Exposure variable selection in heat – health risk assessment and implications for interventions

Introduction: Exposure variable selection is an important component of heat health risk assessment that has received little explicit attention in the scientific literature. The choice between different exposure variables, such as maximum daily temperature or mean daily heat index, can influence our understanding of the causal pathways by which people succumb to adverse health events associated with thermal stress. This knowledge also affects also our ability to design, prioritize, and implement intervention strategies. Methods: We explored how the statistical association between six different health events and warm season weather varied across 54 exposure variables in the chronically hot setting of Maricopa County, Arizona, USA over the period 2000–2012. Variables were evaluated based on cross-validation scores derived from generalized additive time series models. We subsequently tested how different exposure variables performed in their hypothetical application as triggering criteria for an early warning system. Results: We found that exposure variable selection had a large impact on statistical model performance for health events for which heat was known to be a contributing factor (death, hospitalization, and emergency department visit from direct exposure to environmental heat). Exposure variable selection had a negligible impact on model performance for broader categories of health events (all-cause death, cardiovascular death, and death from consequences of heat and dehydration). Variable selection significantly changed the set of days identified for heat warnings, but no single criterion was obviously superior. Conclusions: Rigorous consideration of the exposure variable in heat-health risk assessment can likely provide important guidance for public health preparedness activities related to extreme heat. In our geographic setting, such analysis leads us to recommend that intervention measures that focus on reducing daytime exposure be prioritized.