PM is a uniquely complex mixture that may include solid particles, liquids, and vapors. Sources of PM include geologic sources (eg, sand, salt), metals, and fossil fuel combustion (eg, diesel exhaust particles, black carbon). PM is typically defined by size, such as PM ≤ 10 μm, ≤ 2.5 μm, or ≤ 0.1 μm in aerodynamic diameter (PM10, PM2.5, and PM0.1, respectively), or more qualitatively as coarse, fine, and ultrafine. However, its toxicity varies depending on factors like particle weight and composition, as well as host factors determining the location and density of deposition in the respiratory tract.1 PM toxicity can be further enhanced by exposure to other pollutants like NO2 and O3, with which it is frequently accompanied. PM exerts its effects directly from deposition in the respiratory tract and indirectly from triggering a local inflammatory response that can spread to the systemic circulation.