Spatial variation in the joint effect of heat waves and ozone on respiratory hospitalizations in California, 2004 to 2013

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Extreme heat and ozone are co-occurring exposures that have been shown to independently and synergistically increase risk of respiratory disease. While some studies have quantified the joint effects of temperature and ozone, none assessed its fine spatial variation. We propose the use of a spatial analysis to examine the potential heterogeneity (on the additive scale) in joint effects between heat and ozone at a small geographical scale. Temperature data was downloaded from the National Ocean and Atmosphere Administration Cooperative Observer stations across the United States and ozone data was estimated at the daily level using 24-hour daily means sampled and analyzed by the US EPA Air Quality System. A spatial extension of the case-crossover design was applied to study this association at the zip code level; spatially varying relative risk due to interaction (RERI) was quantified to consider joint effects. A total of 3,858,196 unscheduled respiratory hospitalizations occurred in California from 2004 to 2013 in the May-September period, and mean thresholds for heat waves ranged from 37.75ºC to 39.57ºC. For ozone, average concentrations ranged from 61.66 ppb to 79.90 for the different thresholds considered. When considering the overall joint effect in all of California, the RERIs revealed no additive interaction. However, when considering the joint-effects at the zip code level, some areas show a strong joint-effect and other areas indicate negative interaction. Results indicate unspecific patterns of heterogeneity, but by considering spatial differences in these impacts variation in the effect of ozone, heat waves and joint-effects is apparent throughout California. Results indicate the importance of going beyond average measures to consider spatial variation in the health effects of ozone and heat. This information can be used to inform hybrid early warning systems to protect populations from the deleterious effects of both ozone and heat.