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Fine Particle (PM2.5) Designations

Basic Information

Smoky Mountains obscured by smog

Smog, including PM2.5, obscures the Smoky Mountains.

Particle pollution is a mixture of microscopic solids and liquid droplets suspended in air. This pollution, also known as particulate matter, is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, soil or dust particles, and allergens (such as fragments of pollen or mold spores).

Fine particle pollution or PM2.5 describes particulate matter that is 2.5 micrometers in diameter and smaller - 1/30th the diameter of a human hair.

Fine particle pollution can be emitted directly or formed secondarily in the atmosphere. Examples Sulfates are a type of secondary particle formed from sulfur dioxide emissions from power plants and industrial facilities. Nitrates, another a type of fine particle, are formed from emissions of nitrogen oxides from power plants, automobiles, and other combustion sources.

The chemical composition of particles depends on location, time of year, and weather.

Health studies have shown a significant association between exposure to fine particles and premature death from heart or lung disease. Fine particles can aggravate heart and lung diseases and have been linked to effects such as: cardiovascular symptoms; cardiac arrhythmias; heart attacks; respiratory symptoms; asthma attacks; and bronchitis. These effects can result in increased hospital admissions, emergency room visits, absences from school or work, and restricted activity days. Individuals that may be particularly sensitive to fine particle exposure include people with heart or lung disease, older adults, and children.

EPA issued the fine particle standards in 1997 after evaluating hundreds of health studies and conducting an extensive peer review process. The 1997 annual standard was established as a level of 15 micrograms per cubic meter (µg/m3), based on the 3-year average of annual mean PM2.5 concentrations. The 1997 24-hour standard was established as a level of 65 µg/m3, determined by the 3-year average of the annual 98th percentile concentrations. On September 21, 2006, EPA strengthened the 24-hour fine particle standard from the 1997 level of 65 µg/m3 to 35µg/m3, and retained the annual fine particle standard at 15µg/m3.

At the time the fine particle standards were established in 1997, EPA also issued standard methods for monitoring fine particle levels in the ambient air to determine which parts of the country were subject to unhealthy levels. EPA worked closely with the states and tribes to deploy a nationwide network of more than 1,200 monitors. EPA now uses air quality data from these monitors, and subsequently sited monitors, to inform the area designation process, which plays an important role in identifying for the general public whether the air quality in a given area is healthy.

In June 2007, EPA issued a memorandum outlining the schedule for designating areas under the 2006 revised 24-hour PM2.5 standard and related guidance on the factors to consider in identifying nonattainment areas. The Clean Air Act provides for states and tribes to submit designation recommendations to EPA, and it requires EPA to provide time for consultation in cases where the Administrator plans to promulgate a designation that modifies the state or tribal recommendation. (Tribes are not required to provide recommendations but are invited to do so and participate in the process.)

EPA designates an area as nonattainment if it has violated the fine particle standards over a three-year period, or if relevant information indicates that it contributes to violations in a nearby area. EPA also may designate an area as attainment/unclassifiable, if: 1) monitored air quality data show that area has not violated the fine particle standards over a three-year period; or if 2) there is not enough information to determine the air quality in the area.

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