Hot summer temperatures drive up the number of people suffering mental health emergencies, the most comprehensive study to date has found. The analysis of medical records from millions of US citizens showed an average 8% rise in the rate of emergency hospital visits on days when the temperature was in the top 5% of those recorded across the decade-long study. The effect was seen for almost all mental health conditions, including stress, mood and anxiety disorders, schizophrenia, self-harm, and substance use disorders. The frequency of extreme temperatures is being driven up by the climate crisis, and the researchers said their work could help mental health services predict and prepare for times of greater need. “People are familiar with the risks of extreme heat in terms of dehydration, heatstroke, etc,” said Prof Gregory Wellenius, at the Boston University school of public health in the US and senior author of the research. “What’s really novel is that this study establishes at an unprecedented scale that days of extreme heat can also impact people’s mental health in a substantial way. And this isn’t just for a niche, vulnerable population. This is in every age bracket we looked at, for both men and women, and in every region of the country. Everybody is at risk.” The medical database used for the analysis did not include people without health insurance. “We expect [the 8% rise in risk] is an underestimate of the true burden of disease associated with extreme heat, because the most vulnerable individuals are less likely to be in this database and, frankly, less likely to be treated in the same way for mental health,” Wellenius said. Emergency visits to hospitals are the most severe presentations of mental health disorders, said Prof Amruta Nori-Sarma, also at Boston University and first author of the study: “Even small increases in the number of emergency department visits represent a huge burden to the individuals and to the system.” The study showed the biggest increase in the rate of emergency visits was across the north of the US, rising to 12% in the north-west region. That might appear counterintuitive, said Nori-Sarma, as temperatures in the southern US are hotter: “But we see the opposite.” The reason may be that those living in hotter places are already better adapted, with more access to air conditioners, for example. “This finding is important from a future vulnerability perspective, as we can expect that populations that don’t have those adaptive capacities might be more stressed during times of extreme heat,” she said. According to a report in May 2021, the climate crisis is inflicting huge hidden costs on the mental health of people around the world, such as heatwaves increasing rates of suicide, floods leaving victims traumatised, and loss of food security, homes and livelihoods causing stress and depression. The latest research, which was published in the journal Jama Psychiatry, examined anonymised data on 3.5m emergency hospital visits for mental health disorders made by 2.2 million people from 2010-2019. The study focused on the warmest months, May to September, and covered 2,775 counties, which account for 98% of US citizens. Seven of the warmest years on record for the US have occurred since 2014, the researchers noted. The increased risk was slightly higher for men than for women, the scientists found, potentially because men are less likely to seek early help and may therefore need emergency care more frequently. The only major mental health disorders that did not prompt a rise in emergency visits on hot days were personality and behavioural disorders, which are relatively rare and therefore had a smaller sample. The rise in emergency visit rates was similar for most disorders, the researchers said, indicating that the heat exacerbates existing conditions. This could be due to increasing daytime irritation or discomfort or by disruption of sleep during hot nights. “It’s increasingly clear that a changing climate threatens our bodies and our minds,” said Dr Emma Lawrance of Imperial College London, UK, and not part of the study. “While the effect is relatively small, it has big implications for public health and healthcare systems as climate change increases the number of days of extreme heat; children born today will experience seven times as many heatwaves as their grandparents.” She added: “It’s not just people with more severe symptoms of mental ill-health who are affected by heat – many of us have more symptoms of anxiety, depression and lower mood at higher temperatures. The effects are unequal though, with poorer communities often more exposed to heat. “We need climate mitigation to help prevent worsening of these impacts, and climate adaptation to provide more tree cover and access to green space to prevent urban ‘heat islands’.” The study used a “case-crossover” design, where individuals are followed over a period of time and can therefore serve as their own experimental controls. A person’s increased risk of an emergency visit on a hot day was calculated by comparing that day with the same day of the week in the week before and after. This cancels out other personal factors that might influence the results, such as age or occupation.