In the previous two to three decades, the prevalence of asthma has risen in numerous countries of the world. Correlating with this elevated prevalence of asthma, societies have observed increased air pollution from specific sources. Studies have implicated certain pollutants in asthmatic exacerbation. Particulate matter (PM) is the pollutant most frequently identified with worsening of this airway disease. PM is a temporally and spatially shifting suspension of solids and liquids originating from both natural and anthropogenic sources. Exposures to elevated levels of PM have been associated with asthmatic exacerbations by employing a diverse array of end points, including respiratory symptoms, use of medication, medical visits, emergency room visits, hospital admissions and pulmonary function decrements. Pertinent to worsening of asthma by ambient air PM, there are other particle-associated exposures that similarly precipitate asthmatic exacerbations. These include traffic-associated pollutants, diesel exhaust, emissions from gas and wood stoves, burning of biomass and environmental tobacco smoke. It is widely accepted that the biological effects exerted by all particle exposures result from oxidative stress. This stimulates cell signaling, transcription factor activation and mediator release in the respiratory tract, culminating in inflammation. Other postulated mechanisms for asthmatic exacerbation following PM exposure include an impact on the incidence of infections and adjuvant effects.