Fraction of exhaled nitric oxide (FENO) is a promising non-invasive index of airway inflammation that may be used to assess respiratory effects of air pollution. We evaluated FENO as a measure of airway inflammation after controlled exposure to diesel exhaust or ozone. Healthy volunteers were exposed to either diesel exhaust (particle concentration 300 µg/m3) and filtered air for one hour, or ozone (300 ppb) and filtered air for 75 minutes.