- New research links high temperatures and air pollution to increased suicide risk.
- Nitrogen dioxide primarily affects suicide rates during winter reversals.
- This study revealed an interaction between heat, air pollution, and suicide mortality.
Suicide rates have been rising for decades, and recognizing the effects of heat stress and air pollution can be helpful as policymakers and others seek tools to identify and protect those at risk.
That’s according to a new study by a team at the University of Utah, which found a link between increased year-round heat and suicide risk, as well as specific risks from nitrogen dioxide during winter air inversions. The study, published in Environment International, showed that during warm seasons, nitrogen dioxide and particulate pollution amplify the effect of heat on suicide risk.
“The two weeks immediately preceding suicide are a critical time for intervention,” lead author Amanda Bakian, associate professor of psychiatry at Utah Health University and investigator at the Huntsman Institute for Mental Health, said in a written statement. “So we’re really trying to understand what’s going on in that very short period of time.”
She told the Deseret News that the study comes from a previous paper that analyzed the relationship between short-term air pollution and the risk of suicide deaths in Salt Lake County, which found that the number of suicides increased after short-term exposure to nitrogen dioxide and particulate pollution.
This was important because other studies have come to the same conclusion, but can only show association, not causation. However, the university’s early studies were still conducted in a single location and considered only one environmental exposure (heat or pollution) at a time.
“We are not exposed individually, but rather through a combination of conditions, including air quality and weather factors,” Bakian said. So the next step was to study the mixture and do so over a longer period of time and a wider geographic area. So researchers looked at data on 7,500 suicide incidents across Utah between 2000 and 2016 (the original study was from 2000 to 2010). They looked at multiple factors, including heat, nitrogen dioxide, and particulate pollution, and how they combine to increase suicide risk, focusing on the six days before suicide.
The new study uses a measure of heat stress called wet-bulb temperature, which gives a sense of “how hot it really feels outside,” she said. “It doesn’t just reflect the temperature; it also takes into account humidity, sunlight, and wind. High humidity makes it harder for sweat to dry and keeps you cool. Bright sunlight makes you feel hot, and wind makes you feel cool.”
Bakian said using wet bulb globe temperatures shows how difficult it is to keep the body cool through a combination of factors and is used “to determine when it’s too hot to safely play sports or work outdoors.”
For every 9 degrees Fahrenheit increase in wet bulb global temperature, suicide risk increased by 5%. And it turns out that the period from late March to late September is especially dangerous.
ask for intervention
Bakian said new research shows a year-round relationship between short-term increases in heat and suicide risk. Researchers also found an association between short-term increases in nitrogen dioxide in the winter and suicide risk, and short-term increases in summer heat and suicide risk. She said the study only separated seasons into two categories: “warm” and “cold.”
She said this was likely due to nitrogen dioxide being trapped in the winter season combined with air inversion.
Again, the researchers validated the results of an earlier study that included only Salt Lake County. They then used three approaches to see if there was an interaction between air pollutants and heat. All three showed that the effects of heat are amplified by nitrogen dioxide and particulate matter during warmer months. Bakian said he has seen no other studies in the United States that demonstrate “this kind of interaction or synergistic effect of heat and air pollution on suicide mortality.”
Numerous studies have documented the risks that air pollution poses to physical health, including respiratory and cardiovascular diseases. The expansion to consider other outcomes, such as suicide, is new. “I think what this tells us is that environmental conditions can shape short-term vulnerabilities to a variety of health outcomes. This can lead to not only physical health outcomes, but also serious consequences such as suicide,” she said.
Bakian said the hope is that such research will lead to effective interventions, from improving air quality to providing cooling centers during extreme heat waves.
Future research ideas
He also noted that wildfires are on the rise and that air pollution “looks different” in the summer when wildfires are burning. She said: “Given that air pollution is a little different than what we’ve seen historically, future research will look at that so we can better predict and know what the interaction between heat and air pollution will be.”
They did not consider factors such as gender or age to determine who was most susceptible. This is a future study, she said, but likely a follow-up study.
“We’re so complex and heterogeneous that some people are going to be more sensitive. We really want to figure out who is most sensitive so that we can inform better prevention efforts,” Bakian said.
The study did not include altitude, but future studies may include altitude, she added. Altitude has been implicated in suicide, and since Salt Lake City’s elevation is about 4,500 feet, “so we’re all in mild hypoxia, which is the mechanism that’s thought to be involved in suicide.” She said hypoxia can begin at about 2,500 feet.
She predicted the study would eventually include that and expand beyond Utah.
