How hot weather, air pollution combine to boost suicide risk

As suicide rates have been climbing for several decades and policymakers and others look for tools to identify and protect those at risk, recognizing the impact of heat stress and air pollution could prove helpful.

That’s from a new study by a team at the University of Utah that finds a relationship between increasing heat and suicide risk year-round, along with specific risks from nitrogen dioxide associated with winter air inversions. The research, published in Environment International, showed that the effect of heat on suicide risk becomes amplified in the warm season because of nitrogen dioxide and fine particulate pollution.

“The two-week period just before suicide is a critical time for intervention,” lead author Amanda Bakian, research associate professor of psychiatry at University of Utah Health and a Huntsman Mental Health Institute investigator, said in a written statement. “So we’re really trying to understand what’s happening in that really short-term period.”

She told Deseret News that the research springs from an earlier paper that analyzed the relationship between short-term ambient air pollution and the risk of suicide death in Salt Lake County, finding suicide numbers climb after short-term exposure to nitrogen dioxide and fine particulate pollution.

Other studies reached the same conclusion, which was important because they could only show association and not causation. But the early university study was still in a single location and considered just one environmental exposure at a time — heat or pollution.

“We’re not exposed individually,” Bakian said, but rather to a mixture of conditions, including air quality and weather factors. So the next step was looking at the mixture — and doing it across a longer period of time and a bigger geographic area. So researchers looked at data from 7,500 suicide cases across Utah between 2000 and 2016 (the original study was 2000-2010). And they included multiple components including heat and nitrogen dioxide and fine particulate pollution and how they might combine to raise suicide risk, with a focus on the six days before the suicide.

The new study used a measure of heat stress called wet bulb globe temperature, which captures “how hot it really feels outside,” she said. “It doesn’t just reflect air temperature. It also considers how much humidity, sunlight and wind there is. When it’s humid, sweat doesn’t dry as easily, so your body can’t cool down well. Bright sun can make it feel hotter and wind can help cool you off.”

Bakian said that using wet bulb globe temperature puts multiple factors together to show how hard it is for a body to stay cool and it is used “to decide when it’s too hot to play sports or work outside safely.”

Every 9-degree Fahrenheit increase in wet bulb globe temperature raised suicide risk 5%. And late March to late September proved especially risky.

Seeking interventions

According to Bakian, the new study showed a year-round relationship between short-term increases in heat and risk of suicide. The researchers also found a relationship between short-term increases in nitrogen dioxide and suicide risk in winter and short-term increasing heat and risk of suicide in summer. For the study, they divided the seasons into just two categories: warm and cold, she said.

She said it’s likely the nitrogen dioxide in wintertime pairs up with air inversions because those trap it.

Again, researchers validated findings from the earlier Salt Lake County-only study. Then they used three approaches to see if there were interactions between air pollutants and heat. All three indicated the effects of heat are amplified in the warm season by nitrogen dioxide and also fine particulate matter. Bakian said she hasn’t seen any other study in the U.S. that demonstrates “this sort of interaction and sort of synergistic effects of heat and air pollution on the risk of suicide mortality.”

A lot of research has documented the danger of air pollution exposure to physical health, such as respiratory or cardiovascular disease. Expanding to consider other outcomes, such as suicide, is new. “I think what this helps inform us is that environmental conditions can shape short-term vulnerability to a range of health outcomes, not only physical health outcomes but potentially severe outcomes such as suicide,” she said.

The hope is that such research will lead to effective interventions, from improving air quality to providing cooling centers in times of extreme heat, per Bakian.

Future research ideas

She also noted that wildfires have been increasing and air pollution “looks different” in summertime when those are burning. She said that ”future work will look at that so we can better anticipate and know what the interaction between heat and air pollution might look like given that the air pollution looks a little different than what we’ve seen historically."

They didn’t look at factors like gender and age to figure out who is most susceptible. That’s a study for the future, but she said it will be a likely followup.

“We’re so complex and heterogeneous that there are some people that are going to be more sensitive, and we really do want to figure out who is most sensitive so we can inform better prevention efforts,” Bakian said.

This study didn’t include altitude, but future research might, she added. Altitude has been implicated in suicide and Salt Lake City is around 4,500 feet altitude, “so we’re all mildly hypoxic, which is the mechanism that’s been implicated in the link.” She said hypoxia can begin at about 2,500 feet.

She predicted that the research would eventually include that and also expand outside of Utah.