As temperatures rise, so do rates of mental health problems, new research shows.
A survey study of more than 3 million respondents showed a significant increase in the probability of self-reported psychiatric problems when temperatures were above 80° F; there was a significant decrease when temperatures dipped below 20° F.
“We found a positive relationship between hotter temperatures and self-reported mental distress in the United States,” the investigators, with first author Mengyao Li and colleagues from the University of Georgia in Atlanta, write.
Moreover, they found that individuals are willing to pay several dollars to avoid each additional hot day and its negative impact on their mental health.
The findings were published online March 25 in PLOS ONE.
Since 1880, when global temperature record keeping began, the average global temperature has increased by about 1.4° F; two thirds of this increase occurred in the past 40 years.
The investigators point out that 8 of the top 10 warmest years on record for the contiguous 48 US states have occurred since 1998. The year with the highest temperature on record was 2016, after three consecutive record-breaking years.
It’s well known that higher temperatures can have a deleterious impact on physical health. Heat stress can cause conditions such as heat rash and muscle cramps, but it also has more serious effects on multiple organ systems, including the central nervous system and the circulatory system.
Scientists have begun to examine the impact of higher temperatures on mental health. One study published in 2018 showed an increase in suicide rates in US and Mexican municipalities with a 1.8° F increase in average monthly temperatures.
To examine the effect of temperature on self-reported mental health, the investigators analyzed data from annual Centers for Disease Control and Prevention (CDC) health surveys from 3 million US residents between 1993 and 2010.
An average daily temperature of 60° to 70° F was the “commonly agreed upon temperature range where most humans feel comfortable,” the investigators write.
They evaluated changes in survey results when average temperatures were above or below that range. In the study, days on which a temperature of 80° F or higher was reached were deemed “hot,” and days of 20° F or lower, “cold.”
In general, the probability of individuals reporting mental health difficulties increased with hotter days and decreased with cooler days.
Results showed that one additional day on which the average temperature was above 80° F led to a significant 0.3% increase (P < .05) in the probability of reporting poor mental health.
One additional day on which the average temperature was below 20° F led to a significant 0.8% reduction (P < .01) in the probability of self-reported mental health difficulties for the previous month.
The findings also revealed that individuals are less likely to report mental health problems on unusually cold days. This, the investigators note, is a novel finding and has not previously been reported.
A period of more than 10 consecutive high-temperature days was associated with a marked increase in mental health problems. Similarly, a run of very cold days was linked to a significant reduction in psychiatric problems.
The results also show that individuals who live in warmer regions of the United States don’t necessarily adapt to the effects of hot weather. A sustained period of hot days increased the probability of reporting mental health problems across all regions of the country.
The researchers also examined the economic impact of hot weather and mental health and found that individuals would be willing to pay from $2.60 to $4.60 to avoid an additional hot day and its resulting effect on mental health.
“In both approaches, we wanted to see how much money needs to be involved in order to adjust the adverse effects of hot temperature days,” Li told Medscape Medical News.
She added that the investigators wanted to include the economic analyses because “a monetary value is easier to grasp and perhaps more relevant for policy decisions.”
The researchers were not able to assess community-level factors, such as social isolation and socioeconomic status, that might affect mental health at a county level.
“It would be interesting for future studies to shed light on how community-level factors mediate the effects of climate change on individual mental health and design policies accordingly,” the investigators write.
Li suggested that future research might use temperature data for years since 2010 to study how mental health might correlate with local temperatures.
“The later years actually have higher temperatures compared to previous years,” Li said. “It would be really interesting to perform this kind of study using more recent data…to see whether the relationship gets more intense.”
Commenting on the findings for Medscape Medical News, Elizabeth Haase, MD, a general psychiatrist for the Carson Tahoe Regional Medical Center in Carson City, Nevada, said the research was strong in its use of CDC-supported data and that its findings were consistent with other research on heat and stress.
“They found that in sustained hot weather, heat-related stress goes up, and that is consistent with what we know,” said Haase, who was not involved in the research.
Haase is also an associate professor of psychiatry at the University of Nevada-Reno and is on the steering committee of the Climate Psychiatry Alliance, a group that works to raise awareness of how climate change can affect mental health.
She noted that one of the study’s limitations was that it only used data up to 2010. In addition, she said, it included a wide range of mental problems within its umbrella definition of self-reported mental health problems, which “doesn’t help us divide out the kinds of mental health stress people may feel,” she said.
The study is a good example of the type of research that needs to be conducted to better understand the impact of climate change, Haase noted.
The study authors have reported no relevant financial relationships.
PLoS One. March 25, 2020. Full text