The Regional Air Pollution Control Agency

Particulate Matter

Nature of the Pollutant

Particulate matter is the technical term for airborne dirt. It can be considered in three particle size ranges. The first to be regulated is called total suspended particulates or TSP. It includes a broad range of large and small particles. TSP is generally considered an outdated category, the NAAQS for TSP has been revoked, and we no longer continue to monitor for it. We were well in attainment of the former TSP NAAQS when monitoring was discontinued.

The next category of particulates to be regulated includes solid and liquid particles that are very small, having an effective aerodynamic diameter of 10 microns (10 micrometers, approximately 0.0004 inches) or less, and is known as PM10. Larger particles are kept from the lungs by mechanical means, such as impaction in the nose, throat, and larynx, but these smaller respirable particles can slip past those body defenses and penetrate deep into the lungs where they can harm lung tissue. PM10 can harm such tissues as the linings of the nose, throat, and lungs both by mechanical irritation, similar to getting a speck of dirt in the eye, and by carrying adhering harmful substances deep into the lungs.

We have been monitoring PM10 for over 10 years. The third, and newest, category of particulates to be regulated is called PM2.5 because it is concerned with only particles smaller than 2.5 microns. The latest research shows that these are the particles most damaging to health. The health effects, according to the USEPA, "include premature death and increased hospital admissions and emergency room visits (primarily the elderly and individuals with cardiopulmonary disease); increased respiratory symptoms and disease (children and individuals with cardiopulmonary disease such as asthma); decreased lung function (particularly in children and individuals with asthma); and alterations in lung tissue and structure and in respiratory tract defense mechanisms. " Our PM2.5 monitors were first set up during 1999, and one was added in mid-2001.

The Standards

In 1987 the TSP National Ambient Air Quality Standard (NAAQS) was retired and a health-based standard for PM10 was established. It currently requires a maximum value of 50 µg/m3 (micrograms per cubic meter) as an annual arithmetic mean, and a 24-hour average whose 99th percentile form, averaged over 3 years, cannot exceed 150 µg/m3.

In 1997 the USEPA added a PM2.5 standard to account for health effects of these very fine particles. USEPA states that "Areas will be in compliance with the new annual PM2.5 standard when the 3-year average of the annual arithmetic mean PM2.5 concentrations, from single or multiple community-oriented monitors, is less than or equal to 15 µg/m3.

For the new 24-hour PM2.5 standard, the form is based on the 98th percentile of 24-hour PM2.5 concentrations in a year (averaged over 3 years), at the population-oriented monitoring site with the highest measured values in an area." In addition, the PM10 standard was modified slightly.

See USEPA's Revised NAAQS page for more information about the new/revised standards.

The Monitoring Technique

RAPCA utilizes reference method high-volume samplers to measure the concentration of PM10. A high-volume sampler works like a vacuum cleaner; it draws a volume of air through a filter that captures the particles suspended in the air. The sampler is designed so that only particulates 10 microns or less in aerodynamic diameter are captured.

The filters are dried and weighed both before and after exposure. Drying ensures that their moisture content remains constant. In this way, it can be determined how much particulate matter has been captured during the 24-hour sampling period, from midnight to midnight every six days. The total air volume sampled, of equal importance in determining concentration, is also determined for each filter.

PM2.5 is measured in a somewhat similar fashion to PM10, but is much more tightly controlled. The samplers run at a different frequency from the PM10 samplers. Measurement of one PM2.5 value actually includes the recording of 10 different parameters. The tiny filters are sent to a special PM2.5 lab in Cincinnati for processing. For this reason, PM2.5 data are not available in as timely a fashion as other PM measurements. RAPCA is experimenting with special continuous PM2.5 monitors.