PM2.5 NAAQS Implementation
The term "particulate matter" (PM) includes both solid particles and liquid droplets found in air. Many manmade and natural
sources emit PM directly or emit other pollutants that react in the atmosphere to form PM. These solid and liquid particles
come in a wide range of sizes.
Particles less than 10 micrometers in diameter (PM10) pose a health concern because they can be inhaled into and accumulate in the respiratory system. Particles less than 2.5 micrometers in diameter (PM2.5) are referred to as "fine" particles and are believed to pose the largest health risks. Because of their small size (less than one-seventh the average width of a human hair), fine particles can lodge deeply into the lungs.
Health studies have shown a significant association between exposure to fine particles and premature mortality. Other important effects include aggravation of respiratory and cardiovascular disease (as indicated by increased hospital admissions, emergency room visits, absences from school or work, and restricted activity days), lung disease, decreased lung function, asthma attacks, and certain cardiovascular problems such as heart attacks and cardiac arrhythmia. Individuals particularly sensitive to fine particle exposure include older adults, people with heart and lung disease, and children.
Sources of fine particles include all types of combustion activities (motor vehicles, power plants, wood burning, etc.) and certain industrial processes. Particles with diameters between 2.5 and 10 micrometers are referred to as "coarse." Sources of coarse particles include crushing or grinding operations, and dust from paved or unpaved roads.
In 1997, EPA established annual and 24-hour NAAQS for PM2.5 for the first time. In 2006, EPA revised the 24-hour NAAQS for PM2.5. Additional information on these standards and related implementation activities is provided here.