs and do not carry the same "weight" as PPRTVs.

    Appendix toxicity screening values receive the same level of internal and external scientific peer review as the PPRTVs from the main body of the documents to ensure their appropriateness within the limitations detailed in the document. The RSL work-group regards 'toxicity screening values' as a type of Tier 3 toxicity value.

    Given these considerations, users of the RSL tables should understand and recognize that medium-specific screening levels based upon appendix toxicity screening values generally have more uncertainty. Questions or concerns about the appropriate use of appendix toxicity screening values or RSLs based upon such toxicity values should be directed to the Superfund Health Risk Technical Support Center.

34. Various forms of a chemical, such as salts, acid forms, and ion forms, are commonly listed in the Regional Screening Levels (RSLs). How are the toxicity values, chemical-specific parameters, and analytical methods chosen for these different forms

    Toxicity profiles often present various forms of a chemical in the toxicity value derivation section. When a toxicity value is stated to be applicable for a list of chemical forms, the RSLs incorporate all available chemicals into the database. For instance, the perchlorate IRIS toxicity profile illustrates this approach, assigning the same Reference Dose (RfD) to four perchlorate salts and to the perchlorate ion itself.

    Similarly, some toxicity profiles recommend molecular weight adjustments to derive toxicity values for various forms of the chemical. The thallium PPRTV toxicity profile exemplifies this, with two thallium salt toxicity values derived through molecular weight adjustment. Additionally, the IRIS profile for hydrogen cyanide states that simple salts like sodium and potassium cyanide, which freely dissociate into cyanide, are to be calculated from the RfD by molecular weight adjustment.

    Chemical–specific parameters used in the RSLs are primarily tabulated with values matching the exact chemical structure. This is simple enough for properties like molecular weight.

    Analytical methods may not be available for all salt forms of a chemical. Many salts are listed in the RSLs, but in general analytical methods used would analyze for the component ions. For example, several thallium salts are in the RSL, such as thallium carbonate and thallium chloride, but the analytical methods used would analyze for thallium, carbonate, and/or chloride separately.

    For the PFAS chemical group in the RSLs, toxicity values are applied across the various forms of the respective PFAS, and chemical-specific parameters are applied as appropriate. When the acid form and most salt forms of PFAS are in an aqueous environment, they will dissociate into the PFAS anion and the respective cation (such as hydrogen or sodium). Hence, the same toxicity values are applied to perfluorooctanoic acid (PFOA), ammonium perfluorooctanoate, and perfluorooctanoate. However, chemical parameters are applied as appropriate. For instance, the vapor pressure and water solubility, parameters for PFOS and PFBS (acid forms) are applied to their respective ion forms but not salt forms. Also, analytical methods generally report the acid form concentration, but the method actually analyzes the ion form. 

35. Where can I find Asbestos screening levels?

    The RSL tables provide the asbestos MCL, but it is not included in the calculator picklist. Please consult the asbestos Technical Review Workgroup (TRW) directly for asbestos inquiries. The Asbestos Committee is available to provide technical support for questions concerning the assessment, removal or remediation of asbestos contamination at Superfund sites. Its goal is to support and promote consistent application of the best science in the field of risk assessment for asbestos at contaminated Superfund sites nationwide. The Framework for Investigating Asbestos Contaminated Superfund Sites (PDF) provides a step-wise approach to investigation of and risk management of asbestos.

36. The slope factors for benzene are actually ranges, yet the SL table shows only a single number. Which number was chosen and why?

    The upper end of the slope factor range was chosen. This is because the SL Table is a screening tool, and the consequences of screening out a chemical that could pose a significant risk are more serious than the consequences of carrying the chemical through to the next step of the risk assessment. (At each step of the risk assessment, the risk is further refined using site-specific analysis. Chemicals can always be eliminated from the risk assessment at a later step than the initial screening, if appropriate.)

