HEAT WAVES AND HEALTH OUTCOMES: THE IMPORTANCE OF HEAT WAVE DEFINITION

A better understanding of how heat wave definition impacts the relationship between heat exposure and health, especially as a function of rurality, will be useful for development of effective heat wave adaptation strategies. Here we compare relationships between different heat wave index (HI) definitions and adverse birth outcomes and mortality across urban and rural areas in Alabama. Meteorological forcing fields from the North American Land Data Assimilation System, which are produced through the synthesis of a meteorological model with multiple satellite and in situ observations, together with topographic information, were used to develop 15 zipcode level HIs for the summer months (May-Sept) between 1990 and 2010. A time-stratified case-crossover design was used to define relationships between these 15 HIs and preterm birth (PTB) and non-accidental death (NAD). Heat wave lags, durations of heat waves and timing in season were investigated. Relationships between health outcomes and different HIs were compared by bootstrapping. Interactions with rurality were examined. Our results show associations between health outcomes and heat waves varied across the different HIs. Nine of the 15 HIs evaluated resulted in positive odds ratio point estimates in PTB models and 7 of the 15 HIs evaluated resulted in positive point estimates in NAD models. Results indicate a 32.4% (95% CI: 3.7, 69.1) increase in PTB during a heat wave defined as at least two consecutive days at mean temperature above the 98^th percentile over the time period and a 3.7% (95% CI: 1.1, 6.3) increase in NAD during a heat wave defined as at least two consecutive days at mean temperature above the 90^th percentile. Rurality stratified model results suggest the risk of preterm birth and non-accidental mortality may be greater during heat waves in urban areas defined by Rural-Urban Commuting Area (RUCA) categories. Relative temperature HI measures were better predictors of both adverse health outcomes than absolute measure HIs or HIs with additional meteorological variables commonly used to predict heat stress such as humidity. In summary, our results suggest a relative, temperature only heat wave definition may be the most effective metric for heat wave warning systems in the Deep South.