Particulate matter (PM) is composed of small particles of dust, smoke, and soot that are released or kicked up into the air. PM can also be formed by reactions in the air between chemicals, including some of those that form ozone. Particulate matter smaller than ten microns in diameter, known as PM-10 and PM-2.5 are about one-seventh of the diameter of a human hair. Levels of PM pollution vary depending on seasonal activities, rainfall and wind conditions. The size of PM is important. The smaller the particle, the easier it enters the deepest part of the lungs. PM-2.5 and ultrafine PM are small enough to enter the bloodstream. Blood transports the pollution throughout the body - to organs, tissues, and cells. Ultrafine PM is so very small, it enters and travels within the cells of our body where it can disrupt basic cellular functions. Here is EPA's description of PM. Here is CARB's description.