Particulate matter is the term for solid or liquid particles found in the air.
Some particles are large or dark enough to be seen as soot or smoke.
Others are so small they can be detected only with an electron microscope. The new standard focuses on smaller particles that are likely responsible for adverse health effects because of their ability to reach the lower regions of the respiratory tract. The elderly, children, and people with chronic lung disease, influenza, or asthma, tend to be especially sensitive to the effects of particulate matter. Particulate matter emissions from sources such as fuel combustion, industrial processes, and transportation decreased 17 percent since 1985. Emissions from residential wood combustion decreased 50 percent in the past 10 years.