Temperature and summer mortality: geographical and temporal variations in four Italian cities

Study objective: To investigate geographical and temporal variations in the temperature-mortality relation. Design: The relation between mortality and maximum apparent temperature (T-appmax) in 2003, 2004, and a previous reference period was explored by using segmented regression and generalised additive models. Setting: Four Italian cities (Bologna, Milano, Roma, and Torino), included in a national network of prevention programmes and heat health watch warning systems (HHWWS) were considered. Participants: Daily mortality counts of the resident population dying in each city during summer (June to September). Main results: The impact of T-appmax on mortality differed between cities and varied in the three periods analysed. The geographical heterogeneity of the J shaped relation was seen in the reference period with T-appmax thresholds ranging from 28 degrees C in Torino to 32 degrees C in Milano and Roma. In all cities, the percentage variation in mortality was greatest in 2003. In Torino and Roma a significant increase was seen also at lower Tappmax values that are usually not associated to an increase in mortality (26-28 degrees C). In summer 2004 the exposure levels were similar to the reference period; only in Torino the effect of T-appmax on mortality remained relevant even if reduced compared with 2003, while in Bologna no statistically significant effect was seen for any temperature range. Conclusions: The observed heterogeneous reduction in the impact of temperature on mortality from 2003 to 2004 may be partly explained by the lower levels of exposure. Changes in the ability of individuals and communities to adjust to high temperatures as a consequence of the implementation of public health interventions, based on HHWWS, characterised by a diverse effectiveness, may also have played an important part.