37. The cadmium RSLs are labeled "Cadmium (Diet)" and "Cadmium (Water)." What do I use for soil and where do the toxicity values come from?

    "Cadmium (Diet)" is for food and soil use; "Cadmium (Water)" is for water use only. The diet RfD incorporates a 2.5% gastrointestinal absorption (GIABS) adjustment; the water RfD incorporates a 5% GIABS. These GIABS values are used in the RSL calculations to convert oral RfDs to Dermal RfDs, as directed by EPA Risk Assessment Guidance for Superfund (RAGS) Part E, Supplemental Guidance for Dermal Risk Assessment July 2004. Document (PDF) (186 pp, 4.2 MB) and website.

    The RSLs use the chronic MRL from ATSDR as the Cadmium RfD instead of the IRIS value in response to an EPA memo "Recommendations on the Use of Chronic or Subchronic Noncancer Values for Superfund Human Health Risk Assessments" released in May 2021; this memo recommends using lower tier toxicity values that are more recent, credible, and relevant than the values that would be automatically selected from the RSL toxicity hierarchy. In this memo, it was recommended that the ATSDR chronic RfD of 1E-04 mg/kg-day from 2012 be used over the IRIS values (5E-04 and 1E-03 mg/kg-day for water and diet, respectively) from 1989. The RSLs have adopted this recommendation and retain the GIABS values from RAGS Part E.

38. Why doesn’t cadmium use toxicity values from IRIS?

    The RSLs use the chronic MRL from ATSDR as the Cadmium RfD instead of the IRIS value in response to an EPA memo "Recommendations on the Use of Chronic or Subchronic Noncancer Values for Superfund Human Health Risk Assessments" released in May 2021; this memo recommends using lower tier toxicity values that are more recent, credible and relevant than the values that would be automatically selected from the RSL toxicity hierarchy. In this memo, it was recommended that the ATSDR chronic RfD of 1E-04 mg/kg-day from 2012 be used over the IRIS values (5E-04 and 1E-03 mg/kg-day for water and diet, respectively) from 1989. The RSLs have adopted this recommendation and retain the GIABS values from RAGS Part E.

39. Is the CAS number for Chlordane really for Technical Chlordane and what should I use for screening?

    Previous versions of the RSL tables presented "Chlordane" with CAS 12789-03-6. CAS 12789-03-6 is given by IRIS for Chlordane (technical). CAS 57-74-9 is reserved for nearly pure chlordane which is a mixture of the cis and trans isomers identified in the RSL pick list as alpha (CAS 5103-71-9) and gamma (CAS 5103-74-2) chlordane, respectively. The IRIS profile presents the description as follows:

    The composition of mixtures called technical chlordane varies, as it does from mixtures labeled chlordane (CAS No. 57-74-9). The U.S. EPA (1979) recognizes a mixture composed of 60% octachloro-4, 7-methanotetraydroindane (the "cis" and "trans" isomers) and 40% related compounds as technical chlordane. Infante et al. (1978) reported on analysis of technical chlordane having 38-48% cis- and trans-chlordane, 3-7% or 7-13% heptachlor, 5- 11% nonachlor, 17-25% other chlordane isomers, and a small amount of other compounds. Dearth and Hites (1991) identified 147 different compounds in a preparation of technical chlordane that included cis-chlordane (15%), trans-chlordane (15%), trans-nonachlor (15%), and heptachlor (3.8%). At least, the principal cis- and trans- isomers of chlordane and the component heptachlor may be metabolized to epoxides, oxychlordane and heptachlor epoxide, both of which have been detected in human tissues (reviewed in Nomeir and Hajjar, 1987). Unless otherwise indicated, all studies described in this document were carried out with technical grade chlordane; specific content will be given when available from the individual study.

    The RSLs now present "Chlordane (technical mixture)" with CAS 12789-03-6. All the toxicity values, chemical-specific parameters, and RSLs have not changed.

