Although these contextual variables are known to play a role in the effect of these exposures, no study to our knowledge has considered the role of sociodemographics and neighborhood-level factors in driving the interactive effect between ozone and heat. Green space, for example, has been shown to be a modifier of heat-related health effects ( 14). Furthermore, racial minorities and communities of a low socioeconomic status are also more susceptible to heat-related health effects this is associated with poorer physical health, lower access to air conditioning, and greater neighborhood-level exposure that may increase risk ( 32). Racial discrimination plays a role, as decreased access to primary care, private insurance, and preventive medication of Black residents when compared to their White counterparts likely drive this health disparity ( 31). For example, the ozone-related excess attributable risk was found to be almost two times higher for Black compared to White residents in California for air pollution exposure above federal standards ( 31). Some vulnerable groups are known to be especially susceptible to the effects of ozone and heat. In contrast, no relative interaction between ozone and heat was observed in all but one city in England ( 23). For example, high temperature enhanced the effects of ozone on all-cause mortality in France ( 20) and cardiovascular and respiratory deaths in China ( 22).
Findings differ between these studies, some revealing a strong relative interaction ( 20, 25, 26), while others demonstrate a weaker signal ( 27, 28) or no joint effect ( 29).
Interaction between both exposures suggests that the effect of both ozone and heat drive an increased burden compared to each exposure individually. Studies in Brisbane, Australia, and in the Netherlands suggested that both ozone and heat play a role in increasing excess deaths during a heat wave ( 23, 24).
Due to the comparable meteorological patterns, heat and ozone are co-occurring risk factors, and a number of studies have considered the potential concurrent risks and interaction of these exposures in driving the health burden ( 20– 22). This information can be used to inform early warning systems that consider both heat and ozone to protect populations from these deleterious effects in identified areas.Īmbient ozone increases under high ambient temperature and blazing sunlight, both of which are characteristics of extreme heat events ( 15). Results indicate the importance of going beyond average measures to consider spatial variation in the health burden of these exposures and predictors of joint effects.
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A lower median income, higher percentage of unemployed residents, and exposure to other air pollutants within a zip code drove stronger joint effects a higher percentage of commuters who walk/bicycle, a marker for neighborhood wealth, showed decreased effects. However, when considering the zip code level, certain areas observed strong joint effects. RERIs revealed no additive interaction when considering overall joint effects. A total of 817,354 unscheduled respiratory hospitalizations occurred in California from 2004 to 2013 in the May to September period.
Determinants of the spatial variability of effects were assessed using a random effects metaregression to consider the role of demographic/neighborhood characteristics that are known effect modifiers. Spatially varying relative risks due to interaction (RERI) were quantified to consider joint effects. A within-community matched design with a Bayesian hierarchical model was applied to study this association at the zip code level. We examined heterogeneity in joint effects (on the additive scale) between heat and ozone at small geographical scales. To our knowledge, no joint warning systems consider both risks understanding their interactive effect can warrant use of comprehensive warning systems to reduce their burden. Extreme heat and ozone are co-occurring exposures that independently and synergistically increase the risk of respiratory disease.
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