Exposure to wildfire-specific PM2.5 was associated with increased respiratory hospitalization. In a study with the Medicare population in the Western United States, researchers from the Harvard Clean Air Research Center estimated exposures to wildfire-specific PM2.5 using a global chemical transport model, between 2004 and 2009. About 5 million Medicare participants were exposed to at least one smoke wave (lasting 2 days or longer), including participants in rural areas far from monitoring sites. Compared to matching days without a smoke wave, the risk of respiratory hospitalization increased by 7.2% in that population, when wildfire-specific PM2.5 concentrations exceeded 37 mg/m3. No significant associations were observed below that concentration or for cardiovascular admissions. Despite some limitations, these findings suggest that the respiratory risk from wildfire PM in this elderly population may be stronger than those for PM from other sources.