Both cold and hot temperatures can have a negative effect on human health — and cause death. But is one more potentially harmful to our health than the other?
An international team of researchers recently set out to answer that question. They conducted a statistical meta-analysis of more than 74 million deaths that occurred between 1985 and 2012 in 384 locations in 13 countries, including in 135 U.S. cities.
Their findings, published late last week in the journal The Lancet, are somewhat surprising — and not exactly what those of us living on the Northern Plains will want to hear. For the study found that cold temperatures are associated with about 20 times more excess deaths than hot temperatures.
Specifically, the analysis revealed that about 7.71 percent of the deaths in the study could be attributed directly to weather. But of those excess deaths, most — 7.3 percent — were associated with cold weather. Only 0.4 percent of them were linked to hot weather.
The analysis also found that extreme weather (days when temperatures fell in either the lowest or the highest 2.5 percentile for each location) was responsible for only 11 percent of the weather-related deaths in the study, or 0.86 percent of all the deaths.
Most of the deaths attributed to weather occurred on days that were only mildly colder than the average day in each particular location.
“It is often assumed that extreme weather causes the majority of deaths, with most previous research focusing on the effects of extreme heat waves,” said lead author Antonio Gasparrini, a biostatistician and epidemiologist at the London School of Hygiene & Tropical Medicine, in a released statement. “Our findings, from an analysis of the largest dataset of temperature-related deaths every collected, show that the majority of these deaths actually happen on moderately hot and cold days, with most deaths caused by moderately cold temperatures.”
Gasparrini and his colleagues suggest that the increase in deaths on colder-than-normal days may be due to the extra stress that such temperatures place on the cardiovascular system, raising the risk of heart attack and stroke. Cold temperatures may also weaken the immune system, leaving the body more susceptible to respiratory illnesses.
The results of the study have important implications for public health policy, they add, because the policies of most countries tend to focus primarily on minimizing the health consequences of heat waves.
“Our findings suggest that these measures need to be refocused and extended to take account of a whole range of effects associated with temperature,” said Gasparrini.
This study’s statistical modeling is impressive, and its findings are interesting. But, as the experts who reviewed the study for the “Behind the Headlines” website of Great Britain’s National Health Service (NHS) point out, the findings are somewhat skewed by the fact that extreme temperatures occur on very few days.
So, “even though the relative risk of death is increased on those days,” the NHS experts explain, “the absolute number of deaths is nowhere near as high as on the majority of days.”
The study had other limitations as well. For example, although the researchers adjusted their findings for some confounding variables (such as air pollution and humidity), they didn’t do it for other potentially important ones, such as whether people had access to shelter and heating.
“Since high or low temperatures affect susceptible groups such as unwell, young, and elderly people the most,” write the authors of a commentary that accompanies the study, “ attempts to mitigate the risk associated with temperature would benefit from in-depth studies of the interaction between attributable mortality and socioeconomic factors, to avoid averse policy outcomes and achieve effective adaptation.”
You can read the study and the commentary in full on The Lancet website.