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.