Comparison of the risk for deaths on hot and cold days (relative to that on days with moderate temperatures) at UHIa of +0.5 and -0.5[degrees]C.
We modeled the UHIa modification of these heat- and cold-related mortality risks as interaction (product) terms for each dlnm sub-term:
We present the results from the fitted models as the relative change in these predicted heat (cold) mortality ratios for a UHIa of +0.5[degrees]C compared with that for a UHIa of -0.5[degrees]C (one degree difference).
These IRRs estimate the increased risk on hot days in areas of London subject to the UHI compared with areas typically one degree cooler by the UHIa (and analogously for cold).
The extent to which the curve was displaced by the UHIa ([gamma]) was estimated by calculating likelihoods (deviances) over a grid of candidate values and thereby obtaining the maximum likelihood estimate.
The results of the "shifted spline" analysis are shown in terms of the displacement parameter, [gamma], which, for heat, represents the displacement of the temperature-mortality function for one degree UHIa, for example at UHIa of +0.5[degrees]C compared with that at UHIa of -0.5[degrees]C.
Key to the interpretation of both measures of effect modification by the UHIa is that, in our analyses, the temperature-mortality relationship was based on a single "average" temperature series for London.
In the unadjusted analysis, the point estimate of the heat-related mortality risk at the UHIa of +0.5[degrees]C was 1.208 (95% CI: 1.176, 1.241), slightly higher than the value of 1.203 (95% CI: 1.154, 1.255) obtained at the UHIa of -0.5[degrees]C.
The point estimate results for cold-related mortality suggested a larger relative difference between areas with a UHIa of -0.5[degrees]C compared with those with a UHIa of +0.5[degrees]C in the unadjusted analyses (IRR = 1.020, 95% CI: 0.979, 1.063), but the confidence interval was compatible with no difference.
If the lack of increased heat risk in localities with high UHIa indeed reflects acclimatization such as that observed as "adaptation" over long periods of time in Petkova et al.