How worried should we be about a sneeze? It depends who you ask. In the Odyssey, Telemachus sneezes after Penelope’s prayer that her husband will soon be home to sort out her house-sitting suitors – which she sees as a good omen for team Odysseus, and very bad news for the suitors. In the Anabasis, Xenophon takes a sneeze from a soldier as godly confirmation that his army can fight their way back to their own territory – great news for them – while St Augustine notes, somewhat disapprovingly, that people of his era tend to go back to bed if they sneeze while putting on their slippers. But is a sneeze an omen of anything apart from pathogens, pollen or – possibly – air pollution?
“It’s a physical response to get rid of something that’s irritating your body,” says Sheena Cruickshank, an immunologist and professor at the University of Manchester. “Alongside the obvious nasal hairs that a few people choose to trim, all of us have cilia, or microscopic hairs in our noses that can move and sense things of their own accord. And so if anything gets trapped by the cilia, that triggers a reaction to your nerve endings that says: ‘Right, let’s get rid of this.’ And that triggers a sneeze.”
Plucking an eyebrow hair can generate a sneeze. Photograph: Posed by model; seb_ra/Getty Images
That irritation can be caused by many things: allergens, viruses such as the common cold or flu, and irritants such as dust or pepper. But it’s not the only reason you might sneeze. The trigeminal nerve, which is responsible for all sorts of sensory and motor functions in the facial region, can be triggered by everything from cold air to plucking an eyebrow, sending sneeze signals to nearby nerve endings even when they’re not strictly necessary.
Perhaps most confusingly of all, some people sneeze when they’re exposed to bright light – waggishly dubbed the autosomal-dominant compelling helio-ophthalmic outburst, or Achoo syndrome, by researchers in the 1980s. The exact mechanism still isn’t completely understood, but it is hereditary and congenital.
The visible airflow a sneeze creates travels less than a metre, and only at about 10mph – not the commonly believed 100mph of a Federer second-serve
OK, but why this sort of physiological reflex in particular? Sneezing is one of the most explosive and powerful involuntary actions the human body can perform: your abdominal and chest muscles are forcing air out of your lungs. It isn’t quite as explosive as we once thought: recent studies suggest that the visible airflow a sneeze creates travels less than a metre, and only at about 10mph – not the commonly believed 100mph of a Federer second-serve. Sneezing also doesn’t make your heart skip a beat or make your eyeballs pop out if you purposefully keep your eyes open (both are urban myths). But why does the action need to be so violent?
“It’s a basic protective mechanism,” says Cruickshank. “The first part of any immune response is trying to stop the stuff getting in in the first place, but the next one is to get rid of things that might be damaging to you. It can go awry in the cases of very dusty rooms or allergens, but it’s actually a very sensible response that we see in a lot of mammalian species.”
Los Angeles smog … air pollution can trigger sneezes. Photograph: Justin Lambert/Getty Images
As for what a sneeze can tell us about imminent danger, that’s a question scientists are still keenly interested in. Cruickshank, for instance, has worked with citizen science projects to investigate how pollution interacts with allergies, and, probably not surprisingly, found a significant association between polluted areas and worse, longer-lasting symptoms. “We know pollution damages the linings of your nose and your lungs, and that helps things get in that shouldn’t. So that’s one theory about what’s happening,” she says. “But there’s also evidence emerging that pollution can start to kind of reprogramme your immune system, making you react inappropriately to things.”
How much of a risk does a sneeze itself pose? “That’s very hard to study, because sneezing is an involuntary thing that happens quite infrequently,” says Catherine Noakes, professor of environmental engineering for buildings at the University of Leeds, a specialist in airborne pathogens. “We have a lot more data on the amount and type of particles that we release when we talk, breathe, sing, or cough, because you can make somebody do those things – but we don’t really know much about the relative importance of sneezing.”
Some of the infections that can be transmitted by sneezes include colds, which affect the upper respiratory tract, and flu and Covid, both of which can reach the lungs. Diseases such as tuberculosis, which usually sit in the lungs, can also be spread by sneezes, as well as measles, mumps, rubella, chickenpox, respiratory syncytial virus, glandular fever and adenovirus.
If you’ve got a sneeze-borne illness, and don’t want to pass it on, catching as much of your sneeze as you can in a tissue is the best bet. The “vampire sneeze” – covering your face with a sweep of your arm – is a decent alternative. It’s also worth bearing in mind that if you, or someone else, touches an object after sneezing into their hand, it can transmit disease, but the risk varies: the Lancet, for instance, suggests that the initial risk of Covid transmission from inanimate surfaces was “assumed on the basis of studies that have little resemblance to real-life scenarios”.
So what do you do if you feel at risk from someone else sneezing? “If you’re trying to avoid someone infectious and you can’t get out of the room, your only real alternative is something like an FFP2 mask,” says Noakes. “But really, what I’d say to anyone who’s concerned about being infectious and sneezing a lot is to consider whether they need to be out at all.”
So, a sneeze might be an indicator that you’re in a polluted area, or it could be a harbinger of illness, or simply a sign that you need to lay off the eyebrow-tweezing. Unless it’s accompanied by other symptoms, you probably don’t need to worry about it too much, and it’s unlikely to be an indicator that your soul is in danger of exiting your body.
It’s still nice to say “Bless you”, though, in response to someone’s sneeze. Even with involuntary reflexes, some archaic expressions are just polite.
