Fine Particulate Matter (PM 2.5) in Air - Benchmarks, Standards and Guidelines Established to Protect Public Health

The following is a description of some the benchmarks, standards and guidelines EPA used to evaluate environmental conditions in the aftermath of the World Trade Center disaster.

Airborne particles like smoke and dust are solids and liquid droplets of different sizes suspended in the air. EPA sets the screening level to caution sensitive groups, including children, the elderly, and people with heart or lung disease. For fine particles, that level is 40 ug/m3* (the mass of fine particles in micrograms (a millionth of a gram) in a cubic meter of air) for 24-hour average exposure. This level is based on EPA's Air Quality Index . Above this level, we recommend that sensitive groups reduce their exposure. Shorter term peaks are also of potential concern for sensitive groups. The screening level for the general population is 65 ug/m3 over a 24 hour period. Most healthy adults and children are expected to recover quickly from any symptoms of short-term exposure like coughing, wheezing, or eye and throat irritation. EPA expects the long-term risk from these short-term exposures to be low.

There were two methods for measuring particulate matter (PM) used at the monitoring sites presented on the EPA website.

The filter method is based on ambient air being drawn through a filter which traps particles. This provides a 24-hour integrated sample. After the sample is taken, the filter is collected, weighed in a laboratory, and recorded by laboratory staff. This is the method that is being used for PM10 monitoring at the Park Row, Chambers Street, and Coast Guard sites.

The continuous method is based on ambient air being drawn into a chamber to be analyzed mechanically and the recorded electronically. We installed continuous monitors for PM 2.5 at all of the sites in lower Manhattan. We also had continuous monitors for PM10 at the Albany Street and Wall Street sites.

The primary difference between filter and continuous methods is that the continuous monitors report hourly-average concentrations and a daily average of the 24 one-hour averages, while the filter-based monitors provide one 24-hour average concentration.

* The unit, ug/m3, is the mass of fine particles in micrograms (a millionth of a gram) in a cubic meter of air.

See the Particulate Matter Fact Sheet for more information about particulate matter and how to reduce your exposure.

View Monitoring Data for Fine Particulate Matter (PM 2.5) in Air