Particulate matter (PM), the particulate fraction, is the most harmful component of the air pollution cocktail. It is measured using the PM 10 and PM 2.5 conventions that approximately collect the tracheobronchial and respirable fractions, respectively. Daily increases in PM correspond to increases in deaths and hospitalizations for airway disease and cardiovascular disease 1 or 2 days later. Living in a particle-polluted area also increases the incidence of chronic lung and cardiovascular disease and lung cancer. The underlying causes of these adverse effects are becoming better understood. There is heterogeneity in the potency of the components of PM, with some parts that contribute to the PM mass being essentially nontoxic (e.g., sea salt). The primary combustion-derived nanoparticulate component seems to be especially important in causing oxidative stress and inflammation, which may explain all the observed adverse health effects.