40. What are the sources of chromium toxicity values and how was the IUR derived?

    Beginning in the Fall 2009, we are more strongly encouraging the collection of valent-specific data when chromium is likely to be a COC at the site, and we are no longer calculating default screening levels for total chromium. We are instead calculating screening levels for Cr(III) using toxicity values derived for Cr(III) and using toxicity values derived for Cr(VI) for Cr(VI) screening levels. IRIS Provides two RfC values (8E-6 mg/m³ for chromic acid mists and Cr(VI) aerosols and 1E-4 mg/m³ for Cr(VI) particulates). Our default screening levels use the RfC of 1E-4 mg/m³ for particulates. Review of site specific information may warrant the use of the RfC of 8E-6 mg/m³ when chromic acid mists or dissolved Cr(VI) aerosols are being assessed. All of the toxicity values used for Cr(III) and Cr(VI) come from IRIS, except the oral slope factor for Cr(VI) which was published by the California Environmental Protection Agency.

    The State of California Environmental Protection Agency (CalEPA) determined that Cr(VI) by ingestion is likely to be carcinogenic in humans. CalEPA derived an oral cancer slope factor, based on a dose-related increase of tumors of the small intestine in male mice conducted by the National Toxicology Program. CalEPA determined that Cr(VI) was carcinogenic by mutagenic by mode of action.

    EPA's Office of Pesticide Programs (OPP) made a determination that Cr(VI) has a mutagenic mode of action for carcinogenesis in all cells regardless of type, following administration via drinking water. OPP recommended that Age-Dependent Adjustment Factors (ADAFs) be applied when assessing cancer risks from early-life exposure (< 16 years of age). This determination was reviewed by OPP's Cancer Assessment Review Committee and published in a peer review journal).

    Therefore, the RSL workgroup adopted the Tier III CalEPA value and the OPP recommendation with respect to mutagenicity. More recently, in 2011, external peer reviewers provided input on the EPA's Office of Research and Development Integrated Risk Information System draft Toxicological Review of Hexavalent Chromium. The majority of reviewers questioned the evidence used to support a mutagenic mode of action for carcinogenesis for Cr(VI). Furthermore, in 2011 California Environmental Protection Agency finalized its drinking water Public Health Goal for Cr(VI). CalEPA's Technical Support Document concluded in numerous studies that Cr(VI) is both genotoxic and mutagenic.

    Both of these assessments are considered Tier 3 sources and were used to derive the screening levels for Cr(VI). We applied ADAFs for early life exposure via ingestion and inhalation because OPP's proposed mutagenic mode of action for Cr(VI) occurs in all cells, regardless of type. Application of ADAFs for all exposure pathways results in more health-protective screening levels.

    In the RSL Table, the Cr(VI) specific value (assuming 100% Cr(VI)) is derived by multiplying the IRIS Cr(VI) Inhalation Unit Risk value by 7. This is considered to be a health-protective assumption, and is also consistent with the State of California's interpretation of the Mancuso study that forms the basis of Cr(VI)'s estimated cancer potency.

    If you are working on a chromium site, you may want to contact the appropriate regulatory officials in your region to determine what their position is on this issue.

    The Maximum Contaminant Level (MCL) of 100 µg/L for "Chromium (total)", from the EPA's MCL listing is shown on the total chromium line in the tables.

    For more information, see User Guide Section on Chromium.

41. How was the copper RfD derived?

    Currently the RfD is 0.04 mg/kg-day with a reference of HEAST. Actually, HEAST presents a concentration in drinking water screening level of 1.3 mg/L. In order to use the value to assess oral exposures to other media, we "back out" the adult exposure assumptions (e.g. body weight of 70 kg, ingestion rate of 2 L/day) that go into the calculation of a drinking water screening level.

