The increasing number of deaths caused by heatwaves as the climate changes will not be offset by fewer deaths in milder winters, US research claims.
Mild winters will not cancel out heatwave deaths, the study said
The study in Occupational and Environmental Medicine said the death rate in 50 US cities rose more sharply in very hot spells than very cold ones.
People had already adapted to the cold with central heating, the study said, but remained unused to intense heat.
Many more people currently die in cold weather conditions than hot ones.
In the UK for instance, there are 20,000 cold-related deaths each year and 1,000 heat-related.
Many experts believe we will see fewer deaths as a result of the milder winters climate change may bring, and that getting warmer is far preferable to getting colder.
Researchers at the Harvard School of Public Health looked at the deaths of more than 6.5 million people in 50 US cities between 1989 and 2000.
They found that during two-day cold snaps there was a 1.59% increase in deaths because of the extreme temperatures, but during similar periods of extremely hot weather death rates rose by 5.74%.
While all 50 US cities showed similar rises in deaths when temperatures plummeted, more deaths were seen during extreme temperature hikes in cities with usually milder summers and less air conditioning.
The authors suggest that this is because the use of central heating is widespread, whereas fewer people have air conditioning in their homes.
They did say however that the problem could be reduced by greater access to air conditioning facilities.
Professor Bill Keatinge of Queen Mary's School of Medicine and Dentistry in London said the study's findings did not present any cause for alarm.
People were learning to protect themselves against both the cold and the heat. Even now, he said, "we're seeing fewer cold-related deaths without a big rise in heat-related deaths".
"In the long term we may have to make some lifestyle changes - working at different times for instance, but nothing that is not manageable."