If bright light makes you sneeze you may have ACHOO syndrome, a medical mystery over 2500 years old

• ACHOO syndrome causes uncontrollable sneezing from sunlight, camera flashes, and more.
• Evidence suggests that scientists have been trying to understand the cause as far back as 350 BCE.

What usually makes you sneeze? For most, it's typically dust, pollen, or some other unwelcome invader. However, for a subgroup of people, exposure to bright light can launch an onslaught of sneezes.

This phenomenon is appropriately called ACHOO syndrome, short for autosomal dominant compulsive helio-ophthalmic outbursts.

ACHOO syndrome, aka photic sneeze reflex (PSR), is not serious or life-threatening on its own, says Dr. Louis Ptáček, a neurology professor at the University of California San Francisco.

But depending on your circumstances it could be dangerous in certain situations, like if you operate heavy machinery.

Researchers have yet to identify the exact cause of ACHOO syndrome, but here's what we know so far about this medical mystery.

ACHOO syndrome vs. normal sneezing

Usually, you sneeze when the mucous membranes of your nose or throat get irritated by particles from dust, pollen, spicy foods, or air pollution. It is the body's way to clear the irritants.

With ACHOO syndrome, however, the sneezing is not in response to any sort of particle. Rather, it occurs when someone is exposed to bright light, says Dr. Benjamin S. Bleier, a sinus specialist at Mass Eye and Ear. Sunlight is the most common culprit, but even a camera flash can set the sneezing off.

And it's not just one sneeze. The condition can cause uncontrollable sneezing episodes of at least two or three sneezes in a row. Though, in many cases, people will only experience a prickling, tingling sensation in their nose without actually sneezing, says Dr. Annie Nguyen, an ophthalmologist with Keck Medicine of USC.

What triggers ACHOO syndrome

"The reflex seems to be triggered by a change in intensity of light rather than a specific type or wavelength of light," Nguyen says.

This means that sneezing occurs due to the sudden exposure to bright light after an extended period of relative darkness.

According to Ptáček and Dr. Landon Duyka, an otolaryngologist at Northwestern Medicine Lake Forest Hospital, there are plenty of circumstances where the sneeze reflex may be triggered, which include:

• Watching a film inside the movie theater and then walking out into the bright sunlight.
• Waking up in a dim bedroom and then turning on bright lights.
• Being on a dark stage with a spotlight suddenly turning on.
• Driving out of a tunnel on a sunny day.

ACHOO syndrome can be passed down from parent to child. If one of your parents has it, you have a 50% chance of inheriting the trait. Given that babies can also have the condition, it may be congenital, or present from birth.

Note: If sneezing isn't associated with exposure to light and it's accompanied by fatigue, fever, or congestion, it may be caused by allergies or upper respiratory infections, Nguyen says. Get in touch with a health care provider to seek appropriate treatment.

Causes of ACHOO syndrome

The exact cause of the sneeze reflex is unknown, but many have shared their best guesses throughout the years:

• 350 BCE: The Greek philosopher Aristotle theorized in his "Book of Problems" that the heat of the sun dissolves the moisture inside the nose, and the vapor is then expelled by sneezing. English philosopher Francis Bacon disproved this by facing the sun with his eyes closed where he felt the heat of the sun, but it did not make him sneeze.
• 17th century: Bacon made a theory of his own, suggesting that "the bright light causes tears to form in the eyes, which then drain through the tear ducts into the nose leading to a sneeze," Duyka says. A 1985 study also proposed this mechanism. However, it is unlikely true because people sneeze almost immediately after exposure to bright light, which occurs before the eyes have time to start watering.
• 1990: A more recent theory is that the stimulation of the optic nerve — the nerve that connects the eye to the brain — can trigger a sneeze, Duyka says. When a bright light hits the eyes, the optic nerve constricts the pupils, which in theory triggers a sensation that irritates the nose. The irritation is then sensed by the trigeminal nerve — a nerve that sends sensations from the face to the brain — and causes a sneeze.
• 1995: A 1995 study reported that sneezing in response to light may be associated with having a deviated nasal septum, or when the cartilage and bone separating the nasal cavity is off-center.
• 2010: The condition might also involve the overexcitability of the visual cortex — the part of the brain that processes visual information — in response to light, Nguyen says.
• 2019: A 2019 study theorizes that the photic sneeze reflex may be a part, or a subtype, of the trigeminocardiac reflex, which refers to the sudden onset of disturbances in the parasympathetic nervous system whenever the trigeminal nerve is stimulated.

"Up to 35% of the population have this reflex," says Dr. Bernhard Schaller, professor of medical faculty at the University of Buenos Aires and one of the authors of the 2019 study.

Overall, more studies are needed to fully understand the mechanism behind this condition.

Risks of ACHOO syndrome

Sneezing may be an issue if you do any activity that requires extreme concentration and motor control around large machines or those at high speeds, Ptáček says.

Of course, people with ACHOO syndrome have learned to live with these sudden, uncontrollable sneezing episodes but, Nguyen says it's important to be cautious if you do any of the following:

• Operate heavy machinery
• Work with power tools
• Drive a motorized vehicle, like a car or a motorcycle
• Fly a plane

Since your eyes involuntarily close while sneezing, a series of sneezes could be dangerous, Schaller says. You may lose focus or control and cause an unfortunate accident.

Aside from being potentially dangerous, sneezing at ill-timed moments may also be unprofessional or awkward.

"The reflex can be bothersome to performers or public speakers. Sneezing in public during the pandemic has also become much less socially acceptable," Duyka says.

Insider's takeaway

If you tend to sneeze after being exposed to bright light after a period of darkness, you might have the photic sneeze reflex, aka ACHOO syndrome.

It's not a serious nor life-threatening condition, but you need to take caution if you operate tools, machines, or vehicles that can potentially endanger people.

The exact cause of the condition is still unknown. "As long as the sneezing is not prolonged or causing distress, you probably don't need to see a doctor," Bleier says.