How do heat waves affect mental health?

Extreme heat impacts everything it touches — the body, infrastructure, plant life — and even things it doesn’t. It’s hard to ignore the physical sensations of discomfort and sweat on a hot day, but high temperatures can have a negative effect on mental health, too. Given the record-breaking heat bearing down on the US, Europe, China, and Iran, millions of people may be feeling a change in their mood.

While studying heat waves and mortality in India as a Fulbright fellow, Amruta Nori-Sarma, now an assistant professor in the department of environmental health at the Boston University School of Public Health, realized how much science didn’t understand about the health impacts of extreme heat. Although she was specifically looking to identify heat-related deaths, Nori-Sarma was interested in the health issues that might lead to mortality during extreme heat events. “We know a lot about the physical health impacts of extreme heat,” she says, “but what about the mental health impacts?”

In 2022, Nori-Sarma and her colleagues published a study examining the association between heat and mental health-related emergency room visits among US adults. During the hottest days of the summer, more people went to the emergency room for mental health conditions like substance use disorders, mood and anxiety disorders, stress disorders, other behavioral disorders, and more.

How can people care for their mental health on an ever-warming planet? There are few answers, but Nori-Sarma hopes further research will help illuminate who is most vulnerable to heat-related mental distress and how mental health clinicians can best care for patients when it’s hot.

This interview has been edited and condensed for clarity.

Could you explain the results of your emergency room visit study?

We used a database of commercial health insurance claims across the US to look at adults who were visiting the emergency department between 2010 and 2019 during the summers. The effort was to try and get at this question: What are the different types of mental health outcomes that people experienced because of extreme heat exposure? So in that study, we were comparing the four or five hottest days of each summer to the coolest days of the summer. We saw elevated rates of emergency department visits very broadly across all mental health outcomes. But specifically, we did see elevated rates of emergency department visits for a variety of specific outcomes, including things like substance use disorders, mood [and] anxiety disorders, stress disorders, schizophrenia, self-harm, as well as childhood-onset personality and behavioral disorders.

We saw increased rates of emergency department visits pretty uniformly between men and women, and also pretty uniformly across age groups, which was really interesting. So even younger adults that we might think would be more healthy in the face of these extreme weather events experience greater need for emergency care for mental health issues during heat. One of the other things that I thought was really interesting in this study is that we actually saw higher rates of emergency department visits during summertime heat periods in the northern parts of the US compared to the southern parts of the US, which is kind of counterintuitive, because it’s hotter in the southern US. What that speaks to is maybe there are adaptive measures that are already put in place for communities in the southern US that experience hotter summers — things like air conditioning in homes, for example.

What is going on to cause these mental health issues?

In this current study, we weren’t able to assess at an individual level because all we have is the insurance claims data. We are not able to dig more deeply into the biological mechanisms. But we have a couple of theories.

One that I think is probably the most prevalent is the way that extreme heat disrupts sleep. When it’s hot outside, it’s more difficult to sleep, you’re more uncomfortable. If you’re living in a place where you don’t have access to air conditioning, that might further disrupt your sleep. The reason why I think this is plausible is because all of these different health outcomes that I mentioned earlier are so different from each other. There’s not a strong biological pathway that leads to all of these different outcomes. Substance use disorder is very different from mood anxiety disorders is very different from schizophrenia. The fact that we see similar increases in the rates of emergency department visits for all of these different health endpoints indicates to me that heat is an external stressor that’s somehow exacerbating people’s existing symptoms.

Does heat impact our everyday mood, or does it seem to have the greatest effect on people who already have some of these conditions, like anxiety or schizophrenia?

That’s a good question and another one that’s really difficult to answer with the data that we have, because these are emergency department visits, these are some of the most extreme presentations that people will have in terms of their outcomes. But I think it’s entirely likely that heat is impacting your day-to-day mood. If you have extreme heat, it makes it more difficult to cope with other stressors. If we can catch people when they’re just irritated or anxious, then we might be able to prevent, down the line, an extreme health care need.

