But when the record-breaking heat dome hit on June 25, 2021, it lasted more than a week, pushing temperatures in the typically mild, maritime city past 102 degrees F. Even after dusk fell at around 8:30 pm, nights refused to cool off. Yoo’s ER was soon hit with a surge of patients it was unprepared to handle. As the heat dome peaked, 911 calls doubled. There weren’t enough beds, so paramedics lined the hallways, waiting with patients on stretchers instead of responding to emergency calls—a situation called a “bed block.” Yoo felt the kind of adrenaline rush he’d experienced during the first waves of Covid, but this was the first time he’d treated so many patients with heat-related illness. To cool them down, Yoo kept running back and forth to the ER’s ice dispenser. At one point he pressed the button and heard a buzzing noise, indicating there was none left. “Shit,” he thought to himself, “what do we do now?” One night an ambulance rushed a middle-aged man into the ER. He had been found unconscious on the sidewalk, baking in the evening sun. Delirious and suffering from heat stroke, he didn’t know what city he was in or what year it was. He was so dehydrated that his kidneys began to shut down, but he kept yanking his arm away when Yoo’s team tried to insert an IV. So they strapped him down by his wrists and ankles, covered him in ice packs, and eventually inserted a catheter to deliver cool fluids directly to his bladder, stabilizing him and saving his life. One other thing stuck with Yoo about the man. He was exhibiting the same symptoms as many of the other patients brought in during the heat dome: those of schizophrenia. Yoo’s hospital, St. Paul’s, serves Vancouver’s Downtown Eastside—a neighborhood with some of the highest rates of mental illness, addiction, and homelessness in Canada. That location gave Yoo a concentrated dose of a widespread epidemiological phenomenon. People with schizophrenia are estimated to represent about 1 percent of the world’s population … … but they made up some 16 percent of the more than 600 deaths from heat exposure across British Columbia that week. During a heat wave in Montreal in 2018, the mortality rate was even worse: People with schizophrenia accounted for 26 percent of fatalities. The root cause of these deaths isn’t just the stigma, social isolation, and lack of decent housing that often accompany life with schizophrenia; nor is it merely the disorganized thinking, altered perceptions, and paranoia that can be symptoms of the disorder. It’s all of those things—plus an invisible, less popularly understood set of physiological factors that have little to do with behavior. The bodies of people with schizophrenia, especially those on medication, simply regulate temperature very differently than others do. And for all those reasons, scientists now realize, no other chronic disease appears to put humans at a higher risk of heat-related death. In fact, no other chronic condition even comes close. *** Long before 2021, frontline doctors like Yoo knew that extreme heat put people with schizophrenia in particular danger. “All of us were quite intimately aware that these patients were going to be at higher risk,” Yoo says. But it wasn’t until researchers studied the BC heat dome that science came to crystallize its understanding of the problem’s true scope. For years, public health advisories warned that extreme heat was most dangerous to older adults, infants, and people with chronic illnesses like heart disease, respiratory disorders, and diabetes. But in 2023 a study revealed that schizophrenia was associated with a threefold increase in the odds of mortality during the heat dome. “That's when it became very clear that schizophrenia was the outstanding chronic condition in terms of risk,” says Sarah Henderson, scientific director of environmental health at the BC Center for Disease Control, who coauthored the report. A flurry of studies followed, and in 2025 a group of interdisciplinary experts, including Henderson, published a review of the literature explaining the phenomenon. What they found were layers upon layers of risk factors. At the most biologically fundamental level, people with schizophrenia have wonky thermoregulation—their bodies are less able to cool themselves for a variety of reasons. They have a harder time regulating dopamine, which is associated with our body’s ability to manage its temperature, explains the study’s coauthor Nathalie Kirby, an applied climate and heat-health scientist. A related hallmark of schizophrenia, Kirby says, is an imbalance between the sympathetic and parasympathetic nervous systems, often called the “fight or flight” and “rest and digest” systems, respectively. Typically, these systems are in balance, but someone experiencing psychosis is caught up in a vicious cycle: They may experience hallucinations, tied