Correct Response: C. The Environmental Protection Agency defines particulate matter as "a complex mixture of extremely small particles and liquid droplets." The small size of the droplets and particles keeps them suspended in the atmosphere, where they reduce air quality and can cause serious health problems of the heart and lungs when inhaled. The mixture can contain a variety of human-made and natural compounds, including metals, soot, dust, petroleum residues, sea salt, and a variety of sulfur and nitrogen compounds. Particulate matter may include acidic liquids (sulfuric acid droplets, for example) and gases (A), but it also includes solids and its defining characteristic is its small size. Visible fragments of natural substances (B) are not considered particulate matter and while the mixture does typically include heat-trapping chemicals (D), they are only one possible component of particulate matter.