42. Why was the RfC for trans-1,2 dichloroethylene from the PPRTV removed from the RSLs?

    The trans-1,2 dichloroethylene PPRTV was archived due to inconsistencies in the conclusions regarding RfC derivation. (See this memo (PDF) for further explanation.) The IRIS Toxicological Review of trans-1,2-dichloroethylene was released in 2010.

43. 2,4/2,6-dinitrotoluene mixture has a cancer slope factor, why don't the individual isomers use the same slope factor?

    It was determined for this website that the IRIS toxicological profile did not adequately address this issue.

44. How do I apply the haloacetic acid MCLs?

    The individual haloacetic acids (dichloroacetic acid, trichloroacetic acid, chloroacetic acid, bromoacetic acid, and dibromoacetic acid) all have the MCL of 60 µg/L listed in the RSL table. However, 60 µg/L is the MCL for Total haloacetic acids.

45. What is the inorganic lead residential soil SL?

    EPA has no consensus RfD or SFO for inorganic lead, so it is not possible to calculate SLs using the same methodology as was used for other chemicals. EPA considers lead to be a special case because of the difficulty in identifying a level of exposure, or dose, below which adverse effects are unlikely, which is needed to develop a RfD.

    The science on lead exposure has evolved to demonstrate that lead exposure is more dangerous to children’s health than we previously understood when we last updated our residential soil lead guidance in 1994. In January 2024, EPA issued its Updated Soil Lead Guidance for CERCLA Sites and RCRA Corrective Action Facilities.

    The updated guidance recommends a residential soil screening levels for lead in soil at residential sites to 200 ppm. However, for remedial response, a residential soil lead screening level of 100 ppm should be used if an additional source of lead is identified, such as lead service lines, lead-based paint, or non-attainment areas where concentrations of lead in air exceed the National Ambient Air Quality Standards. The residential inorganic lead soil screening level will be updated in the Tables with the May 2024 updates.

    For more information see Addressing Lead At Superfund Sites.

46. For manganese, IRIS shows an oral RfD of 0.14 mg/kg-day, but the SL Table uses 0.024 mg/kg-day. Why?

    The IRIS RfD includes manganese from all sources, including diet. The explanatory text in IRIS recommends using a modifying factor of 3 when calculating risks associated with non-food sources, and the SL table follows this recommendation. IRIS also recommends subtracting dietary exposure (default assumption in this case is 5 mg). Thus, the IRIS RfD has been lowered by a factor of 2 x 3, or 6. The table now reflects manganese for "non-food" sources.

47. Why is there no oral RfD for mercury? How should I handle mercury?

    IRIS gives oral RfDs for mercuric chloride and for methylmercury, but not for elemental mercury. Therefore, the SL Table follows suit. Consult your toxicologist to determine which of the available mercury numbers is suitable for the conditions at your site (e.g., whether mercury is likely to be organic or inorganic.)

48. Where did the cancer toxicity values for carcinogenic PAHs come from?

    Many of the PAH cancer toxicity values are calculated relative to benzo[a]pyrene, which has an IRIS slope factor and inhalation unit risk value. The relative potency factors (RPFs) for other PAHs can be found in Provisional Guidance for Quantitative Risk Assessment of Polycyclic Aromatic Hydrocarbons. The RPFs are listed in Section 2.3.6 of the User's Guide. There are other PAH toxicity values from California EPA derived without RPFs.

    The IRIS Profile gives the following instructions for RPF application:

    "It (BaP) also serves as an index chemical for deriving relative potency factors to estimate the carcinogenicity of other PAH congeners, such as in EPA's Relative Potency Factor approach for the assessment of the carcinogenicity of PAHs (U.S. EPA, 1993)."

    and

    "The inhalation unit risk for benzo[a]pyrene is derived with the intention that it will be paired with EPA's relative potency factors for the assessment of the carcinogenicity of PAH mixtures. In addition, regarding the assessment of early life exposures, because cancer risk values calculated for benzo[a]pyrene were derived from adult animal exposures, and because benzo[a]pyrene carcinogenicity occurs via a mutagenic mode of action, exposures that occur during development should include the application of ADAFs (see Section 2.5)."

