How heat affects the mind Policy and infrastructure changes are urgently needed to protect our mental health from the impact of high temperatures

How heat affects the mind Policy and infrastructure changes are urgently needed to protect our mental health from the impact of high temperatures By Stephanie Pappas Date created: June 1, 2024 16 min read Vol. 55 No. 4 Print version: page 42 Climate Change Mental Health Population Health 20 collage of images showing people dealing with extreme heat Temperatures are rising as summer kicks off, and psychological scientists are sounding the alarm on what to be aware of in warmer temperatures. It remains to be seen whether summer 2024 breaks the records of last summer: In the United States, heat domes baked the Midwest; El Paso, Texas, saw weeks without a day below 100°F; and Tampa Bay, Florida, issued its first extreme heat advisory. In November 2023, Phoenix reported that at least 569 people had died because of heat-related reasons over the summer. Extreme heat days are an inevitable consequence of a warming world, and things are not cooling down. Globally, 2023 was the hottest year on record, and the Met Office—the United Kingdom’s national weather service—predicts that 2024 may be worse. It could even be the first year on record to surpass 1.5°C of warming above the preindustrial era. The physical consequences of heat, such as heat stroke and heat exhaustion, are well-known. But heat has psychological consequences as well—consequences ranging from irritability to impulsivity to trouble concentrating. The impacts can put already-vulnerable people in crisis during heat waves but may also lead to general mental health impacts and increased friction within society. A 2023 report by APA and ecoAmerica on children and youth and climate change found that young people are particularly at risk, given the potential impact of heat-related mental health effects on developing brains (Clayton, S., et al., Mental Health and Our Changing Climate: Children and Youth Report, 2023). “Extreme heat can make people more depressed or irritable, it can bring on psychotic outbreaks, and people on certain psychiatric medications are more sensitive to heat,” said Jane Gilbert, the world’s first Chief Heat Officer, a government position established in 2021 in Miami-Dade County. Gilbert is one of several local-level policymakers around the country dedicated full time to handling the fallout from extreme heat. As municipalities try to deal with this consequence of climate change, psychologists are simultaneously working to understand the ways in which heat affects people’s moods, behaviors, and cognition. As they unravel the scope of the damage, they’re becoming increasingly vocal about the need to respond. “The way we are headed right now, things are only going to get worse,” said Kim Meidenbauer, PhD, an assistant professor of psychology at Washington State University who studies the effect of heat stress and other environmental factors on cognition and emotion. “If we don’t even understand the scope of the effect heat is having on us, that bodes poorly for our ability to protect people from the negative psychological consequences.” [Related: How does climate change affect mental health?] construction worker pour water on his head to cool off Heat and mental health The most immediate dangers from heat are physical. When the body loses too much water and salt via sweat, the result is heat exhaustion, marked by dizziness, clammy skin, and a rapid pulse. Untreated heat exhaustion can rapidly progress to heat stroke, which happens when the body’s cooling system completely breaks down, leading to a catastrophic rise in internal body temperature. This hyperthermia is very hard on the brain: Between 10% and 28% of heat stroke survivors experience persistent brain damage, according to a 2022 review (Bouchama, A., et al., Nature Reviews, Vol. 8, 2022). Almost all psychotropic medications, except for benzodiazepines, can impair the body’s ability to handle heat, raising the risk of heat exhaustion and heat stroke. “One sixth of Americans are on these medications,” said Joseph Taliercio, PhD, a clinical psychologist at Cognitive & Behavioral Consultants in New York who is also active in the Joint Advocacy and Outreach Committee of the Climate Psychiatry Alliance and in the Climate Psychology Alliance–North America. Though the medication labeling mentions this side effect, not all patients are specifically warned by their doctors about the risk. Heat stroke is an extremely serious condition that can kill nearly two thirds of those who experience it during heat waves, according to the 2022 review. But less deadly consequences of heat can also be far-reaching. homeless tents set up along a street Heat seems to exacerbate mental health symptoms across many diagnoses (Charlson, F., et al., International Journal of Environmental Research and Public Health, Vol. 18, No. 9, 2021). A U.S. study of mental health-related emergency department visits found an 8% increase in mental health visits on the hottest days of summer compared with the coolest days. The increase ranged from 5% to 11% across different disorders (Nori-Sarma, A., et al., JAMA Psychiatry, Vol. 79, No. 4, 2022). Specific mental health conditions exacerbated by heat included substance use disorders, anxiety and stress-related disorders, mood disorders, schizophrenia and related disorders, self-harm, and