Suicide rates in the U.S. have been steadily rising since the late 1990s, leading to an urgent need to understand what's putting people at risk. Risk factors include high air pollution and hot temperatures, which have each been individually associated with increased suicide risk in the days after exposure.
Now, a new study has found that, in the hot summer months, high levels of air pollution can make unusually hot days much riskier.
"The two-week period just before suicide is a critical time for intervention, so we're really trying to understand what's happening in that really short-term period," says Amanda Bakian, PhD, research associate professor of psychiatry at University of Utah Health, Huntsman Mental Health Institute investigator, and the senior author on the study.
"Many environmental factors have been associated with short-term suicide risk, but very little has been done to consider what's happening in a mixture."
Higher temperatures, higher risk
Heat stress alone is associated with higher suicide risk all year round, but especially during the warmer months, the study found.
The researchers analyzed over 7,500 suicide cases across Utah from 2000-2016, cross-referencing the approximate levels of heat stress, nitrogen dioxide, and fine particulate air pollution at the decedent's address. While previous studies had looked at temperature alone, this study used a measure of heat stress called wet bulb globe temperature, which includes wind speed, humidity, and cloud cover to determine a more accurate measure of how heat actually affects the body.
Heat exposure was associated with higher risk of suicide. For every 9°F increase in wet bulb globe temperature, the risk of suicide increased by 5%. The risk was especially amplified during the warmer periods of the year, from late March to late September.
Air pollution makes hot days more dangerous
For an air pollutant called nitrogen dioxide, produced at high levels by fossil fuel power plants and gas-powered vehicles, the story is more complicated. "We found a strong synergistic effect of heat and nitrogen dioxide, which is one of the traffic-related air pollutants, particularly during the warm season," says Dirga Lamichhane, PhD, postdoctoral researcher at U of U Health and first author on the study. "The effect of heat stress is especially high when the NO2 level is very high."
In other words, during the warm season, nitrogen dioxide on its own isn't associated with higher risk of suicide-but higher levels of nitrogen dioxide can amplify the risk associated with wet bulb globe temperature. During the warm season, if levels of nitrogen dioxide are high, every 9°F increase in wet bulb globe temperature is associated with a nearly 50% increase in suicide risk.
During the cold season, when air pollution tends to be much worse overall in Utah, high levels of nitrogen dioxide are associated with higher suicide risk regardless of temperature.
Understanding risk to improve interventions
The researchers emphasize that the links between heat stress, air pollution, and suicide are only associations-they don't know whether these environmental conditions directly cause increases in risk.
As the climate shifts, these patterns of risk could change in unexpected ways, requiring further study. "This data was collected between 2000 and 2016," Bakian explains. "Since then, we've seen the advent of wildfire season in Utah, so that could potentially change nitrogen dioxide and fine particulate matter measures in the summertime. Now that there's more seasonal variability in air pollution in Utah, that raises questions about what that synergism with high temperatures looks like."
In other future work, the researchers hope to help figure out who's most vulnerable to environmental changes by investigating how genetic risk factors for suicide may interact with environmental factors.
"Future studies could link environmental data with genetic markers to see the interaction between temperature and some of the genetic factors and see the contribution of each factor to suicide risk. This may really help to understand and predict suicide."
The authors add that, while air quality and heat interventions historically haven't been considered in the context of suicide prevention, their results suggest that environmental policies could help prevent suicide. And better prediction of who is at risk for suicide and when could enable just-in-time interventions to save lives.
