Why does the sun make people sneeze?

When you step outside from a dark building into the full glare of the sun, do you suddenly have the urge to sneeze? If so, you're not alone — as many as 1 in 3 people display this reaction.

This reflex, called the photic sneeze reflex, has been observed since at least the fourth century B.C., when the ancient Greek philosopher and scientist Aristotle posed the question, "Why is one more apt to sneeze after looking at the sun?"

The photic sneeze reflex is also known as "autosomal dominant compelling helio-ophthalmic outburst syndrome," according to the Cleveland Clinic, or ACHOO. This reflex is an autosomal dominant genetic trait, meaning a person has a 50% chance of inheriting this reaction to light if one of their biological parents has it, according to the National Center for BioTechnology Information.

Photic sneezing usually happens after exposure to a bright light such as sunlight and most often when moving from darkness to light, such as after turning on the lights in a dark room. "It's not triggered by a special wavelength of light — it's caused by a change in light intensity," Dr. David Lang, chairman of the Department of Allergy and Clinical Immunology in the Respiratory Institute at the Cleveland Clinic, told Live Science.

The severity of the photic sneeze reflex can vary from person to person. For some, photic sneezes only happen occasionally. For others, bright lights can trigger uncontrollable sneezing multiple times in a row, the Cleveland Clinic noted.

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A 1995 study found that 33% of about 370 patients questioned at an Alabama eye clinic experienced photic sneezing. A more recent 2019 study in China discovered around 25% of about 3,400 people examined displayed the reaction.