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Many studies showed that the effects of temperature vary over time. The aim of the paper is to evaluate the temporal change in the effect of heat on mortality in Italy in the last 12 years after the introduction of the national heat plan.Distributed lag non-linear models were used to estimate the association between maximum apparent temperature and mortality in 23 Italian cities included in the national heat plan in four study periods, one before the introduction of the heat plan (1999–2002) and three after the plan was in place (2005–2016). The Relative Risks and impact, in terms of attributable fraction and number of heat-related deaths, were estimated for mild (20th and 75th percentile) and extreme temperatures (75th and 99th percentile) in each study period during summer. We also estimated the potential change in the number of heat-related deaths between the first and subsequent periods by applying the dose-response relationship estimated in the first period to the mortality and exposure observed in the other periods.Although summer temperatures are increasing in Italian cities, a reduction in heat-related mortality is observed progressively over time. After the introduction of the heat plan, a reduction in the effect of extreme temperatures was observed, while for mild summer temperatures the effect remains high (RR 1.23; 95% CI: 1.15 to 1.32). By the second period, when all cities had implemented local prevention programs for a number of years a further reduction in the effect of both extreme and mild temperatures was observed. The heat attributable fraction related to extreme temperatures declined from 6.3% in the period 1999–2002% to 4.1% in the last period (2013–2016). Considering the entire temperature range the total number of heat-related deaths spared over the entire study period was 1900.Considering future climate change and the health burden associated to heat waves, it is important to promote adaptation measures by showing the potential effectiveness of heat prevention plans.