    Relative Potency Factors for Carcinogenic Polycyclic Aromatic Hydrocarbons

    Compound	RPF
    Benzo(a)pyrene	1.0
    Benz(a)anthracene	0.1
    Benzo(b)fluoranthene	0.1
    Benzo(k)fluoranthene	0.01
    Chrysene	0.001
    Dibenz(a,h)anthracene	1.0
    Indeno(1,2,3-c,d)pyrene	0.1

49. Why is the tap water screening level for Perchlorate of 14 μg/L different from the preliminary remedial goal (PRG) of 15 μg/L calculated by the Office of Solid Waste and Emergency Response in its January 8, 2009, guidance (PDF) ?

    As described in the OSWER memorandum, the Agency issued an Interim Drinking Water Health Advisory (Interim Health Advisory) for exposure to perchlorate of 15 µg/L in water. A health advisory provides technical guidance to federal, state, and other public health officials on health effects, analytical methods and treatment technologies associated with drinking water contamination. The Interim Health Advisory for perchlorate was developed using EPA's RfD of 7E-04 mg/kg-day and representative body weight, as well as 90th percentile drinking water and national food exposure data for pregnant women in order to protect the most sensitive population identified by the National Research Council (NRC) (i.e., the fetuses of pregnant women who might have hypothyroidism or iodide deficiency).

    The NCP (40 CFR 300.430(e)(2)(A)(1)) provides that when establishing acceptable exposure levels for use as remediation goals (for a Superfund site), consideration must be given to concentration levels to which the human population, including sensitive subgroups, may be exposed without adverse effects over a lifetime or part of a lifetime, incorporating an adequate margin of safety. As a result of the publication of the Interim Health Advisory for perchlorate, OLEM (formerly OSWER) recommends that where no federal or state applicable or relevant and appropriate (ARAR) requirements exist under federal or state laws, 15 µg/L (or 15 ppb) is recommended as the PRG for perchlorate when making CERCLA site-specific cleanup decisions where there is an actual or potential drinking water exposure pathway. However, where State regulations qualify as ARARs for perchlorate, the remediation goals established shall be developed considering the State regulations that qualify as ARARs, as well as other factors cited in the NCP (see 40 CFR 300.430(e)(2)(i)(ff)). Final remediation goals and remedy decisions are made in accordance with 40 CFR300.430 (e) and (f) and associated provisions.

    Preliminary remediation goals are the starting points in the development of final cleanup levels at sites. As at all sites addressed under the NCP, these goals may be modified, depending on physical characteristics of a site, State laws and guidance, and other site specific factors, such as additional exposure routes.

    The tap water screening level of 14 µg/L is based on EPA's RfD and using standard RSL equations.

50. Since an earlier FAQ said that route to route extrapolations were not used by the RSLs to develop toxicity values, how were the inhalation unit risks derived for Polychlorinated biphenyls (PCBs)?

    Although it is true that route to route extrapolations (oral to inhalation or inhalation to oral) of toxicity values are not used by the RSLs, support for these inhalation unit risk values for PCBs is found in the IRIS assessment on PCBs. IRIS presents the oral slope factors for high, low and lowest risk in section II.B.3. of the IRIS Assessment . The IRIS high risk oral slope factor (SFO) is 2; low risk is 0.4; and lowest is 0.07 (mg/kg-d)^-1. IRIS states, "For inhalation of evaporated congeners, the middle-tier slope factor can be converted to a unit risk estimate and ambient air concentrations associated with specified risk levels." and "For inhalation of an aerosol or dust contaminated with PCBs, the slope factor for "high risk and persistence" should be used instead." So, take the "middle tier" SFO of 0.4 and divide by body weight over inhalation rate (70 kg/20 m³) and divide by 1000 µg/mg and you get 1.E-04 (µg/m³)^-1 IUR for low risk IUR. For the high risk take the SFO of 2 and divide by body weight over inhalation rate (70 kg/20 m³) and divide by 1000 µg/mg and you get 5.7E-04 (µg/m³)^-1 for high risk IUR. For the lowest risk take the SFO of 0.07 and divide by body weight over inhalation rate (70 kg/20 m³) and divide by 1000 µg/mg and you get 2E-05 (µg/m³)^-1 for lowest risk IUR.

