Respirable particulate matter (PM) is associated with harmful cardiopulmonary effects in humans. To test the hypothesis that individuals with certain susceptibility factors have heightened inflammatory and airway responses to PM exposure, a single-blind randomized dose crossover human study of controlled exposure to filtered air (FA), and concentrated ambient particles (CAPS) was conducted. We enrolled 10 mild- moderate asthmatic GSTMI null subjects, 10 mild- moderate asthmatic GSTMI present subjects and 10 healthy GSTMI present subjects to determine the short-term effects of CAPS exposure in individuals likely to be at risk for adverse effects. Outcome measures included symptom scores, physiologic measures (vital signs, spirometry, exhaled nitric oxide, heart rate variability) as well as serum, sputum, and nasal lavage samples for inflammatory biomarkers.