Methods to Estimate Acclimatization to Urban Heat Island Effects on Heat- and Cold-Related Mortality

BACKGROUND: Investigators have examined whether heat mortality risk is increased in neighborhoods subject to the urban heat island (UHI) effect but have not identified degrees of difference in susceptibility to heat and cold between cool and hot areas, which we call acclimatization to the UHI. OBJECTIVES: We developed methods to examine and quantify the degree of acclimatization to heat-and cold-related mortality in relation to UHI anomalies and applied these methods to London, UK. Methods: Case-crossover analyses were undertaken on 1993-2006 mortality data from London UHI decile groups defined by anomalies from the London average of modeled air temperature at a 1-km grid resolution. We estimated how UHI anomalies modified excess mortality on cold and hot days for London overall and displaced a fixed-shape temperature-mortality function (shifted spline model). We also compared the observed associations with those expected under no or full acclimatization to the UHI. RESULTS: The relative risk of death on hot versus normal days differed very little across UHI decile groups. A 1 degrees C UHI anomaly multiplied the risk of heat death by 1.004 (95% CI: 0.950, 1.061) (interaction rate ratio) compared with the expected value of 1.070 (1.057, 1.082) if there were no acclimatization. The corresponding UHI interaction for cold was 1.020 (0.979, 1.063) versus 1.030 (1.026, 1.034) (actual versus expected under no acclimatization, respectively). Fitted splines for heat shifted little across UHI decile groups, again suggesting acclimatization. For cold, the splines shifted somewhat in the direction of no acclimatization, but did not exclude acclimatization. CONCLUSIONS: We have proposed two analytical methods for estimating the degree of acclimatization to the heat-and cold-related mortality burdens associated with UHIs. The results for London suggest relatively complete acclimatization to the UHI effect on summer heat-related mortality, but less clear evidence for cold-related mortality.