What should individuals keep in mind as they care for their mental health in this time of unprecedented heat?

One of the things that’s really important is feeling like you’re prepared in case an extreme heat period happens — so knowing what resources are available to you. Cities, like Boston, have cooling centers, in case you’re caught in a location where there’s not good cooling during an extreme heat period. Not spending a lot of time outdoors in the sun or, if we have to, making sure we’re hydrated.

The other thing that I think is really important is relying on our social networks. It’s about checking in with ourselves, but also checking in on your friends and family, checking in on your neighbors, making sure that the people around you know what resources are available and are being taken care of during periods of extreme heat.

You mentioned that gender and age didn’t impact the rate of emergency room visits in your study. But are there any other factors that might make somebody more vulnerable for heat-related mental distress?

Yes. The study that we conducted with this dataset has led to a lot of additional questions that we can answer, but we only have access to commercial insurance beneficiaries. What about people who are on public insurance? Or what about people who don’t have health insurance? How are those folks coping with extreme heat? This is really important. This is a next step for our research. This is one of the most vulnerable populations: people who are of low socio-economic means who may be leveraging emergency departments as a front line for their health care needs, because that’s the best access to care that they have.

Other than that, diving more deeply into people who are receiving different forms of treatment for their ongoing mental health conditions and how that might be impacting both their experience of extreme heat and also their mental health conditions. That’s another really important, vulnerable population that we need to focus on. There’s medications that affect our body’s ability to thermoregulate. Anti-schizophrenia medications are ones that come to mind. People who have schizophrenia and who are taking medication for it might have a reduced ability for their own body to thermoregulate. That’s why sometimes you see people in extreme heat who are wearing layers and layers of jackets and clothing. It’s because their body has lost the ability to thermoregulate due to the medication usage.

How does summer seasonal affective disorder (SAD) differ from winter SAD?

I’m not as familiar with wintertime seasonal affective disorder. My understanding of SAD during the winter is that it’s very much correlated with the number of daylight hours that we have and the body’s ability to hormonally regulate and cope with reduced periods of daylight. I think that seasonal affective disorder in the summer is more correlated with the overwhelming nature of the heat exposure. Even if people are able to be out in the daylight for longer periods during the summer, the fact that it seems more difficult to do that if it’s extremely hot outside and you’re limited to an air-conditioned space, I think that’s the mechanism for summertime disorders.

Is there anything you find surprising or misunderstood about heat and how it affects our mental health?

A thing that people don’t tend to think about as much is that a lot of people right now, and especially a lot of young people, are already facing additional levels of anxiety and stress around climate change. Having that baseline level of anxiety already elevated because you’re anticipating future events is also a problem. It might limit our ability to cope as these events are occurring. It might reduce our resilience in the face of future summertime extremes. One of the things that I’m really interested in is what are the ways in which this anticipation of ongoing climate change is also impacting people’s need for emergency care or other types of health care.

What other questions do you want to answer in your research?

We’re working on the question of who are the most vulnerable in society and trying to see what resources we have to understand the impact of extreme heat exposures on people who experience homelessness or housing insecurity, people who don’t have access to health insurance, people who are repeat visitors for psychiatric emergency services. We’re doing that on a smaller scale in the greater Boston area.

The other thing that I think is a really important factor is what role do clinicians play in keeping their patients safe. Are clinicians aware of the elevated need for emergency care during extreme heat waves? What can we do to support clinicians so that they’re understanding the impact that climate and extreme heat are having on their patients? We’re starting by talking with the clinicians. The translation of our results would be seeing more patients in the emergency department when it’s hot. Do physicians experience that? And if they do, what are the things that they are noticing about their patients when they’re coming in for emergency department visits? We can get their perspective on what are the manifestations of these different mental health outcomes during extreme heat. When do we see them? And how can we provide better services for people in advance?