Heat tolerance and mortality by birth-area

Background: The temperature-mortality relationship follows a wellknown ‘J-V shape’ pattern where the value of Minimum Mortality Temperature (MMT) is used as a proxy of population heat-tolerance. In Italy, during the fifties a huge amount of unemployed moved from the South to the industrializing northwestern regions. We analysed mortalitytemperature relationships among residents died in a northern city, during 1980-1989, by birth-area groups: northern, middle, southern Italy. Methods: Since 1981, mortality data have been linked to census records as part of a longitudinal population study. Meteorological dataset was part of MISA study, the Italian meta-analysis on short term effects of air pollution on health. Log-linear models have been used to fit daily death count data as a function of different explanatory variables: months, weekdays, holiday, influenza epidemics, temperature and relative humidity. Results: Among residents died in the city 21% were born in the South, 8% in the middle and 71% in the North; the death’s daily means were respectively 4.5, 1.7 and 15.1. Elderly over 65 years were 68% among Southerners and about 78% among other groups. It’s likely that, in the eighties, some retired immigrants came back to native areas. Preliminary analyses confirm the V-shaped curve in each group. The curve for those born in the North shows two possible breakpoints approximately at 19°C and 25°C, while both curves for immigrants, born in the Middle or Southern Italy, show only one breakpoint (or MMT) at around 20°C. The left slope of each curve is substantially null, suggesting that the cold effect is negligible. At warmer temperatures, over the MMT, the slope is rather pronounced and steeper in the group of natives in northern areas; the percentage increase of mortality risk, for an increase of 1°C during hotter days, decreases with the latitude of the birth area: it is 6.9% (95%CL 4.2;9.7) for residents born in the North, while it is 4.0% (95%CL 1.9;6.1) for immigrants from Middle and 2.6% (95%CL 0.9;4.3) for those born in the South. Conclusions: This preliminary result confirms what already seen in the city of Milan, Lombardy, suggesting that heat tolerance in populations could be modulated by outdoor temperature experienced early in life, and no complete adaptation occurs if external environmental temperatures increase. Further analyses are on going, and will be resented, by age at migration and duration of residence, available in this data set