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 neighbourhoods subject to the urban heat island (UHI) effect, but not identified degree of difference in susceptibility to heat and cold between cooler and hotter area, which we call as acclimatization to the UHI. OBJECTIVES: To develop methods to examine and quantify the degree of acclimatization to heat- and cold-related mortality in relation to UHI anomaly, then apply 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 modelled air temperature at 1km grid resolution. We estimated how (i) UHI anomalies modified excess mortality on days cold and hot for London overall and (ii) displaced a fixed shape temperature-mortality function ('shifted spline' model). For each we also compared observed associations with those expected under no or full acclimatization to the UHI. RESULTS: The relative risk of death on hot compared to normal days differed very little across UHI decile groups. A 1(o)C UHI anomaly multiplied the risk of heat death by 1.004 (95% CI 0.950, 1.061) (interaction rate ratio) compared with 1.070 (1.057, 1.082) expected if there were no acclimatization. The corresponding UHI interaction for cold was 1.020 (0.979, 1.063) against 1.030 (1.026, 1.034) expected. Fitted splines for heat shifted little across UHI decile groups, again suggesting acclimatization. For cold, they shifted somewhat in the direction of no acclimatization, but not excluding acclimatization. CONCLUSIONS: We have proposed two analytical methods for estimating the degree of acclimatization to the heat- and cold-related mortality burdens associated with UHI. The results for London suggest relatively complete acclimatization to the UHI effect on summer heat-related mortality, but less clear evidence for cold. Language: en