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 common in the spring and early summer than other times of year.

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.

“I think it points toward a strong link between allergic rhinitis and mental health,” said Christopher Lowry, a professor of integrative physiology at the University of Denver.

Many allergy sufferers might, when their mucous membranes are burning and their sinuses feel like they’re clogged with Silly Putty, say they want to die. And indeed, some of Postolache’s studies have found links between increases in allergy symptoms, aggression, and mood disturbances. But the connection doesn’t seem to be driven by allergy-induced misery alone. Instead, it appears to be caused by an inflammatory chain reaction that allergens set off in the body.

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.

These cytokines, then, may play a role in the angst and impulsiveness that drives people to take their lives. Indeed, Postolache and others found elevated cytokine levels in the brains of suicide victims.

Obviously, not everyone who suffers from allergies will be driven to suicide. Much depends on one’s predisposition to both mood disorders and allergies themselves. Those exposed to lots of pollen early in life might be protected, Postolache said.

Similarly, few suicides are driven entirely by allergies. “Think of suicide like a pie chart,” said Erick Messias, a psychiatry professor at the University of Arkansas. Various factors, like a family history, or a job loss, or owning a gun, are each potential slices of the pie. “The moment the pie closes, you have a suicide attempt. For some, [pollen allergies] might be a make-or-break deal that closes the gap.”

Despite the growing body of evidence—or perhaps because of it—some of the allergy-suicide studies seem contradictory. In 2010, Messias found a link between allergies and suicidal ideation, but not suicidal attempts—something he attributes to the fact that far more people think about suicide than actually try it. In 2011, Postolache, Aarhus University professor Ping Qin, and others found that suicide victims were more likely than others to have a history of severe allergies, but this was true only among those with no history of mood disorders. Qin told me that could be because getting treatment for a mood disorder might also improve allergy symptoms, since the two ailments have related biological mechanisms. But in another study in 2013, they found that air-pollen counts increased suicide risk, but in men, the effect was stronger among those with a mood disorder. Yet another study, meanwhile, failed to replicate the link between suicide rates and air-pollen levels.

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.

Still, some researchers say future studies are needed to prove the connection. “We’re nowhere near being able to say with confidence that allergens cause suicidality,” Ian Dawe, chair of the Ontario Hospital Association task force on suicide-prevention standards, told the Globe and Mail last year.

In his future research, Postolache plans to see if administering probiotics to animals can help with allergy symptoms, mood changes, and suicidal behavior. The thinking is that if our immune systems, spoiled by overly sterile environments, are overreacting to weeds and trees, a little bit of “good” bacteria might calm their tempers.

In the meantime, he and others recommend that primary care doctors ask their severe-allergy patients how they’re doing emotionally. “We psychiatrists should pay more attention to the body,” Messias said, “and people who pay attention to the body should pay more attention to the mind.”