Exposure to airborne fine particles (PM2.5) is implicated in excess of 50 000 yearly deaths in the USA as well as a number of chronic respiratory illnesses. Despite intense interest in the toxicity of PM2.5, the mechanisms by which it causes illnesses are poorly understood. Since the principal source of airborne fine particles is combustion and combustion sources generate free radicals, we suspected that PM2.5 may contain radicals. Using electron paramagnetic resonance (EPR), we examined samples of PM2.5 and found large quantities of radicals with characteristics similar to semiquinone radicals. Semiquinone radicals are known to undergo redox cycling and ultimately produce biologically damaging hydroxyl radicals. Aqueous extracts of PM2.5 samples induced damage to DNA in human cells and supercoiled phage DNA. PM2.5-mediated DNA damage was abolished by superoxide dismutase, catalase, and deferoxamine, implicating superoxide radical, hydrogen peroxide, and the hydroxyl radical in the reactions inducing DNA damage.