Does the temperature outside decide a person's temperament inside? On extremely hot summer days, adults are at an increased risk of visiting emergency rooms for mental health crises related to substance use, anxiety, stress, and more.

According to a new study led by Boston University School of Public Health researchers, during periods of extreme heat, clinicians were expected to see an increase in patients requiring mental health services. Published in the journal JAMA Psychiatry, the study found that days with higher-than-normal temperatures during the summer season in the United States were associated with increased rates of emergency department (ED) visits for any mental health-related condition, particularly substance use, anxiety, and stress disorders, and mood disorders.

The impact of heat on physical health has been well documented, but few studies have examined the effects of extreme heat on mental health. This nationwide study was the largest and most comprehensive analysis of daily ambient temperature and mental health-related ED visits among US adults of all ages. As days of extreme heat are expected to increase due to worsening climate change, the findings fill a critical gap in research and provide evidence-based support for proactive interventions and policy solutions that can reduce heat-related crises.

"Emergency department visits represent some of the costliest interactions within the healthcare system," said study lead author Dr Amruta Nori-Sarma, assistant professor of environmental health at BUSPH. "Addressing the needs of the most vulnerable to pre-empt some of these visits can have a positive impact on individual health and costs, as well as preserve healthcare resources for other emergencies."

The new findings should prompt healthcare providers to prepare for an increased need in mental health services during times when extreme heat is predicted, Nori-Sarma said. "When heatwaves are forecasted, clinicians and public health experts may use our findings to prepare especially for outreach to patients with existing mental health conditions."

The general public can also benefit from this insight, said study senior author Dr Gregory Wellenius, professor of environmental health and director of the Climate and Health Program at BUSPH. "On days of extreme heat, it is important that we each take the precautions necessary to take care of ourselves and our loved ones," he said, which can include checking on neighbours or family members who may be susceptible to health impacts of heat exposure.

For the study, Nori-Sarma and colleagues obtained medical claims data on mental health-related ED visits from OptumLabs Data Warehouse, which contained de-identified, longitudinal health information on more than 200 million commercial and Medicare Advantage enrollees throughout the US. The researchers analyzed approximately 3.5 million ED visits among 2.2 million adults ages 18 or older who had commercial or Medicare Advantage health insurance during the warm season (May to September) from 2010 to 2019.

Days of extreme heat--defined as temperatures above the 95th percentile of temperature distributions by county--were most strongly linked with ED visits for childhood-onset behavioural disorders and substance use disorders, followed by anxiety, stress-related, and somatoform disorders, and mood disorders. Extreme heat was also associated with ED visits for schizophrenia.

The researchers found that the impact of heat on mental health was similar across age groups and evident in both men and women and in every region of the country. "These results show that heat can profoundly impact the mental health of people regardless of age, sex, or where they live," said Wellenius. The authors found the impact of heat was slightly higher in the Northeast, Midwest, and Northwest.

Although those regions generally have lower temperatures than the southeast or the south-western US, "that is exactly why the populations in these areas might suffer the most during times of high temperatures," Nori-Sarma said. "They don't necessarily have the skills or resources in place to cope during times of extreme heat. Heat events will become even more extreme as the climate continues to warm, so it's doubly important to identify the populations that are most vulnerable and to help them adapt to warmer summertime conditions."

In future studies, the researchers have aimed to identify public health strategies that will help alert people to the risks posed by extreme heat and better protect the most vulnerable community members. Subsequent research will also explore the impact of elevated temperatures on mental health during longer periods of time (i.e. heat waves), as well as the impact on vulnerable groups that this study did not assess, including the uninsured, low-income, and various race/ethnicities, and those experiencing less urgent situations.

The continuing effects of COVID-19 on mental health will also shape this work. Lockdowns, social isolation, and general uncertainty during the early days of the pandemic increased the need for--and limited the availability of--mental health services simultaneously as EDs were overwhelmed with patients experiencing physical emergencies, Nori-Sarma said.

"As we approach the upcoming summer season, it is important to keep in mind that the combination of stressors--pandemic and climate--might exacerbate existing mental health conditions," she said. "The mental healthcare system should plan accordingly."