Females may be more vulnerable to extreme high temperatures such as the heatwave gripping the UK than males, experts have suggested.
The heatwave plan for England states that those at high risk include the over-75s, babies, young children, people with severe physical or mental illness, and females.
The document does not explain why females are on the list, but the UK Health Security Agency pointed to a study in the Netherlands that looked at mortality after various heatwaves and found elderly women are at higher risk than men. The researchers said the results were not simply down to age.
“When equal ages were considered, mortality rates were still 15% higher for females,” the team wrote of their analysis of data from the 2003 heatwave in France.
Another study by Dutch and German researchers, looking at 23 years of temperature data from the Netherlands together with daily mortality figures, also found differences between the sexes.
“Heat-related mortality was higher in females than in males, especially in the oldest age group (≥ 80 years) under extreme heat,” they said.
The team said the findings did not seem to be down to elderly people being more vulnerable to heat and women generally living longer than men.
Hein Daanen, a professor of exercise physiology at VU University in Amsterdam and an author of the study, said the team speculated that the reduced sweat production in females may play a role.
“Roughly, elderly [people] sweat half the amount compared to youngsters and females half that of males,” he said. In other words, the study notes, “the ability of older females to lose heat from the body is the lowest”.
The team said the stress placed on the cardiovascular system by heat may also be involved. “Cardiovascular strain is reportedly higher in females, potentially explaining their higher mortality risk in the heat,” they wrote.
Among other possibilities, the team note that elderly women may be at increased risk because they may be more likely to live alone – a known risk factor in hot weather – and they may be less active overall than men but more likely to be active in the household.
“Continuing these activities during heatwaves while being less physically fit puts females more at risk for overheating and cardiovascular strain than males,” the team wrote.
Ollie Jay, a professor of heat and health at the University of Sydney, also noted that studies had shown the maximum sweat rate in young healthy women tended to be lower than their male counterparts.
“Whether or not this is why more females are present in the mortality and morbidity data in heatwaves, we do not really know yet. It is also not clear if there is any interaction of this sex effect with age,” he said.
Mike Tipton, a professor of human and applied physiology at the University of Portsmouth, said women may be more at risk from heatwaves because they have higher core temperatures after ovulation, while they are often smaller than men and hence have a bigger surface area to mass ratio, meaning they heat up more quickly.
But Jay said: “The most likely answer is that it is a combination of factors and we need to do more research to understand what is going on.”