(Washington, D.C.): Particulate matter (PM) remains one of the most significant air pollutants in terms of public health impact. PM exposure affects the cardiovascular, respiratory, and nervous systems, causing illness and premature death. And recent research has shown that health effects are associated with PM levels below current federal air quality standards. While the health risks from particle pollution have been understood for some time, it is less widely recognized that most of our exposure occurs indoors.