Health Effects: Particulate soot (particularly "fine" particles - see below) has been linked to tens of thousands of premature deaths every year. It also is associated with increased emergency room visits, asthma attacks, decreased lung function and other respiratory problems. Those most at risk include the elderly, people with cardiopulmonary disease such as asthma, and children.

Environmental Effects: Particulate soot is a major cause of reduced visibility in many parts of the United States. It also can cause damage to paints and building materials.

Sources: Particulate matter (PM) is the general term for the mixture of solid particles and liquid droplets in the air. Some particles are large enough to be visible as smoke or soot. Others are so small they can be detected only with an electron microscope. "Fine" particles (less than 2.5 micrometers in diameter, known in the jargon of air pollution as "PM 2.5") result from motor vehicles, coal-burning electric power plants, factories as well as from residential fireplaces and wood stoves. Larger "coarse" particles come largely from windblown dust, vehicles traveling on unpaved roads, and crushing and grinding operations. Some particles are emitted directly from their sources, for example, smokestacks and cars. In other cases, gases such as sulfur oxides and nitrogen oxides interact with other compounds in the air to form fine particles. These tiny bits of soot can travel hundreds of miles downwind of the original pollution sources.

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