childhood behavioral disorders. The most affected regions were the Northeast, Midwest, and Northwest, the researchers found, perhaps indicating the dangers of at-home heat stress; these areas are less likely than the South to have housing with air-conditioning built in. Those findings probably represent just a small segment of those whose mental health is affected by heat, said study leader Amruta Nori-Sarma, PhD, an assistant professor of environmental health at the Boston University School of Public Health. “We’re only looking at emergency department visits, so we’re only looking at the most extreme presentations,” Nori-Sarma said. “There might be a lot of other people who see a primary-care physician or a mental health provider. Or there might be a lot of people who are having symptoms who will never come into contact with the medical system.” Just as heat waves are physically dangerous to older adults, mental health symptoms linked to heat seem to hit this population hard, according to a recent systematic review and meta-analysis led by Jingwen Liu of the University of Adelaide School of Public Health. That review found a 2.2% increase in mental health-related mortality and 0.9% increase in mental health morbidity associated with every 1.8°F rise in ambient temperature (Environment International, Vol. 153, 2021). The greatest mortality overall was among those with substance use disorders. People living in subtropical and tropical environments and people over the age of 65 were also disproportionately affected. Though seasonal mood problems are mostly associated with winter, there is also a subset of people who struggle with summer-related major depression. “It’s understudied,” said Kelly Rohan, PhD, a clinical psychologist at the University of Vermont who studies seasonal affective disorder (SAD). Summer-related depression appears far less common than winter SAD, which is linked to short daylength in the winter months. Instead, it seems tied to people’s experiences of heat and humidity, Rohan said. “Some of the same neurotransmitters, like norepinephrine, dopamine, and serotonin, that are involved in mood regulation are also involved in thermoregulation,” she said, “so maybe there is some dysfunction in one or more of those neurotransmitter systems that confers a risk of depression as well as the inability to tolerate heat and humidity.” There may also be a behavioral or cognitive component to summer depression, Rohan said, as people who can’t stand the heat might isolate themselves at home or fall into a spiral of dread about the long, steamy summer months. Summer-type SAD differs from winter SAD in that people with winter SAD tend to report weight gain, food cravings, and sleeping too much, while people with the summer version of the disorder tend to report weight loss, loss of appetite, and insomnia. There is no specific treatment for summer depression, Rohan said, though it likely responds to antidepressants and therapy in the same way as typical major depressive disorder. [Related: 4 tips for protecting your mental health this summer] Aggressive reactions Heat has also been linked to changes in behavior and cognition. Criminal justice researchers have long noted correlations between warm weather and crime, a link that may be due to activity and opportunity. A recent study that examined the relationship between large temperature swings and crime in 28 U.S. cities found that unexpectedly warm days yielded a 4% increase in robberies, a 9.4% increase in aggravated assaults, and a 19.4% increase in homicides compared with the average (Thomas, C., & Wolff, K. T., Journal of Criminal Justice, Vol. 87, 2023). Unusually cool days had lower incidences than average of all three types of crime. Though studies of crime have often focused on summer, the research found that unseasonably warm days in winter saw unusual increases in robberies and homicides. This finding suggests that people’s routines may play a large role in these temperature-dependent crime spikes, the authors wrote, since people are more likely to be out on warmer days, increasing the opportunities for crime and conflict. There are mixed laboratory results as to whether heat leads directly to aggression. Craig Anderson, PhD, a psychologist at Iowa State University, has found evidence that people may interpret ambiguous interactions as more aggressive in hot temperatures (Advances in Experimental Social Psychology, Vol. 32, 2000). “When you perceive other people behaving more aggressively, that increases the likelihood that you’ll see yourself as being provoked and responding in a more hostile manner,” Anderson said. Heat could potentially stymie decision-making and provoke irritability by disrupting sleep, by causing discomfort, or simply by increasing the load on the brain as it struggles to work in less-than-ideal circumstances. A recent economic decision-making experiment run in California and Nairobi suggests that the real-world impacts may be subtle and complex. In that study, researchers found that hot conditions made participants more likely to decide to destroy the assets of other players in a game—a choice that held no benefit for themselves (Almås, I., et al., NBER Working Paper 25785, 2023). This effect was driven, however, by a subset of players in Nairobi who were part of an ethnic group that had recently been politically marginalized in a highly contested election. “Our study provides