What facilities are required to report to the NEI?

The emissions reporting thresholds are in the Air Emissions Reporting Rule (AERR), 40 CFR Part 51, Subpart A, Appendix A, Table 1, which require state/local air agencies to report emissions as point sources for facilities that meet or exceed these emissions thresholds. 

The AERR requires states/locals to report to EPA, and the state/local rules require facilities to report to them.

While some states/locals use the same thresholds as the AERR, other states have lower limits and also require Hazardous Air Pollutants to be reported.  Usually, the states/locals also send in these smaller point sources voluntarily and they are included in the NEI as well.  A select few tribes also report point sources.

When the emissions are not included in the National Emissions Inventory as point sources, they are captured as nonpoint sources as a county total for emissions processes, called source classification codes (SCCs).