The Indoor Microbiome

The quality of air indoors is impacted by all the living things in it. All of the living microorganisms inside an indoor environment, and their by-products, are known collectively as an indoor microbiome. Research on indoor microbiomes has rapidly expanded, leading to an increased public interest. This page describes the indoor microbiome and its relationship with indoor air quality (IAQ).

On this page:

Microbiomes Reports from the National Academy of Science, Engineering and Medicine

What is the Indoor Microbiome?

The indoor microbiome is a complex community made up of all the living microorganisms found in an indoor environment and their fragments and by-products. The microbiome that surrounds us is diverse and dynamic. It is made up of bacteria, viruses, fungi, and other single-celled organisms. These microorganisms may be growing or inactive, and may be present in different life stages, such as spores or growing cells. The indoor microbiome also includes non-living things such as fragments of microorganisms or materials and chemicals produced by microorganisms.

What’s a “Normal” Indoor Microbiome?

image of mold on a ceiling
Visible microbial growth of any kind should be addressed

Every building has a microbiome, but there is no normal or typical indoor microbiome found in all homes or other buildings. The indoor microbiome in an individual home or building depends on many things, including the people and pets that occupy it and their behaviors; the type, design, maintenance, building material, location of the home or building; and climate and other local conditions.

In general, testing for microorganisms, or for odors or chemical compounds released by microorganisms, is not necessary. Testing and sampling may help determine which microorganisms are present, but it is generally expensive and is usually done for research purposes. Remember, there is no standard for a “normal” or “healthy” indoor microbiome and some microorganisms will always be present indoors. While we know that diverse populations of microorganisms can be found indoors, we still have much to learn about how they interact with each other, the indoor environment, and the human and animal building occupants.

It is important to note that properly maintained indoor environments should not have visible mold or other microbial growth. If mold growth is visible it should be addressed promptly.

How Can the Indoor Microbiome Affect My Health?

Human beings encounter microorganisms every day, and those microorganisms can have neutral, adverse, or beneficial effects depending on the person, their health, the microorganism, and many other variables. For example, some microorganisms or their products are linked to allergies or asthma, but exposure to some microorganisms early in life may also protect against asthma and recurrent wheeze in children. Much remains unknown about the direct and indirect connections between humans and the indoor microbiome. Many ongoing research projects aim to understand the interactions between microbiomes, the indoor environment and human health.

Where Do the Microorganisms in the Indoor Environment Come from?

animation depicting the sources of indoor microbiomes - human shedding, pet shedding, ourdoor air/soil, and indoor mold growth

The animation above shows how microorganisms enter, leave, and move around the indoor microbiome. Most microorganisms indoors come from people, pets, pests, air, water, and areas of active microbial growth indoors. Once they settle on surfaces, microorganisms can re-enter the air by resuspension, which is stirring up settled dust or dirt. This happens as people and pets move around the home or do activities. Some building features and conditions can impact the growth of microorganisms and their resuspension indoors, such as building materials, temperature and humidity, condensation or water leaks, and airflow patterns. How microorganisms move with the air indoors may be influenced by heating, ventilation, and air conditioning (HVAC) systems, window and door opening, and fan usage, among other factors. 

Video: Particles in Your Indoor Air

Particles in Your Indoor Air and Strategies to Improve Indoor Air Quality

How Should I Maintain an Indoor Space, Taking the Indoor Microbiome into Consideration?

Remember, there is no standard for a normal indoor microbiome and some microorganisms will always be present indoors. Microorganisms may have neutral, adverse, or beneficial effects in different people. In many cases, it is not yet clear what actions are likely to have specific effects on the indoor microbiome or how to predict what impact those actions could have on human health. While some products may claim to alter the indoor microbiome to make it healthier, research has not yet shown any health benefits from intentionally adding microorganisms directly to the air or the indoor environment.

Some actions that can be taken to maintain a generally healthy indoor environment include:

  • Control moisture, including humidity, condensation, and water leaks.
  • Promptly dry areas that are wet and keep areas that are supposed to be dry, dry.
  • Clean up visible mold.
  • Regularly clean surfaces, including floors, walls, fixtures, and furnishings. For most routine cleaning of hard surfaces, damp wiping with clean water or water and a detergent is sufficient.
  • Using doormats at exterior doors and taking off your shoes when you enter your home can reduce the dirt and other contaminants that get tracked in from outside.
  • Ventilation, such as opening windows to bring in outdoor air, can help to lower the concentrations of indoor air pollutants in your home.
  • Consider using portable air cleaners that are properly sized to the room and well located. For more information, see EPA’s Guide to Air Cleaners in Your Home.
  • Get your HVAC system inspected regularly and upgrade filters if possible. You may need to consult a professional HVAC technician to determine the highest efficiency filter that will work best for your system.

Learn more about improving indoor air quality.

If your indoor space was in a flood or related event please see, Flood Cleanup to Protect Indoor Air and Your Health.

How Does Water or Moisture Affect the Indoor Microbiome?

Water or moisture indoors is often the most important factor for any microbial growth. Sources such as excess humidity, condensation, or water leaks, can encourage the growth of mold and other microorganisms and may attract or support other unwanted pests (such as insects or mice). Water or moisture present for long periods of time in areas that can’t dry out, or on materials that absorb water, such as carpeting or drywall, will increase the likelihood of mold growth. Controlling moisture and regular cleaning can help to reduce the growth of indoor microorganisms.

Microbiome Reports from the National Academy of Science, Engineering and Medicine (NASEM)

Microbiomes of the Built Environment

Cover of the Microbiomes of the Built Environment Report

The National Academies of Science, Engineering, and Medicine (NASEM) released a report co-sponsored by the EPA in 2017, Microbiomes of the Built Environment, that examines microbial communities in built environments and the impacts of these microbial communities on health. The study describes the state of scientific research and proposes a research agenda with areas of focus to move the field from research to application.

Report Brief for:

Visit the NASEM for additional information. 

Environmental Chemicals, the Human Microbiome, and Health Risk: A Research Strategy

Cover of Environmental Chemicals, the Human Microbiome, and Health Risk: A Research Strategy

(NASEM) released a report, co-sponsored by the EPA in 2017, Environmental Chemicals, the Human Microbiome, and Health Risk: A Research Strategy, that proposes a research strategy to advance understanding of the interactions between environmental chemicals and the human microbiome and the implications of those interactions on human health risk. The report also highlights key aspects of the human microbiome and its relation to health, describes potential interactions between environmental chemicals and the human microbiome, reviews the risk-assessment framework and reasons for incorporating the proposed research, describes methods for studying the microbiome, and identifies barriers for research and opportunities for collaboration.

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Last updated on March 20, 2024