new evidence that thermal stress and the political context together might be jointly responsible for generating observed patterns of violence through a mechanism embedded in the human mind,” the researchers wrote. Though it can be difficult to isolate the effect of temperature alone in real-world studies, recent research also indicates that heat exposure may increase self-reported impulsivity. In a study spanning several Chicago neighborhoods, Meidenbauer and her team asked participants to report their heat exposure as well as their levels of impulsivity and other traits and found that experiencing heat was linked to higher self-reported impulsivity. “Being exposed to heat stress in one’s home in particular was robustly related to whether participants reported themselves as being more impulsive,” Meidenbauer said (PsyArXiv Preprints, 2023). Exposure to the urban heat effect—in which concrete and asphalt hang on to heat, raising temperatures in cities compared with greener surroundings—was also linked to self-reported impulsivity, the researchers found, though less strongly. It’s possible that access to air-conditioning somewhat mitigates the urban heat effect, Meidenbauer said, but that those without the resources for sufficient cooling at home have little escape from the heat. woman sitting at a bus stop A Boston University study of 115 U.S. metro areas found that homes in dense urban cores are less likely than homes in other areas to be air-conditioned (Romitti, Y., et al., PNAS Nexus, Vol. 1, No. 4, 2022). The likelihood of a given home having air-conditioning was inversely correlated with the percentage of people in the neighborhood who identified as Black, African American, Hispanic, or Latino. And areas with low median income and high percentages of residents without high school degrees were also least likely to have air-conditioned homes. Heat stress is also linked to problems with cognition. In one within-subjects study conducted in Shanghai, 36 healthy participants were set up at workstations and exposed to temperatures ranging from 75.2°F to 82.4°F. Their physiological responses were regularly recorded as they worked through a series of standard office tasks like typing and doing basic arithmetic. Across almost all tasks, cognitive performance declined as the workers experienced hotter temperatures, with cognitive performance decreasing by up to 10% at the hotter temperatures compared with the coolest (Lan, L., et al., Indoor Air, Vol. 32, No. 1, 2022). An actual heat wave provided an opportunity for experimentation in 2016, when 44 students in Boston area buildings were followed for 12 days of summer swelter. About half the students lived in air-conditioned buildings, while the other half did not have air-conditioning. Each morning, the students did two cognitive tasks, a STROOP color-word test and a simple addition and subtraction game. Their performance results showed that those without air-conditioning took 13% longer to respond on both tasks (Laurent, J. G. C., et al., PLOS Medicine, Vol. 15, No. 7, 2018). Large-scale population studies reveal how these cognitive effects may play out in the educational system. An analysis of standardized testing data from 58 countries and 12,000 U.S. school districts found that each school day above 80°F was associated with reduced achievement by 0.07% of a standard deviation (Park, R. J., et al., Nature Human Behaviour, Vol. 5, 2021). The effect was largely driven by poorer schools, with hot school days associated with reduced achievement by 0.12% of a standard deviation in low-income school districts and no significant reduction in high-income districts. In places like Florida, where all schools have air-conditioning, heat stress on students at school is a less pressing problem than home heat stress, Gilbert said. Low-income families may have inadequate air-conditioning or may not be able to afford to cool their homes sufficiently, meaning students are often exposed to heat at home. But in places that were largely temperate before the climate started warming, many school buildings lack cooling. One such city is Watsonville, California, an agricultural city in central California of about 50,000 people. The rising temperatures there are starting to affect students, said Nancy Faulstich, a former educator and the founder of the local climate justice nonprofit Regeneración. “We did a survey in 2017, and one of the comments in the survey was from a high school teacher who was monitoring temperature in the classroom,” Faulstich said. “He said it was over 95 degrees for days in a row, at times over 100—when you’ve got 28 to 34 high school students crowded in a classroom and it’s 100 degrees in there, how can you learn?” Cooling down For local governments, heat mitigation is one of the primary ways to attempt to soften the psychological impacts of heat. “When you cool down a neighborhood, it benefits your physical and your emotional health,” said Ali Frazzini, a policy adviser with a background in public health at the L.A. County Chief Sustainability Office. Policies range from those directed toward individuals, such as making public service announcements and maintaining cooling centers in the summer, to those focused on making city infrastructure more climate resilient, such as planting trees and installing green infrastructure. Because of the known socioeconomic disparities in heat impacts, many of these policies are