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Particulate matter is composed of small solid and liquid particles suspended in the ambient air, and research studies have associated exposure to elevated levels of these particles in the air with damaging health effects.

The information on these pages describes the research being done by EPA to better understand how particles are emitted into the air or how they form in the air from gaseous pollutants, how they are transported, how people are exposed to them, and the health effects people may experience after they breathe in these particles.

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What is Particulate Matter, and Why and How Do We Study It?

Particulate matter (PM) in the ambient air comes from a variety of sources and has an equally broad range of size, composition, and other characteristics. Numerous scientific studies have provided evidence that PM in the ambient air is associated with increases in daily mortality, even at levels below the National Ambient Air Quality Standards (NAAQS, usually pronounced as "nacks") for PM that were in place before 1997. In that year, EPA set new NAAQS for PM that included standards for PM2.5, particles smaller than 2.5 µm in aerodynamic diameter. PM2.5 is composed of particles that are directly emitted and particles that are products of chemical reactions of gases in the atmosphere. Examples of directly emitted particles (both PM2.5 and larger, or "coarse," particles) include those from combustion sources such as residential wood combustion, agricultural open burning, coal and oil fired power plants and industries, as well as dust particles from roads and fields. Particles formed in the atmosphere are referred to as "secondary" particles, and the majority of these particles in many areas of the country are from gases from fuel combustion in automobiles, trucks, and power plants such as sulfur dioxide (SO2) and nitrogen oxides (NOx) released by anthropogenic and natural sources.

Many questions remain about particles and why they are associated with adverse health effects, including premature mortality and higher instances of respiratory illness. EPA is conducting research to better understand which attributes of particles may be causing these health effects and who may be most susceptible to their effects, how people are exposed to PM air pollution, how particles form in the atmosphere, and what the contributions are from various sources in the different regions of the country. Three research laboratories and two research centers in EPA's Office of Research and Development are participating in these studies, through work by EPA scientists and researchers at universities and research institutes across the nation. Health research is being conducted by EPA's National Health and Environmental Effects Research Laboratory, and by five PM Research Centers supported by the National Center for Environmental Research. Studies of how people are exposed to ambient particles are being conducted by the National Exposure Research Laboratory, whose scientists are also working to find better means of predicting how particles and gases transform and move in the ambient air, developing air quality models for use by decision makers, improved approaches to accurately measure the levels of PM in the air, and methods to more accurately link particles collected in ambient air to their originating sources. The National Risk Management Research Laboratory is conducting tests to measure the size and composition of particles emitted from different types of sources, including industrial boilers, the burning of biomass in fields and forests, and diesel engines, as well as measuring emissions of gases such as ammonia that can react with other gases in the air to form particles. The research conducted by EPA scientists in each of these areas is expanded by drawing on the expertise of researchers across the country through grants developed and awarded by the National Center for Environmental Research. These efforts, and research results from around the world, are collected and evaluated by the National Center for Environmental Assessment, which develops EPA's official review of the scientific evidence of health and environmental damage from exposure to ambient PM, called the Air Quality Criteria Document for Particulate Matter.

Details on EPA's research and assessment efforts are found by accessing the links below, which describe the ongoing work at each Laboratory and Center.

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