According to a recent study, there is an association between hotter temperatures and an increase both in the number of hospital visits for mental health reasons and in suicide rates.

Suicide is one of the leading causes of death in the United States and globally.

According to the National Institute of Mental Health, in 2017, suicide claimed the lives of 47,173 people in the US, which is more than double the number of homicides.

Of course, behind every suicide, there is a convoluted, interlinked web of causal factors.

Unpicking the vast array of potential risk factors that can link to suicide is challenging work.

However, because suicide rates in the US have steadily increased from 2001 to 2017, understanding these factors is more pressing than ever.

A group of scientists from the University of Massachusetts Amherst and California Polytechnic State University, San Louis Obispo, is interested in the role of climate. The investigators recently published the results of their latest study in the Journal of Health Economics.

Data mining on a grand scale

Looking for a relationship between environmental conditions and mental health is not a new idea. However, as the climate crisis takes center stage in public discourse, the role of climate in psychological well-being has entered the spotlight.

Earlier studies have identified links between temperature and mental health, but, to date, much of this research has focused on relatively short periods and only looked for associations rather than causal factors.

Also, the findings have been contradictory, and not all studies have reached the same conclusions. The researchers behind the current study hope to address some of the earlier shortfalls and produce a definitive answer. They have attempted this by collecting and analyzing vast amounts of information.

To investigate, the scientists used several different sources. Firstly, they gathered data from Californian emergency departments, collating information about visits relating to mental health diagnoses from 2005 to 2016.

Secondly, they gained access to information about suicides in the US from 1960 to 2016.

Thirdly, they took data from a nationally representative survey that had more than 4 million respondents between 1993 and 2012. This information included self-reported mental health statuses.

The authors also collated information about other factors that might influence any relationship between temperature and mental health, including access to air conditioning, availability of mental health services, insurance coverage, accessibility of substance abuse treatment, and income levels.

The authors were able to develop "fine spatial and temporal scales," drilling down to the temperatures in each month in each county of the US.

Temperature and mental health

Overall, the authors concluded that cooler temperatures decrease the level of adverse mental health outcomes and that warmer temperatures increase negative health outcomes. They provide further detail:

"Our main estimates imply that increasing average monthly temperature by 1°F leads to a 0.48% increase in mental health [emergency department] visits and a 0.35% increase in suicides."

They also show that their estimates remain stable over time—in other words, people do not seem to adapt to changes in temperature very quickly. The authors explain that they found "no evidence of effective adaptation to the identified effects anywhere—or among any group—in the US."

Similarly, the authors also showed that their estimates remained stable even when they accounted for levels of air conditioning adoption and socioeconomic status.

Importantly, the relationship remained significant both in areas with higher average temperatures and in regions with lower average temperatures.

In other words, hot temperatures influenced mental health, even in populations accustomed to the heat.

For many years, scientists have studied how climate affects mental well-being. However, it is often difficult to link temperature directly to health outcomes. As an example, a 2017 study that focused on India found that suicide rates there peak in tandem with increasing temperatures.

However, the author noted that the suicide rate only increases when the temperature spike occurs during the growing season. At this time, the higher temperature reduces crop yield and produces economic hardship, which, the author believes, might increase the risk of self-harm. So, in this case, temperature increases do not directly influence suicide rates.

The role of sleep

The authors of the recent study believe that the link between increasing temperatures and poor mental health outcomes in the US might be due to sleep disturbances.

Dipping into other datasets, they found that the sleep disruption resulting from high temperatures closely mirrors temperature's effect on mental health outcomes. The authors write:

"While it is possible that temperature independently affects both sleep and mental health in a similar manner, we argue that this is not likely to be the case as other research documents a strong link between poor sleep and measures of mental health that are unrelated to temperature."

Overall, the results of this in-depth analysis make for somber reading. Of course, the reason why people carry out this type of study is to identify risk factors in the hope that it might be possible to do something to reduce that risk.

With this in mind, the authors write that "[a] direct policy recommendation stemming from our research is for mental health providers to ensure patients get adequate sleep during periods in which sleep is likely to be disturbed (such as a heat event)."

The authors note that their study only focuses on temperature and that they hope that future work might probe other environmental factors that could influence mental health outcomes.