focused on those who are financially precarious, Gilbert said. “It’s gotten cost-prohibitive for many households to keep their homes cool,” Gilbert said. “Or if the wall unit breaks down in July and they can’t afford to replace it or the landlord doesn’t fix it, they can be in a dangerous situation.” family sitting outside in the shade Miami-Dade County has a financial assistance program for low-income residents and maintains a list of resources for Florida incentives and rebates for energy savings and home cooling. The county’s HOMES plan, a housing affordability initiative announced in 2022, includes $7 million for retrofitting and weatherizing older homes and includes requirements that new construction funded by the county include cool roofs—light-colored roofs made with materials that reflect instead of absorb heat, which can reduce cooling costs considerably. The city is also focused on improvements to the power grid, especially since hurricanes can leave residents sweltering without electricity for days or weeks. In Los Angeles, city planners are looking at cool surfaces, such as roads coated with heat-reflecting materials, and other strategies to reduce the urban heat effect, Frazzini said. “Right now, one of our big areas of focus is just increasing the tree canopy,” which can provide shade and mitigate temperature spikes, she said. “Many of our most vulnerable neighborhoods have poor tree canopy.” Across the United States, the link between poverty and vulnerability to heat comes up repeatedly. In Meidenbauer’s analysis of Chicago’s neighborhoods, people reporting high economic hardship were also likely to live in areas impacted the most by the urban heat effect. In Miami, data from heat-related hospital visits also show disparities. “We have ZIP codes that have 4 to 5 times the rates of heat-related illness compared to other ZIP codes,” Gilbert said. “The top correlating factors are high poverty rates, high land surface temperatures, families with children, and a high percentage of outdoor workers.” Unhoused individuals are also at high risk during heat waves, Gilbert said. Not only is it difficult for people without shelter to escape the heat of the day, many people who are unhoused also have mental health diagnoses that may be exacerbated by heat. These disparities aren’t limited to urban areas. In Watsonville, tree cover is at only about 8%, Faulstich said, and much of the housing stock for agricultural workers is in poor condition without sufficient cooling. The city is working with the nonprofit conservation organization Watsonville Wetlands Watch to develop a 40-year plan for sustainable tree cover in the city, with a goal of eventually reaching 30% canopy. Heat doesn’t happen in a vacuum. With heat comes drought, and with drought comes wildfire, which can lower air quality. Pollution is also linked with both mood and cognition problems. One study in China that used portable monitors to measure participants’ exposure to air pollution in real time found that exposure to fine particulate matter (P.M. 2.5) impaired executive control on cognitive tasks (Ke, L., et al., Environment International, Vol. 170, 2022). In a longitudinal population study in Sweden, exposure to fine particulates was linked to cognitive decline in adults over the age of 80 (Grande, G., et al., Journal of Alzheimer’s Disease, Vol. 80, No. 2, 2021). And a meta-analysis of studies between 1974 and 2017 found that particulate exposure was linked to depression, anxiety, and suicide (Braithwaite, I., et al., Environmental Health Perspectives, Vol. 127, No. 12, 2019). [Related: Extreme heat and poor air quality are threatening childrens’ mental health] Researchers also worry about the knock-on effects of mass migrations caused by climate change, which have the potential to increase intergroup conflict. Climate refugees, especially children, experience stressors that are well-known to impact mental health and cognition, Anderson said. And large-scale migration can lead to political conflict and violence that further impact those involved. According to the UN Refugee Agency, 110 million people are currently displaced around the globe because of persecution, conflict, and violence, a number that has steadily risen from about 32.3 million in 2008. “The proportion of the world’s population that is exposed to a number of these risk factors is going to increase,” Anderson said. “In fact, it already has increased.” The good news, Anderson said, is that psychological research on heat, mental health, and violence is starting to get more traction outside of psychology: “It’s only recently that we’ve started publishing [heat research] in climate change journals,” he said. There are still urgent questions that psychologists need to answer, Meidenbauer said. She and her team are looking at how uncomfortable, but not dangerous, levels of heat might affect cognition. They’re also interested in trying to unravel individual differences in heat tolerance. People report becoming uncomfortable in heat at very different temperatures, she said, and it’s a combination of warmth and discomfort that seems to cause psychological symptoms. There is also a dearth of research on ways to reduce the negative impacts of heat, and that needs to change quickly, she said. “Knowing that it’s only going to get worse from here,” she said, “really emphasizes the importance of understanding the effects so that we can intervene.”