    Aroclor 1016 is considered to be in the lowest risk tier and the other Aroclors on the RSL table are considered to be in the high risk tier.

    Example calculation:

    Take 1,4-Dioxane for instance. The IRIS equation for ingestion is as follows: (TR*BW*1000)/(SFO*IR) or (0.000001*70*1000)/(0.1*2) = 0.350 ug/L while the hypothetical , adult only, RSL equation would be (TR*AT*LT*BW*1000)/(SFO*EF*ED*IR) or (0.000001*365*70*80*1000)/(0.1*350*26*2.5) = 0.898 ug/L. Notice that the RSL equation uses the AT, LT, EF and ED exposure parameters to time-adjust exposure.

    The actual RSL cancer ingestion equation not only time adjusts but also age-adjusts. (TR*AT*LT*1000)/(SFO*EF*((EDc*IRc/BWc)+(EDa*Ira/BWa)) or (0.000001*365*70*1000)/(0.1*350*((6*0.78/15)+(20*2.5/80)) = 0.78 ug/L

51. [TCDD] Why was the TCDD (Dioxin) oral slope factor of 130,000 (mg/kg-day)^-1 (or 1.3E-04 (pg/kg-day)^-1) chosen?

    Several Tier 3 sources were available for TCDD oral slope factors. In the RSL hierarchy, CalEPA is the first Tier 3 source to have an oral slope factor, so it was selected. Below are tier 3 oral slope factor sources that can be considered:

    • EPA's Office of Health and Environmental Assessment (EPA 1985) developed an oral cancer slope factor of 1.56E-04 (pg/kg-day)^-1. This was based on the combined incidence of lung, palate, and nasal carcinomas and liver hyperplastic nodules or carcinomas in female rats in the study by Kociba et al. (1978).
       
    • EPA (1997a) (EPA's Health Effects Assessment Summary Table, or HEAST) provides an oral SFO of 1.5E-04 (pg/kg-day)^-1. The citation for the SFO in HEAST lists EPA (1985) as one of the sources for the HEAST value.
    
    
    • California (CalEPA) (1986, 2002) developed an oral cancer slope factor of 1.3E-04 (pg/kg-day)^-1. This is based on the occurrence of hepatocellular adenomas and carcinomas in male mice in a study by the National Toxicology Program (NTP 1982).
    
    
    • Michigan (MDEQ 1998) utilizes an oral cancer slope factor of 7.5E-05 (pg/kg-day)^-1, which is based on a re-analysis of the histological slides of livers from female rats from the Kociba et al. (1978) study using the liver tumor classification scheme proposed by NTP in 1986 (Maronpot et al. 1986, EPA 1990).
       
    • Minnesota (MNDOH 2003) uses an oral cancer slope factor of 1.4E-03 (pg/kg-day)^-1, which is based on the draft re-evaluation of the exposure-response data for liver cancer in female rats reported in the draft EPA (2003b) dioxin reassessment.
       
52. What toxicity values are used for TCE?

    It is recommended that a regional risk assessor be consulted when evaluating TCE in any medium especially when less than chronic exposure scenarios are considered. The Superfund program issued a Compilation of Information Relating of Early/Interim Actions at Superfund Sites and the TCE IRIS Assessment (PDF) (3 pp, 929 K) memo in August 2014. Several regions have issued their own guidance as well.

