Extreme temperatures and mortality:: Assessing effect modification by personal characteristics and specific cause of death in a multi-city case-only analysis

BACKGROUND: Extremes of temperature are associated with short-term increases in daily mortality. OBJECTIVES: We set out to identify subpopulations and mortality causes with increased susceptibility to temperature extremes. METHODS: We conducted a case-only analysis using daily mortality and hourly weather data from 50 U.S. cities for the period 1989-2000, covering a total of 7,789,655 deaths. We used distributions of daily minimum and maximum temperature in each city to define extremely hot days (>= 99th percentile) and extremely cold days (<= 1st percentile), respectively. For each (hypothesized) effect modifier, a city-specific logistic regression model was fitted and an overall estimate calculated in a subsequent meta-analysis. RESULTS: Older subjects [odds ratio (OR) = 1.020; 95% confidence interval (CI), 1.005-1.034], diabetics (OR = 1.035; 95% CI, 1.010-1.062), blacks (OR = 1.037; 95% CI, 1.016-1.059), and those dying outside a hospital (OR = 1.066; 95% CI, 1.036-1.098) were more susceptible to extreme heat, with some differences observed between those dying from a cardiovascular disease and other decedents. Cardiovascular deaths (OR = 1.053; 95% CI, 1.036-1.070), and especially cardiac arrest deaths (OR = 1.137; 95% CI, 1.051-1.230), showed a greater relative increase on extremely cold days, whereas the increase in heat-related mortality was marginally higher for those with coexisting atrial fibrillation (OR = 1.059; 95% CI, 0.996-1.125). CONCLUSIONS: In this study we identified several subpopulations and mortality causes particularly susceptible to temperature extremes. This knowledge may contribute to establishing health programs that would better protect the vulnerable.