Got a cold? Are you allergic to something? These seem to be routine questions any time I sneeze, but they always leave me feeling somehow falsely accused — I have neither a cold, nor allergies (as far as I can tell) and yet, there's this assumption that if there's a sneeze, it must be one or the other.
That's one of many myths surrounding the sneeze. Other myths (or are they facts?) about sneezing are that you cannot keep your eyes open during a sneeze; that your heart stops when you sneeze; that looking at a bright light can trigger a sneeze, and that rubbing the bridge of your nose can prevent it.
And then there are what might best be called superstitions, for example: "Sneeze before seven, company before eleven;" or "Always say 'God bless you' when you sneeze, to keep the devil from flying down your throat." These superstitions attach significance to the number of times you sneeze, when it happens, or various blessings that should accompany a sneeze.
So there is plenty of advice and folklore built around sneezing. For such a common reflex — found throughout the animal kingdom — it is surprising how little is known about it. Let's take a look at what we do know.
The medical term for a sneeze is "sternutation" (though I've never heard that term used!), a complex reflex involving nerves in the nose that detect swelling of the nasal membranes, particles of a certain size, or substances to which you are allergic.
Particularly good stimulators of sternutation are pepper, cold air and dust. These triggers send messages to the throat, chest and abdomen to contract in a particular sequence to forcefully rid the nasal passage of its contents. For unknown reasons, perhaps as protection from flying particles, the muscles of the face, including the eyelid muscles that briefly close the eyes, are also in the loop. The end result is that the offending agents in the nasal passages are forcefully expelled.
And forceful it is: Particles exiting the nasal passages have been clocked at more than 100 miles per hour. The contents of the upper airways and mouth also contribute to the expelled materia. In fact, while dogs and cats (and perhaps other animals) sneeze primarily through their noses, most of the expelled material during a human sneeze departs by way of the mouth. That's why covering both your mouth and nose when sneezing is important.
The underlying conditions most associated with sneezing are allergies or infections. Dust, tree and grass pollen, dander (often from pet hair) and molds are among the most common allergic triggers.
Viruses that inhabit or invade the nasal passages can cause swelling of the membranes, increased mucus and runny nose. They are much more common triggers of sneezing than bacteria, fungi or other infections. In fact, the virus that classically causes the common cold get its name ("rhinovirus") from its tendency to take up residence in the nose — the prefix "rhino-" comes from the Greek rhis, meaning "nose." If you have a cold as the cause of your sneezing, the particles projected into the environment may be teeming with potentially infectious viruses.
It is hard to know what to believe, but at least one of the myths I have learned about sneezing is actually true. For up to 30% of the population, looking at a bright light can trigger sneezing. It's called "photic sneezing" and why it occurs is unknown.
There are theories that a bright light somehow triggers the other nerves involved in the sneeze reflex by the flood of signals into the retina (at the back of the eye), the pupil (at the front of the eye) or from squinting. That is, it may be a situation when signals that are separate in most people are "crossed" in those with photic sneezing. At least it isn't harmful, and it can even be useful: When you feel you are about to sneeze but just can't, look briefly at a light; often that will encourage the sneeze and relieve that itchy, "about-to-sneeze" feeling. If you are one of those who are troubled by fits of sneezing after coming out of a darkened movie theater into the light, keep your sunglasses handy; one study found they could prevent the photic sneeze.
Most of the other rumors about sneezing are not true. Your heart does not stop when you sneeze, and with great effort you could probably keep your eyes open during a sneeze. While the tradition of saying "Bless you" or "Gesundheit" (meaning "health" in German), or some variation on this theme, apparently originated from superstitions that sneezing was a sign of approaching danger or even death, most people who sneeze have an excellent prognosis!
Many things remain unanswered when it comes to sneezing. As mentioned, the reason that some people sneeze in response to bright light is a complete mystery. But even more basic than that is the question of why we sneeze at all. Because humans expel more from their mouths during sneezing than from their noses, the idea that it rids the nasal passage of irritating substances may not be the whole story. It is possible that evolution has maintained this reflex through human development even if it is rather ineffective.
I could find little reliable information about how to stifle a sneeze through physical maneuvers or distraction — if there is a way to predictably prevent a sneeze, it is a mystery to me. And finally, why does each person sneeze somewhat differently with sort of a "sneeze fingerprint"? The answers to these questions are not readily available — and they may never be.
Sneezing is a phenomenon that is common to all humans and is widespread in the animal kingdom as well. It may play an important role in maintaining health in ways that we don't currently understand. It is rarely a sign of serious illness or impending disaster as feared by previous generations. On the other hand, it can be remarkably annoying, as anyone with significant seasonal allergies or a bad cold can tell you. Perhaps the most important "take-home" message I can offer is to mention again the importance of covering your mouth and nose when you have a cold that triggers a sneeze.
Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.