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The impact of heat on death rates begins at relatively low temperatures during hot weather, finds research in the Journal of Epidemiology and Community Health.
Researchers analysed temperature readings from the Meteorological Office and death rates from the Office of National Statistics for London between 1976 and 1996.
They found that death rates associated with heat started rising at about 19 degrees Centigrade (66 degrees Farenheit), and once above 21.5 degrees Centigrade (about 71 degrees Farenheit), the rate increased by almost 3.3 per cent for every degree rise in temperature.
The 1976 heatwave, which lasted 15 days, triggered an average increase in death rate of 4.7 per cent for every degree rise. And the number of excess deaths was estimated to be almost twice as high as that during any other heatwave.
But the researchers found that, overall, the largest increases in deaths occurred during the longest periods of heat, when average temperatures were relatively high. And hot days occurring in late May and June may have greater impact than hot days later on in the year, say the authors, even though the highest average temperatures were recorded for July and August.
One possible explanation for this is that people might not yet have acclimatised, say the authors. Alternatively, it might be down to the ‘harvesting phenomenon,’ whereby clusters of susceptible people die sooner than would be expected, followed by a drop-off in deaths later in the year, as the pool of people at risk diminishes.
The effects of temperature rises on death rates were immediate, with the numbers of deaths dropping off up to three days later. Elderly people are likely to be the most vulnerable, say the authors, and those with respiratory disease. Hot days pushed up the rates of death from respiratory disease by around 5.5 per cent fore very degree rise above 21.5 C.
Cold weather still claims more lives than hot weather, the authors say, but conclude that deaths during periods of hot weather can be substantial.
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