Seasonal variations in mortality caused by cardiovascular diseases in Norway and Ireland

Seasonal variations in mortality resulting from cardiovascular diseases (CVD) have been demonstrated in many countries, with the highest levels observed during the coldest months of the year. We studied the seasonal changes in CVD mortality in Norway and the Republic of Ireland, two countries which are demographically quite similar, but climatically different; we also examined the relation between CVD mortality and air temperature.

Registered monthly data for mortality from CVD for the period 1985-1995 were obtained from the Norwegian Central Bureau of Statistics and the Irish Central Statistics Office. Meteorological data were provided by the Norwegian Institute of Meteorology and Met Eireann, in Ireland. Monthly mortality ratio for both men and women aged 60 and older was calculated from the mortality date. Mean monthly air temperatures for the two countries were calculated from the meteorological data.

For the 10-year period investigated, the lowest and highest monthly mortality ratios were on average found in August and January, respectively, and mean excess winter mortality, expressed as the difference between the August and January values for the entire 10-year period, was 22% (Norway) and 35% (Ireland). However, when the percentage difference in the months with the respective highest and lowest mortality ratios were calculated for each year, the average of these differences for each of the 10 individual years was 29% and 45%. Mortality ratio was found to increase much more steeply with decreasing air temperature in Ireland than in Norway.

Although the seasonal variation between CVD mortality in both countries is similar, the different relation with climatic conditions may result from differences in housing standards, allowing outdoor temperatures to have a greater influence on indoor temperature in Ireland than in Norway.