With all the stress and anxiety we’re experiencing during this coronavirus pandemic, perhaps we should be grateful that it’s also not hot outside. For according to a new study published last week in the journal PLOS One, Americans are more likely to report stress, depression and other psychological problems when temperatures climb above 80 degrees Fahrenheit.

The study also found that Americans are less likely to report such problems when temperatures fall below 20 F.

“In general, the probability of reporting mental health difficulties decreases with cooler days and increases with hotter days,” the authors of the study conclude.

Interestingly, the longer the hot days continue, the more likely they’ll have a negative effect on mental health. That wasn’t true of the cold days.

“Hotter temperatures really tend to get to people after about 10 consecutive days, while cooler days have an immediate effect,” explain two of the study’s authors, Susana Ferreira and Travis Smith, in an article they wrote about the research for the Conversation. Both are professors of economics at the University of Georgia.

Global warming has given a new impetus to research on the impact of ambient temperatures on mental health. In recent years, a growing body of scientific literature has reported that rising temperatures are associated have a negative effect on mental health, including an association with higher rates of suicide.

As background information in the current study points out, since 1880, the average global temperature has risen by about 1.4 degrees F, with most of that increase having taken place during the past 40 years.  Nineteen of the 20 warmest years have all occurred since 2001.

“The promotion of mental health has — for the first time — been included in the United Nations Sustainable Development Agenda as a goal to be reached by 2030,” write Ferreira and Smith. “In a rapidly warming world, temperature increases pose a challenge to achieving that goal of ‘good health and well-being.’”

“Our study attempted to gauge the magnitude of that challenge by quantifying the effect of temperature on self-reported mental health,” they add.

How the study was done

For their study, the researchers used data on 3 million Americans who participated between 1993 and 2010 in the Behavioral Risk Factor Surveillance System (BRFSS), an ongoing state-based system of health surveys conducted annually under the direction of the Centers for Disease Control and Prevention (CDC). One of the survey questions ask participants to provide the number of days during the previous month when, because of stress, depression or other negative emotions, they felt their mental health was not good.

The researchers also relied on county-level daily temperature data from the National Oceanic and Atmospheric Administration (NOAA), matching it to the date and the location of the survey responses.

After analyzing all that data, the researchers found that “compared to the human comfortable temperature range of 60-70° F, additional cold days in the past month reduce the probability of self-reported mental health difficulties while additional hot days increase such probability.”

“Specifically,” they add, “one additional day with an average temperature below 20° F leads to a 0.8% reduction in the probability of self-reported mental health difficulties for the past month; one additional day with average temperature above 80° F would lead to a 0.3% increase in that probability.”

“We also wanted to know the economic cost of increasing temperatures in terms of potential changes in mental health,” Ferreira and Smith write in their article for the Conversation.  “Our estimate: The average American would be willing to pay between US $2.60 and $4.60 to avoid an additional very hot day (over 80° F) in the past month in order to maintain [their] current mental health status.”

Limitations and implications

The study is observational, which means it can’t prove a direct connection between heat waves and poor mental health.  It also relies on self-reports of mental health, and those reports may not be accurate.

On this topic, however, we are confined to observational studies. “Clearly, we cannot randomly assign individuals to locations with different temperatures to identify a clear causal impact of temperature on self-reported mental health,” Ferreira and Smith point out.

Still, the study’s findings are supported by other research. And, as its authors note, there is at least one possible explanation for the effect that temperature has on sleep: Cooler temperatures tend to improve the quality of sleep.

But this is just the beginning, not the end of this area of research.

“We would like to further explore which demographic and socioeconomic groups are particularly vulnerable to hotter days,” the researchers say. “We would also like to know how community-level factors — like social cohesion and neighborhood environment — and individual adaptation actions — like air conditioning and migration — mediate the effects of temperature on individual mental health.”

FMI: PLOS One is an open-access journal, so you can read the study in full its website.