Why does the sun make us sneeze?

“Sunshine on my shoulders makes me happy,” the late John Denver sang. “Sunlight in my eyes can make me cry.”

“Sunshine on my shoulders makes me happy,” the late John Denver sang. “Sunlight in my eyes can make me cry.”

Lovely lyrics. But as a kid, I thought it would have made more sense for Denver to sing, “Sunlight in my eyes can make me sneeze.” Because for somewhere between one in 10 and one in three people, sunlight has exactly that effect.

It’s called “photic sneezing,” and it’s nothing new: Aristotle wondered about it in the 4th century BC (although he thought it was brought on by heat, not light). But millennia later, we still don’t know exactly why it happens, as New Scientist writer Richard Webb recently discovered.

The usual explanation for regular sneezing is that it serves to expel unwanted material from the airway. A regular sneeze begins with an irritation in your nose. This excites the trigeminal nerve, which sends impulses to the “sneezing center” in the brainstem, the primitive part of the brain that triggers our involuntary reflexes.

The sneezing centre sends impulses along the facial nerve ordering the nasal passages to secrete fluid, and simultaneous impulses along the spinal cord to the respiratory muscles, prompting them to take a quick, deep breath (“Ah-”), then expel it with great force (a 150-km/h “Choo!”). The abdominal, chest, vocal cord and throat muscles are all in on the act, as is the diaphragm and even the eyelids (you always close your eyes when you sneeze).

With today’s sensitive brain-scanning equipment, you’d think it would be a simple matter to track down the precise location of the “sneezing center.” But you’d be wrong. And since scientists can’t pin down the specific neurons involved, they can’t pin down how photic sneezing arises, either.

They do know a few things about it. In 1964 Henry Everett, a consulting psychiatrist at the Johns Hopkins University Hospital in Baltimore, Maryland, made one of the first systematic studies of the condition, questioning 75 of his patients and 169 of his students in detail about their sneezing habits. Among other things, he asked those with photic sneezing (18 per cent of the patients and 24 per cent of the students) if they had any close relatives who reacted to sunlight the same way, and found that 80 per cent of the sneezers said they did, compared to only 20 per cent of the non-sneezers.

This strongly suggests photic sneezing has a genetic component, and further studies have borne that out. In fact, its inheritance is consistent with transmission via a dominant gene, meaning you only need one copy of it from either parent. This is known as autosomal dominant transmission, so photic sneezing has the irresistible scientific name of “autosomal-dominant compelling helio-opthalmic outburst,” the acronym for which is, of course, ACHOO.

But that still doesn’t answer the question of why sunlight should make us sneeze, and as Webb found, at the moment nobody actually has an answer.

Not that there aren’t theories. Other things unrelated to nasal irritation can cause sneezing, after all. A study last year revealed there are patients who sneeze when they have an orgasm — or even simply in response to sexual thoughts.

Maybe all of these strange causes of sneezing come about because of a kind of short-circuit in the brainstem, where all kinds of reflex actions are triggered, so that various stimuli that trigger unrelated reflexes such as blood flow to the genitals or squinting against a bright light also trigger, quite by accident, a sneeze.

The genes that create these short-circuits are more nuisances than threats to survival, and so have been preserved by evolution.

As our tools for studying the brain improve, maybe we’ll figure it out. In the meantime, if sunlight makes you sneeze, try to follow Aristotle’s example:

Just be philosophical about it.

Edward Willett is a Regina freelance writer. E-mail comments or questions to ewillett@sasktel.net. Visit Ed on the web at www.edwardwillett.com.

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