Particulate Matter in Ambient Air

Particulate matter (PM) is a pollutant as a mixture of solid particles, liquid droplets and mixture of solid and liquid particles in the air, such as dust, dirt, soot, or smoke. These particles less than 10 micrometers in diameter cause serious health problems, because they can get into human’s lung, and some may even get into bloodstream. Some coarse particles are emitted directly from sources, such as construction sites, unpaved roads, pestisides, smokestacks or fires. Others form in complicated reactions in the atmosphere of chemicals such as sulfur dioxides and nitrogen oxides that are emitted from power plants, industries and automobiles. These particles make up most of the fine particle pollution in the air. These fine particles are the major cause of reducing visibility known as hazeness in the sky.

US Environmental Protection Agency (EPA) regulates inhalable particles up to 10 micrometers in size.

EPA’s national and regional rules to reduce emissions of pollutants that form particle pollution will help state and local governments meet the Agency’s national air quality standards.

The Clean Air Act requires EPA to set National Ambient Air Quality Standard (NAAQS) for six pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, PM, sulfur dioxide (see table attached). The Clean Air Act established two types of national air quality standards for particle pollution. Primary standards set limits to protect public health, including the health of “sensitive” populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings.

The nation’s air quality standards for particulate matter were first established in 1971 and were not significantly revised until 1987, when EPA changed the indicator of the standards to regulate inhalable particles smaller than, or equal to, 10 micrometers in diameter (PM₁₀). Ten years later, after a lengthy review, EPA revised the PM standards, setting separate standards for fine particles up to 2.5 micrometers in diameter (PM_2.5) based on their link to serious health problems ranging from increased symptoms, hospital admissions and emergency room visits for people with heart and lung disease, to premature death in people with heart or lung disease.

The 1997 standards also retained but slightly revised standards for PM₁₀ which were intended to regulate “inhalable coarse particles” that ranged from 2.5 to 10 micrometers in diameter. PM₁₀ measurements, however, contain both fine and coarse particles.

EPA revised the air quality standards for particle pollution in 2006. The 2006 standards tightened the 24-hour fine particle standard from the level of 65 micrograms per cubic meter (µg/m³) to 35 µg/m³, and kept the annual fine particle standard at 15 µg/m³. The Agency decided to retain the existing 24-hour PM₁₀ standard of 150 µg/m³. The Agency revoked the annual PM₁₀ standard, because available evidence did not suggest a link between long-term exposure to PM₁₀ and health problems.

The Clean Air Act requires EPA to review the latest scientific information and standards every five years.

National Ambient Air Quality Standards

⁽¹⁾ Not to be exceeded more than once per year.

⁽²⁾ Final rule signed October 15, 2008.

⁽²⁾ Final rule signed October 15, 2008.

⁽³⁾ Not to be exceeded more than once per year on average over 3 years.

⁽⁴⁾ To attain this standard, the 3-year average of the weighted annual mean PM2.5 concentrations from single or multiple community-oriented monitors must not exceed 15.0 µg/m3.

⁽⁵⁾ To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area must not exceed 35 µg/m3 (effective December 17, 2006).

⁽⁶⁾ To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.075 ppm.  (effective May 27, 2008)

⁽⁷⁾ (a) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm.
    (b) The 1997 standard—and the implementation rules for that standard—will remain in place for implementation purposes as EPA undertakes rulemaking to address the transition from the 1997 ozone standard to the 2008 ozone standard.

⁽⁸⁾ The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is < 1.