As we enter the second heatwave of the summer, we’re more clued up on how to look after our health in the short-term, taking care to avoid heat-stroke and dehydration. However, the perils of the hot spell won’t be over when the weather breaks. As changes in our climate become permanent – forecasters predict we could have a 40-degree spell every three years – scientists are now warning us of the longer-term effects of climate change on our wellbeing.
“A growing body of evidence shows that heat affects our mental health as well as our physical health,” says Prof Gregory Wellenius, director of the Centre for Climate and Health at the Boston University School of Public Health. “Heat often leads to dehydration or electrolyte imbalances and people who spend a lot of time outside, pregnant women, or those taking certain medications that reduce their ability to sweat, are more at risk.”
Here are some areas to watch out for...
We could put on weight
Acute dehydration is a serious condition, but studies have also identified a link between chronic dehydration and obesity. According to the US National Health and Nutrition Examination survey, which collected data from 9,500 adults, those who did not drink enough had a higher body mass index.
“There are a lot of relatively unexplored health effects of the interaction between weather, climate change and obesity,” says Dann Mitchell, professor of climate science at Bristol University. One theory is that when our body gets dehydrated, it triggers a chemical reaction stimulating the production of the hormone vasopressin, which in turn initiates the production of fat. This is an evolutionary survival mechanism found in many mammals, as animals use fat to survive when water is not available.
New research from Tel Aviv University found that when men are exposed to sunlight, they release increased amounts of the hormone ghrelin from fat stores in the skin – which makes them hungrier. A study of 3,000 men showed that they increased their food intake by an average of 300 calories per day over the summer months. While not enormous, it would be enough to cause weight gain.
Mitchell also points out that people are simply much less likely to do exercise when hot. “We do know that heatwaves make you more thirsty, which tires you out, and so there’s a logical link to doing less exercise,” he says.
We’ll be prone to skin flare-ups
Dann Mitchell has found that certain skin conditions – particularly eczema – can be exacerbated by sudden fluctuations in heat. This backs up a 2021 study from Dutch scientists which found that a third of children with difficult-to-treat eczema tend to report flare-ups during the warmer months in spring and summer. The possible causes for this are sweating, heat rashes and exposure to increased pollen – all of which can irritate the skin. “Your skin gets used to a certain temperature, and then it can become irritated by rapid changes over a short timescale,” says Mitchell.
There is also evidence to suggest that climate change may be increasing the frequency of skin breakouts in people who are prone to acne. This is because acne is linked to increased oil production, sweating and clogged pores, which become more common as temperatures rise.
Strategies to fight this include liberal use of sun cream, staying in the shade using electric and misting fans to keep the skin hydrated, and self-dousing with a water spray or sponge.
Our mental health could suffer
It’s normal to feel tired and grumpy during excessively hot weather, but the combination of humidity and temperature can have an even more serious impact on mental health.
According to Dr Emma Lawrance, a mental health innovations researcher at Imperial College London, the function of the brain gets interrupted by high temperatures, meaning that people find it harder to reason and solve cognitive tasks. Broken sleep also decreases our emotional regulation.
Studies have shown that hot weather can lead to an increase in violent crime and domestic violence during heatwaves, with just a one or two degree Celsius temperature rise resulting in a 3-5 per cent spike in assaults.
The levels of a brain chemical called serotonin, which helps keep our impulses in check, are affected by higher temperatures. While it is often assumed that hot weather improves our mood, the fluctuations in serotonin and other hormones mean that heatwaves increase symptoms of depression and anxiety. Studies have found that suicide rates increase by 1-2 per cent for every one degree Celsius increase in monthly average temperature.
“You get more cases of suicide or attempted suicide during heatwaves,” says Dr Lawrance. “And worsened symptoms, or more hospitalisations for people with mental illnesses.”
Common drugs used to treat mental illness are less effective in hot weather. Lithium, a widely used mood stabiliser for people with bipolar disorder, works differently if the user is dehydrated. Antipsychotics can actually suppress thirst, exacerbating symptoms of heat stress.
As heatwaves become more common, Dr Lawrance feels that there is an important need to include people with mental health problems among the vulnerable groups. “Mental wellbeing is often not considered as much as physical health,” she says. “But the way that we think and feel at higher temperatures seems to be affected. And we know that people with vulnerabilities to mental health seem to be particularly prone to these effects.”
Women need to take care in late-pregnancy
Women in the latter stages of pregnancy are thought to be more sensitive to heat as their ability to naturally regulate their body temperature is less efficient. A number of studies have identified a much greater risk of pre-term birth – when a baby is born prematurely before 37 weeks of pregnancy – as well as stillbirths and other birth abnormalities during heatwaves.
Scientists still do not fully understand this connection. However, Jason Kai Wei Lee, director of the Heat Resilience and Performance Centre at the National University of Singapore, explains that an inflammatory response to heat stress could play a role in triggering pre-term birth. Compromised blood flow as a result of heat exposure could also limit the supply of nutrients and oxygen to the womb.
“For obvious reasons, there are limited experimental studies in humans that have analysed the effect of high temperature in pregnant women and the foetus,” says Lee. “We need to better understand the exact mechanisms in order to intervene or at least derive evidence based advice to protect pregnant women and inform them of the potential risks of exposure to high temperature.”
Some of the NHS advice to pregnant women during hot spells is to wear loose, light clothing, and do foot exercises to reduce swelling and cramp in the legs and feet. Immersing the feet in cold water can also help stabilise body temperature.
We’ll need to drink to avoid kidney stones
Sharp twinges in your back or abdomen could be a sign of kidney stones, a surprisingly common heatwave ailment which can affect anyone who is not consuming enough fluids.
During spells of hot weather, the University Hospital Southampton HS Foundation department has reported a rise in the number of patients being hospitalised with renal colic, a crippling pain resulting from the collection of crystals inside the kidney, which can develop into hard lumps. This is only likely to continue based on projections of climate change-induced temperature gains.
Researchers in the US have predicted that the percentage of the American population living in high-risk zones for kidney stones will grow from 40 per cent back in 2000, to 56 per cent by 2050 and 70 per cent by 2095.
Kidney stones occur because we sweat more during hot weather meaning that our urine is less diluted. If you are not replacing that sweat with water intake, the urine in the kidneys will contain a far higher concentration of minerals and salts which slowly accumulate.
Because of all this, people who already suffer from kidney impairment are particularly vulnerable during hot weather. “We know that heat often leads to dehydration or electrolyte imbalances,” says Prof Wellenius. “In people with compromised kidney function, these changes or stressors can push them to needing medical attention.”
Asthma sufferers will need to take care
The combination of increased pollutants and pollen levels in the air during a heatwave can also be particularly problematic for asthma sufferers. According to a survey conducted by the charity Asthma + Lung UK, 72 per cent of respondents found that their asthma has flared up as a result of the hot weather this summer, while 28 per cent had an asthma attack directly triggered by last month’s heatwave.
“When pollen levels are at their highest this can be deadly for those with lung conditions like asthma who can suffer serious symptoms and have life-threatening attacks,” says Andy Whittamore, clinical lead for Asthma + Lung UK.
There are concerns that extreme heat can cause a chemical reaction in air pollutants, which produces ground-level ozone, a toxic gas that can even cause mild lung damage. One study suggests that global deaths associated with ozone formed as a result of climate change, have increased by 5 per cent in the past 20 years, while people who live in major cities and those who spend a lot of time outside such as construction workers, being more vulnerable to this.
More pollen also means more hayfever, although the Met Office comments that if a heatwave becomes particularly sustained, the supply of pollen can run out altogether, which can give sufferers some relief.