There's really never a good time to sneeze. Judgmental eyes note how well you've covered your mouth. And that second and third sneeze? That's when the co-workers start scooting their chairs away from your cubicle, fearful of the contaminated air that surrounds you.
So why can't we just sneeze once and be done with it? According to Popular Science, it's because we're wimpy sneezers whose first go at clearing our nasal passages is apparently low on our list of evolutionary achievements.
Sneezes are caused by irritation in the lining of the nasal cavity, which releases inflammatory histamines. These in turn stimulate the nerve endings that produce the actual action of sneezing — the explosive push of air that is meant to clear irritants from the nose. Follow-up sneezes occur because the irritant is still there. Sometimes it takes successive efforts to blow out that last lingering particle. For those who suffer from allergies to things like pollens and pet hair, it's very common to need multiple sneezes to do the trick. Other non-illness-inducing sneeze-causers include perfume, dust and even cold air.
Unfortunately, when irritants are blown from the nose, it often comes with mucus. And this is probably why sneezers get a bad rap. If the sneezer is sick with a virus like a cold or the flu, the infected mucus can spread the illness. But don't assume that someone sneezing a bunch is carrying a virus. As mentioned, airborne allergens can easily cause a multi-sneeze attack.
Then again, you don't even need to have an irritant in your nose to sneeze. Popular Science reports that there is something called "photic sneeze reflex," which is a genetic condition that causes one to sneeze simply by looking at bright lights. It's estimated to affect 18 to 20 percent of the population, and while scientists aren't exactly sure why it happens, they think it may have to do with the pupil's rapid adjustment to brightness.