to a dopamine imbalance in the brain, that make neutral experiences seem scary. This suppresses the parasympathetic nervous system, which in turn produces a fight-or-flight response and worsens the initial psychosis. Along the way, someone with increased fight-or-flight activity is predisposed to adverse cardiovascular events. It becomes harder for them to control the blood vessels and sweating that help with thermoregulation. “Most deaths during extreme heat are actually cardiovascular in nature,” Kirby says. “So if someone isn't able to control their cardiovascular system very well, they're going to have a much higher risk when it's hot outside.” Another challenge that faces people with schizophrenia—one that Yoo was already aware of in the ER back in 2021—is that treatment for the disorder can include antipsychotic medications that prevent people from sweating, further depressing their ability to dissipate heat. “A lot of the antipsychotic medications have an anticholinergic effect,” Yoo says, describing physical side effects that include increased heart rate and decreased sweating. “That means they’re going to be at even higher risk of cooking their organs.” Finally, heat can also cause schizophrenia symptoms to flare up, even for someone whose symptoms are well managed. They may feel increased anxiety, dread, and fear, which further aggravates the fight-or-flight system, Kirby says. Beyond internal dysregulation, people with schizophrenia often show a lack of insight into their own health (though this can be improved with medication). In a heat wave, they may not be aware that their body is heating up, so they don’t drink water or attempt to cool themselves. They may do just the opposite—wearing multiple layers instead of shedding them. Paranoia, another marker of schizophrenia, may also cause some people to avoid exposure to media, including news. “They are left in the dark, they don't know that a heat wave is coming,” says Liv Yoon, an assistant professor at the University of British Columbia’s School of Kinesiology. She led a study interviewing dozens of people with schizophrenia who survived the heat dome. “I cannot have news,” one woman told the authors. “News is triggering for me, impacting my mental health a lot. I don't own a radio, I don't watch TV, or the internet or anything.” When the heat dome arrived, the woman had no warning. Another of Yoon’s research subjects was a Vancouver-based artist named Sandra Yuen, who says she likewise avoids the media. Unless she gets a notification from her weather app, “somebody has to tell me” that a heat wave is coming, she says. Yuen, who in 2010 wrote a memoir about her life with schizophrenia, has been on various antipsychotic medications since she was a teenager—so she can’t really say how her body might regulate temperature differently without them. But Yuen can report this much: “I'm very sensitive to the heat.” She sweats very little, constantly has dry mouth, and has a hard time staying hydrated. “I might perspire a bit, but it’s very hard for me to cool off.” Yuen wonders what would happen in a major heat wave if the power went out and she had no air-conditioning. In the past when she’s been caught without AC on hot days, she’s felt trapped indoors and fallen asleep. “Hopefully,” she says, “I’ll wake up.” *** What can protect people with schizophrenia from extreme heat? None of the experts WIRED interviewed suggest that anyone go off their antipsychotic meds, even though they may exacerbate some risks. (Of the people with schizophrenia who died in the BC heat dome, 80 percent had been dispensed an antipsychotic medication in the past 90 days, compared to 55 percent of those who survived, according to a 2023 article published in the British Columbia Medical Journal.) There’s a small trade-off, says Kirby, but antipsychotic drugs help manage mental symptoms, and changing medications suddenly can be dangerous. “Keeping people consistent during extreme heat is probably best,” she says, “and someone communicating with their health care provider about what to do is probably the best way forward.” The experts did emphasize that one entirely preventable risk factor stands out: lack of access to fans, air-conditioning, and other forms of cooling. The BC Coroners Service found that 98 percent of deaths during the heat dome occurred indoors, often in housing that lacked mechanical cooling. “If the people taking the antipsychotics had a safe indoor temperature, they would be fine,” says Henderson. (According to the research, only one predictor of death from extreme heat is stronger than schizophrenia, and that’s another closely related condition: poverty.) Public cooling centers can help, but only if people actually go. Yoon interviewed one woman who lived in an un-air-conditioned rental apartment that cooked in the afternoon