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London Cold Is A Bigger Killer Than London Heat Or The Cold Of The Bulgarian Capital, Sofia

July 25, 2003

London cold turns out to be a bigger killer than London heat, and a bigger killer than the cold of Sofia, the capital of Bulgaria, reveals a comparison of temperature effects between the two capitals, reported in the Journal of Epidemiology and Community Health. But ultimately, summer is good and winter is bad for health, concludes the study.

The researchers set out to explore the effects of temperature on death rates in two very different European capital cities, by comparing the northerly maritime climate of London with the more extreme climate of the considerably less affluent Sofia, Bulgaria's capital.

They looked at the associations between average two day and two weekly temperatures and deaths from all causes in Sofia for the years 1996 to 1999 and in London for the years 1993 to 1996. Daily records of air pollution were also assessed.

During the time periods under study, almost 45,000 people died in Sofia while almost 257,000 people died in London. Temperatures ranged from minus 6 degrees to 34 degrees Centigrade in London and from minus 17 degrees to 37 degrees Centigrade in Sofia.

While average winter temperatures were very different, average summer temperatures were similar, although there were three heat waves in Sofia, but only one in London over the time periods studied.

In London, for every degree fall below the cut off point of extreme cold (just over 5 degrees C), deaths increased by over 4%, while in Sofia, deaths increased just under 2% for every degree fall below the cut off point (minus 0.46 degrees C). There were more cold days in London than in Sofia.

But the effects of the cold extended over a considerable period in London, peaking after a lag of three days, but still very evident after 22 days. This, the authors suggest, means that cold temperatures in London harm the general population and that the adverse effects are not confined to people who are close to death.

The cut off points for heat were similar in both cities at around 21 degrees C. But the deleterious effects of heat were greatest in Sofia, where, for every one degree rise above the cut off point, deaths increased by 3.5%, compared with below 2% in London. This was in spite of more hot days in London.

The effects of heat tended to be fairly immediate in both cities, occurring on the day itself and continuing perhaps another day or two. But while these levels dropped back to normal in London, in Sofia they fell below normal, suggesting that heat had already brought forward the death of those close to the end of life, an effect referred to as "harvesting."

British Medical Journal (BMJ)