    IRIS has recently released a Toxicity Assessment for TCE. IRIS suggests that the kidney risk be assessed using the mutagenic equations and the liver and non-Hodgkin lymphoma (NHL) be addressed using the standard cancer equations. In order to make the calculator display the correct results for TCE, the standard cancer and mutagen equations needed to be combined. Since TCE requires the use of different toxicity values for cancer and mutagen equations, it was decided to make a toxicity value adjustment factor for cancer (CAF) and mutagens (MAF). The adjustments were done for oral (o) and inhalation (i). These adjustment factors are used in the TCE equation images presented in section 4 of the User's Guide. The equations used to generate adjustment factors are presented below. The adjustment factors are based on the adult-based toxicity values and these are the cancer toxicity values presented in the Generic Tables.

    

    The calculator, if run in default mode, will now produce accurate TCE RSLs for all land uses. The old three step process is no longer necessary.

53. How are total petroleum hydrocarbon (TPHs) considered in calculating RSLs?

    Traditionally, hydrocarbon-impacted soils at sites contaminated by releases of petroleum fuels have been managed based on their total petroleum hydrocarbon (TPH) content. TPH refers to the total mass of hydrocarbons present without identifying individual compounds. In practice, TPH is defined by the analytical method that is used to measure the hydrocarbon content in contaminated media. Since the hydrocarbon extraction efficiency is not identical for each method, the same sample analyzed by different TPH methods will produce different TPH concentrations.

    The hazard and health risk assessments that are typically conducted to support risk management decisions at contaminated sites generally require some level of understanding of the hydrocarbon chemical composition present in the contaminated media. Traditional TPH measurement techniques, however, provide no specific information about the detected hydrocarbons. Because TPH is not a consistent entity, the assessment of health effects and development of toxicity values for mixtures of hydrocarbons are problematic. In fact, many risk assessors prefer to analyze and assess the individual chemical constituents rather than rely on TPH data; consult with your regional risk assessor for site-specific recommendations. In cases where TPH data are used, more details about the provisional TPH toxicity values are provided below.

    In 2022, the Superfund Health Risk Technical Support Center National Center for Environmental Assessment Office of Research and Development published a document that provides the data, methods, and assumptions for deriving Provisional Peer-Reviewed Toxicity Values (PPRTVs) for six fractions of petroleum hydrocarbons. The six TPH fractions were assigned representative compounds for determination of toxicity values and chemical-specific parameters to calculate RSLs. 

    The PPRTV (PDF) paper was the principal source for the derivation of these values. The carbon ranges and representative compounds for the RfDs, RfCs, and chemical-specific parameters are listed in the table below. An average of the chemical-specific parameters for 1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, and 1,2,3-trimethylbenzene was calculated for the medium aromatic fraction. This is because all three are representative compounds for chronic and subchronic RfDs and RfCs. An average of the chemical-specific parameters for n-heptane, n-hexane, and cyclohexene was calculated for the low aliphatic fraction. This is because n-heptane represents the chronic RfC, n-hexane represents the subchronic RfC, and cyclohexene represents the the chronic and subchronic RfDs. 

    The carbon ranges, presented in the PPRTV assessment, may not exactly match the carbon ranges from laboratory results for TPH fractions. Additionally, the carbon ranges, presented in the PPPRTV assessment, may not match laboratory results for DRO, GRO, ORO, or RRO analysis. The TPH carbon ranges used in the RSLs are not intended to screen against DRO, GRO, ORO and RRO analysis. While the carbon ranges for TPHs and range organics may not exactly match, with direction from your regional risk assessor, reasonable assignment may be achieved.

