Early in his career, Teodor Postolache, a psychiatry professor at the University of Maryland, was struck by a peculiar trend that comes up again and again in suicide research. Across decades and in various countries, suicide is much more commonin the spring and early summer than other times of year.

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Now, Postolache and other researchers believe they have found a curious link between the season and self-harm: pollen-induced allergies.

In 2005, Postolache and his collaborators found that the suicide rate among young women doubled during peak pollen season, and the rate among older women went up by more than four-fold. Last year, researchers in Texas similarly found that suicide attempts in women rose with daily tree pollen counts in the Dallas area. And just last month, a paper published in Environmental Research found that increased pollen in the air raised the risk of suicide in women in Tokyo—meaning this dark trend might apply across cultures.

There are several ways in which a severe reaction to airborne allergens might tip the scales for someone at risk for suicide, but here’s one. When a speck of pollen from the air comes into contact with immune cells in the nose, the cells release cytokines—molecules that cells use to communicate messages to one another. Postolache and others believe cytokines might drift through the nose to enter the brain. There, the cytokines might disrupt the brain’s delicate chemical soup, shifting the balance from feel-good chemicals to toxic ones that may trigger anxiety and impulsive behavior. Besides the nose, cytokines might also influence the brain by traveling through nerves, or by prompting immune cells to mistakenly attack healthy brain cells.

Postolache attributes the clashing results to the disparate methodologies used in the studies. Allergies and mental illness have so many contributing factors—like the type of tree or the patient’s genetic vulnerability to suicide—that slight tweaks in data-gathering and analysis can produce dramatically different results.

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