Heat waves aren’t just physically harmful – research shows they can harm mental health, too

The record-setting heat that blasted much of Europe this week had immediate and obvious effects on public health. More than 2,000 people died from heat-related causes, like heat stroke or dehydration, in Spain and Portugal alone, and the London Ambulance Service was fielding 400 calls an hour during the hottest periods.

But as scientists have begun to discover, heat waves harm mental health, too. From increases in suicides to spikes in aggressive behavior, research is just starting to reveal how and why extreme heat can impair mental health, especially in those with underlying psychiatric conditions.

On days that drastically exceed normal highs, emergency room visits across the U.S. for substance use, anxiety, schizophrenia and other mental illnesses increase by as much as 8 percent, according to a study in JAMA Psychiatry published earlier this year. Hot weather can also stir aggressive behavior, a link scientists have long noted. Laboratory studies show that heat increases irritability and hostility, which could explain, in part, why violent crime tends to rise in summertime.

And suicide rates, which peak during the spring and summer, become even more pronounced during heat waves. For every degree Celsius increase in temperature above the average for a given county in the U.S. and Mexico, suicides rose by about 1 to 2 percent, according to a 2018 study in Nature Climate Change.

These wide-ranging effects are likely to become more pronounced as heat waves become more frequent and intense. Grid spoke with Laurence Wainwright, a mental health researcher at the University of Oxford, about the acute impacts of heat on mental health, who they impact most and how they might worsen as the planet warms.

This interview has been edited for length and clarity.

Grid: The U.K. just went through a record-setting heat wave. What’s it been like there?

Laurence Wainwright: On the hottest day, walking out on the street in Oxford, it felt like I was back in Australia, where I grew up, with this baking heat just coming down. There’s this semi-dystopian feel too, since the streets were nearly empty.

G: I think a lot of people think about the health effects of extreme heat in terms of its impact on our body. Why is it important to expand this into mental health too?

LW: Firstly, the body and the brain are inseparably interconnected, just as physical health and mental health are. And so we can’t really talk about one without talking about the other.

In the last 10 years, it’s become much more acceptable for people to talk about their struggles with mental health and living with a psychiatric illness. I think we can open this up further to talk about climate change and realize that there are very serious problems here, especially for people with underlying psychiatric illnesses, due to the extreme weather events caused by climate change, particularly heat waves.

G: What sorts of impacts can extreme heat have on mental health? In what ways are people with preexisting mental illness more vulnerable?

LW: Roughly speaking, for the general population without any sort of underlying psychiatric illness, heat waves potentially have a minor to moderate impact on overall levels of mental health. Some people may feel more down than usual, more anxious, more easily agitated and quicker to lose their temper. This is not the case for everyone, but some otherwise healthy individuals in a population will experience minor, occasionally moderate, mental health impacts during heatwaves.

But, it’s complex because we often use the term “mental health” in a fairly loose, ambiguous way. Sometimes people frame it as being about their cognitive functioning. They could be saying, “Well, you know, my mental health is not so great today because I’m having a hard time thinking — I can’t think straight.” We obviously know that heat waves cause serious problems in terms of cognitive function, information, recall, memory, processing speed and so on. So, it could well be that the impacts there are considerably less; a lot of it has to do with cognitive functioning rather than mental health, per se.

But for those with underlying psychiatric illnesses, there are a host of problems. Mortality rates go up by a couple of percentage points during a heat wave, levels of hospital presentation and/or admission go up, levels of suicide go up. There’s a possibility of exacerbation of symptoms or switching into a certain phase of a condition, for instance, relapsing into a manic or hypomanic state in bipolar disorder or a depressive episode in depression. Multiple studies have shown a significant correlation between high temperature and severity of symptoms in schizophrenia. And there are very complex relationships here between a host of different biological, environmental, socio-economic factors, intersecting with one another. There’s an awful lot that we don’t yet know, but the numbers are pretty clear that heat waves pose problems for those with underlying psychiatric illnesses.

G: What’s known about the links between extreme heat and aggressive behaviors?

LW: There’s an awful lot we don’t know here. But what we do know from the evidence is that violent crime increases during extreme heat. The police here in the U.K., they generally dread very hot weather because they know that there is going to be typically an increase in incidences of violence, assaults and so forth. Interestingly, however, some categories of crime go down during heat waves, such as petty theft, but generally violence goes up during heats.

G: Why might that be?

LW: There’s a hypothesis around serotonin, one of the neurotransmitters, and the way in which serotonin potentially modulates aggressive behavior and helps with self-constraint. Some people hypothesize that the interplay between serotonin, aggression and heats might be partially responsible for the increase in levels of violent crime during heat waves. But the situation here, as with most things, is probably multifactorial. There’s socio-economic, behavioral, psychological, biological, environmental factors all meshing together.

G: Are there other hypotheses for why extreme heat seems to have these mental health impacts?

LW: Yeah, sleep is a really big one as well. If we’re tossing and turning because of the heat, we’re not getting enough sleep. A couple of nights of broken sleep can be all that it takes, for some people, for the state of someone’s psychiatric illness to worsen.

We also know that some of the side effects of some medications used in psychiatry are made worse in the heat. In a few cases, it seems that the effects of severe heat might have a minor to moderate impact on the efficacy of the drug or slightly change the way it works. So if someone’s very dehydrated, or the body temperature is very high, medication might be interacting with the body in a slightly different way. And as a result, it might not be doing what it’s supposed to do.

For antipsychotics, the drugs we use to treat bipolar and schizophrenia, thirst perception can be diminished. If we’re taking this class of drug and are out and about in the heats, the incidence of suffering from a heat-related condition could go up quite significantly.

In summary, there’s an awful lot going on and an awful lot that we don’t know, but the writing’s on the wall in the sense that the outcomes are worse, that hospitalizations and mortality go up in heat waves.

G: Climate scientists expect heat waves to become more frequent and intense. What do we know about what sort of cumulative impact that might have on our mental health?

LW: We know that heat waves cause a higher presentation to hospital and higher rates of suicide. And as heat waves become more frequent and severe, we’re going to have a lot worse outcomes.

For the population in general, there’s this phenomenon of eco-anxiety, particularly anxiety in young people, about the climate. The studies in this country have shown that every time that there’s a severe weather event, typically a heat wave, young people feel more anxiety about the climate and their future. And that’s quite concerning. It’s tough, we have to maintain a level of optimism and not get overwhelmed by the enormity of the task. Heat waves give us an opportunity to learn how to improve public health messaging and action, and make sure that health systems are actually equipped to be able to manage heat events, which are going to become more frequent and more severe in the future.

So there’s a lot we can take away from this last couple of days in the U.K., and better prepare ourselves for what’s to come, and overall realize that the best thing we can do here is take serious action on climate change.

Editor’s note: The above story deals with suicide. If you have suicidal thoughts, the National Suicide Prevention Lifeline is 988 (En Español: 1-888-628-9454; Deaf and Hard of Hearing: 1-800-799-4889). For suicide prevention resources from the Centers for Disease Control and Prevention, click here.

Thanks to Lillian Barkley for copy editing this article.