A predictive model relating daily fluctuations in summer temperatures and mortality rates

Abstract

Background: In the context of climate change, an efficient alert system to prevent the risk associated with summer heat is necessary. The authors' objective was to describe the temperature-mortality relationship in France over a 29-year period and to define and validate a combination of temperature factors enabling optimum prediction of the daily fluctuations in summer mortality.

Methods: The study addressed the daily mortality rates of subjects aged over 55 years, in France as a whole, from 1975 to 2003. The daily minimum and maximum temperatures consisted in the average values recorded by 97 meteorological stations. For each day, a cumulative variable for the maximum temperature over the preceding 10 days was defined. The mortality rate was modelled using a Poisson regression with over-dispersion and a first-order autoregressive structure and with control for long-term and within-summer seasonal trends. The lag effects of temperature were accounted for by including the preceding 5 days. A "backward" method was used to select the most significant climatic variables. The predictive performance of the model was assessed by comparing the observed and predicted daily mortality rates on a validation period (summer 2003), which was distinct from the calibration period (1975-2002) used to estimate the model.

Results: The temperature indicators explained 76% of the total over-dispersion. The greater part of the daily fluctuations in mortality was explained by the interaction between minimum and maximum temperatures, for a day t and the day preceding it. The prediction of mortality during extreme events was greatly improved by including the cumulative variables for maximum temperature, in interaction with the maximum temperatures. The correlation between the observed and estimated mortality ratios was 0.88 in the final model.

Conclusion: Although France is a large country with geographic heterogeneity in both mortality and temperatures, a strong correlation between the daily fluctuations in mortality and the temperatures in summer on a national scale was observed. The model provided a satisfactory quantitative prediction of the daily mortality both for the days with usual temperatures and for the days during intense heat episodes. The results may contribute to enhancing the alert system for intense heat waves.

Figure 1

Fluctuations in daily observed and estimated mortality rates during three summers (1975, 1976, 1983), France. Fluctuations in daily observed (black) and estimated mortality rates by model with 1 group^a (right column, blue), 3 groups^b (right column, red) and 4 groups^c (right column, green) for minimum (left column, broken blue) and maximum (left column, red) temperatures in France from June through September, 1975, 1976 and 1983. X-axis: days from 1st June to 30th September (122 days). Y-axis: daily mortality rate (deaths/100,000/day). ^a 1 group: G2; ^b 3 groups: G2, GMA, GCum1_t; ^c 4 groups: G2, GMA, GCum1_t, Gcum1_t-2.

Figure 2

Fluctuations in daily observed and predicted mortality rates, in France in the summer of 2003. (a) Fluctuation in daily minimum (broken blue) and maximum (red) temperatures, and observed mortality rates (black) in France from 1^st June to 30^th September, 2003. (b) Fluctuations in daily observed (black) and predicted mortality rates, by model with 1 group^a (blue) and 4 groups^b (red), and by model with cubic spline functions for minimum and maximum temperatures ^c (green), in France from 1^st June to 30^th September, 2003. X-axis: days from 1st June to 30th September (122 days). Y-axis: daily mortality rate (deaths/100,000/day). ^a 1 group: G2; ^b 4 groups: G2, GMA, GCum1_t, Gcum1_t-2; ^c model with natural cubic spline functions (6 df) for minimum and maximum temperatures of same-day and 2 lag days.