MORTALITY AS A FUNCTION OF TEMPERATURE - A STUDY IN VALENCIA, SPAIN, 1991-1993

Background. Increased mortality is associated with both very low and v ery high ambient temperatures. This study assesses the relationship be tween daily numbers of deaths and variations in ambient temperature wi thin the city of Valencia. Methods. The daily number of deaths from al l causes (total deaths and only those occurring in people aged over 70 ), as well as those deaths from specific causes (e.g. cardiovascular a nd respiratory diseases, malignant tumours and all causes except exter nal ones) occurring within the city of Valencia were related to the av erage daily temperature using autoregressive Poisson regression contro lling for seasonality, day of the week, holidays, air pollution, influ enza incidence, and humidity. Temperature was measured within the regr ession model as two complementary variables: 'Heat' and 'Cold'; also t aken into account were their delayed effects up to 2 weeks after measu rement. Results. Graphical analysis revealed a relationship between te mperature and mortality according to the time of year. For the cooler months (November-April), the temperature at which mortality was lowest was the 'minimum' (i.e. around 15 degrees C), while for the warmer mo nths (May-October), it occurred at around 24 degrees C. Because of thi s, a stratified analysis was undertaken with different values for the 'Heat' and 'Cold' variables according to which of the two seasons was involved. During the colder months of the year, higher temperatures te nded to exert a rapid influence on mortality and the lower temperature s a more delayed relation. During the hot season it is the heat variab le which more clearly manifests an effect, and this is prolonged over the two following weeks. Variations also occur according to age and ca use of death. The effect of temperature is greater in persons aged ove r 70 years of age, and it is also greater in cases of circulatory and respiratory diseases. Conclusions. A statistically significant associa tion has been found between temperature and mortality. This relationsh ip is not monotonic, but mortality increases in proportion to the vari ance in ambient temperature from a range of temperatures that varies f rom winter to summer.