Investigation has repeatedly demonstrated an association between exposure to ambient air pollution particles and numerous indices of human morbidity and mortality. Individuals with chronic obstructive pulmonary disease (COPD) are among those with an increased sensitivity to air pollution particles. Current and ex-smokers account for 80 to 85% of all those with COPD. The human breathing in an urban site with a significant level of particulate matter (PM) may be exposed to 720 microg daily. A single cigarette introduces 15,000 to 40,000 microg particle into the respiratory tract of the smoker. It is subsequently confounding why such a relatively small mass of airborne PM should have any biological effect in the patient with COPD, as these individuals are repeatedly exposed to particles (with a similar size and composition) at perhaps a thousandfold the mass of ambient PM. Regarding this increased sensitivity of COPD patients to air pollution particles, there are several possible explanations for this seeming contradiction, including correlations of PM levels with other components of air pollution, an accumulation of multiple independent risk factors in a patient, changes in individual activity patterns, disparities in dosimetry between healthy subjects and COPD patients, and some unique characteristic of an ambient air pollution PM. Regardless of the underlying mechanism for the increased sensitivity of COPD patients, exposures of these individuals to elevated levels of PM should be discouraged. To provide a greater awareness of PM levels, the U.S. Environmental Protection Agency now includes levels of air pollution particles in an air quality index.