sun during the Vancouver heat dome; she knew cooling centers were open but didn’t want to use them. One reason was that she had hallucinations that made her feel conspicuous around other people. She felt cowed enough by the stigma that she stayed home. As Yoon interviewed survivors, this kind of experience came up again and again. One participant told Yoon that society had made progress reducing the stigma around anxiety and depression but not schizophrenia. Yuen, for her part, has encountered strangers who assume people with schizophrenia are violent or use drugs and others who don’t believe they are truly ill at all. “You doubt yourself,” she says. “You doubt your own brain.” Another participant in Yoon’s research—one who’d had negative experiences with the health care system—told her outright that neurotypical folk “don’t care if people with schizophrenia die.” Hearing this “was devastating,” Yoon says. “They learn to have lower expectations of other people.” Over the course of her research, Yoon came to believe that creating new interventions for people with schizophrenia won’t work if they don’t address stigma. *** In the years since the heat dome, scientists have pieced together a complex, composite portrait of how the average victim with schizophrenia came to be at such high risk of death. They were likely at home alone, in an older building that heated up easily and lacked mechanical cooling. They were less likely to have a social network to check on them. They likely avoided the news, so they didn’t know a heat wave was coming. When the heat wave hit, their body could not thermoregulate effectively, and antipsychotics further increased heat sensitivity. Even if they were aware of what their body needed, they didn’t feel comfortable going to a cooling center, because they faced social stigma. And they didn’t feel safe interacting with the medical system, because they’d had bad experiences in the past. British Columbia’s medical community is determined to prevent the next mass-casualty event and set an example for other places that might be caught off guard by a heat dome. Since 2021, the province has pledged $148 million to hire some 600 more paramedics and increase ambulance services, and BC’s Center for Disease Control placed people with schizophrenia at the top of the list of high-risk conditions during heat waves. The British Columbia Schizophrenia Society developed a check-in program for caregivers, and Kirby and her fellow researchers compiled a guide that includes a checklist of steps to take before a heat wave. BC updated its emergency alert system to push cell phone alerts when extreme heat is coming, as it already did for wildfires and floods. And the province launched a free air conditioner program that enrolls people based on income, delivering AC to 40,000 people. Henderson says the province is also beginning to work on engaging pharmacists to alert people to heat risk when they dispense antipsychotic medications. Deploying behavioral health workers at cooling centers also seems to help. During the 2021 heat dome in Portland, Oregon—where the heat reached over 110 degrees F, much higher than in Vancouver—a behavioral health specialist named Laurel Wonder was stationed at one of the city’s “weather activation centers.” Without asking about diagnoses, Wonder’s decades of experience allowed her to spot people who likely had schizophrenia. On day three of the heat dome, a staff member saw a middle-aged man walking down the block in clothing soaked with sweat. Staff could see he was suffering from heat-related illness and that he was hesitant to come inside. Wonder said they had to actively invite him in by saying, “Please, let us help you. Please, let us get you water.” When he came inside, he was initially only able to utter single words. The man had schizophrenia, Wonder would eventually realize. They called an ambulance, handed him a cooling towel and water, monitored his water consumption, and handed him fresh clothes. It took time for the ambulance to arrive, because the city was experiencing simultaneous heat emergencies. Wonder was concerned that the man might leave, but he stayed. Keeping the man safe until he made it to the hospital felt like a “huge accomplishment,” she says. It may have saved his life. Back in Vancouver, Yoo says he continues to worry about people with schizophrenia as the planet heats up. “We probably will be seeing more of these extreme heat events in North America every single year,” he said when I first spoke to him in May. In July, sure enough, a massive heat dome stretched across much of the continent, including large parts of Canada. It didn’t go as far west as Vancouver, but the emergency room at St. Paul’s Hospital is more prepared than it used to be; it now has a chest freezer full of cold packs in addition to the ice machine.