    TPH Fractions	Number of Carbons	Equivalent Carbon Number Index	Representative Compound (RfD/RfC)	Representative Compound (Chemical Parameters)
    Low aliphatic	C5-C8	EC5-EC8	cyclohexene/n-heptane and n-hexane	average of cyclohexene, n-heptane, and n-hexane
    Medium aliphatic	C9-C18	EC>8-EC16	midrange aliphatic hydrocarbon streams	n-nonane*
    High aliphatic	C19-C32	EC>16-EC35	white mineral oil	white mineral oil
    Low aromatic	C6-C8	EC6-EC<9	benzene	benzene
    Medium aromatic	C9-C10	EC9-EC<11	1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, and 1,2,3-trimethylbenzene	average of 1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, and 1,2,3-trimethylbenzene
    High aromatic	C10-C32	EC>11-EC35	benzo[a]pyrene	benzo[a]pyrene

    *A medium aliphatic representative compound was not listed in the PPRTV paper so n-nonane was selected by the RSL work-group to represent the chemical-specific parameters.

    Within the Superfund program, when TPHs are considered in the site characterization, they are assessed in supplemental health risk assessments only for potential noncancer health effects. Therefore, starting with the May 2014 update the RSLs are no longer calculated using cancer toxicity values. Most of the carcinogens in the TPH carbon range are individually listed on the RSL table. Combining TPH and individual constituent cancer risks would be overly protective.

54. How do I apply the trihalomethane MCLs?

    The individual trihalomethanes (bromodichloromethane; bromoform; dibromochloromethane, chloroform) all have the MCL of 80 µg/L listed in the RSL table. However, 80 µg/L is the MCL for Total Trihalomethanes.

55. IRIS presents 2 types of toxicity values for vinyl chloride yet the SL table shows only a single number. Which number was chosen and why?

    The vinyl chloride calculations were based on the examples given in the Toxicological Review for vinyl chloride, which appears on IRIS. IRIS presents "continuous lifetime exposure during adulthood" and "continuous lifetime exposure from birth" slope factors and inhalation unit risks. Because the equations used on this website show the individual lifetime segments, the "continuous lifetime exposure during adulthood" toxicity values are chosen.

    The examples in the Toxicological Review indicate that, during childhood, both pro-rated and non-pro-rated risks should be generated using the lower slope factor or IUR. When estimating the risk using this method and considering the lifetime segments during childhood and adulthood, it is clear that the cancer risks early in life are higher than those that would be generated if the typical pro-rated risks were simply generated using the lifetime SFO or IUR. This finding is consistent with the IRIS assessment's statements that cancer risk is increased during early life.

    Over the course of a 70-year lifetime, the risk generated using the pro-rated and non-pro-rated segments, along with the lower SFO or IUR, generally exceeds the risk generated using only pro-rated exposure and the lifetime SFO or IUR. However, the former risk estimates trend closer and closer to the latter as life advances, and converge at about the 70-year mark.

56. Why is the White Phosphorus CAS the generic phosphorus CAS of 7723-14-0 when it should be 12185-10-3?

    IRIS uses the name, "white phosphorus" with the generic CAS of 7723-14-0. The RSLs make every effort to retain the CAS of the toxicity source. The chemical properties for white phosphorus were only taken from the ATSDR MRL Toxicity Profile (PDF) (248 pp, 4.66 MB).

57. Where do the RfDs and RfCs for the xylene congeners come from?

    The IRIS RfD and RfC values for "xylene, mixture" are used as surrogate values for the individual congeners. The earlier RfD values for some xylene isomers were withdrawn from our electronic version of HEAST. The IRIS Toxicological Review for xylene (2003) has a RfC value that replaces values from Cal EPA.

    For links to guidance on vapor intrusion, see https://www.epa.gov/vaporintrusion. The EPA 2002 interim draft Vapor Intrusion Guidance can be found there.

    The residential and industrial air regional screening values can be used to screen chemicals that are detected in the air (e.g., indoor and outdoor) from a variety of sources.

For assistance/questions please use the Regional Screening Levels (RSLs) contact us page. For general risk assessment questions, separate from the RSLs, please use the link below.