Dr Checkoway and colleagues at the University of Washington tested the primary hypothesis that increases in daily fine particle levels were related to increased risk of out-of-hospital sudden cardiac arrest. Sudden cardiac arrest and questionnaire data, and exposure data were obtained from the Puget Sound Clean Air Agency (Seattle WA). The investigators used a case-crossover study design; for each case of sudden cardiac arrest, a time period when the person was disease free was selected as a matched "referent" period. The exposure status at the time of disease onset, the "hazard" period, was compared with exposure during the referent period for that subject. The authors also examined potential sources of bias in case-crossover studies of air pollution.