Short-term effects on mortality of hot and cold weather in Sweden, 1901 – 2009

Daniel O. Åström, Umeå University
Bertil Forsberg, Umeå University
Sören Edvinsson, Umeå University
Joacim Rocklöv, Umeå University

The on-going climate change is projected to increase the frequency of extreme weather events. The short-term effects of hot and cold weather and impact on mortality have been thoroughly documented. We investigated if short-term sensitivity to heat and cold temperature extremes changed in Sweden from the beginning of the 20th century until present time. We collected daily mortality and temperature data for the period 1901-2009 in Stockholm, Sweden. Heat extremes were defined as days with temperatures above the 98th percentile of mean lag 0-2, cold extremes were defined as days with temperatures below the 2nd percentile of mean lag 0-25 on the basis of prior studies. The relationship between extreme heat/cold temperatures on all-cause mortality, stratified by decade, sex and age was investigated by time series modelling adjusting for time trends. There was a statistically significant increase in the relative risk of mortality during heat extremes for the total population and for women for all decades investigated, as well as the entire period. There was a declining trend for effects of heat extremes, with a levelling off during the last three decades. A statistically significant increase in the relative risk of mortality during cold extremes was found for the entire period, with a more disperse pattern than that for heat regarding individual decades. Although the relative risks of mortality during extreme events appear to be levelling off, they still pose a threat to public health, with women and the population above 65 becoming the largest and most susceptible groups.