How concerned should we be about a sneeze? The answer varies widely depending on whom you consult. In ancient literature, sneezing often carried significant meaning. In Homer’s “Odyssey,” Telemachus sneezes following Penelope’s prayer for the return of her husband, Odysseus. She interprets this as a favorable sign for Odysseus and a dire omen for her suitors. Meanwhile, in Xenophon’s “Anabasis,” a soldier’s sneeze is seen as divine affirmation that the army will successfully return home. Contrastingly, St. Augustine observes that during his time, people would return to bed if they sneezed while putting on their slippers, suggesting a more superstitious view. But today, is a sneeze anything more than a sign of pathogens, allergens, or perhaps even air pollution?

“It’s a physical response to expel something that’s irritating your body,” explains Sheena Cruickshank, an immunologist and professor at the University of Manchester. “We all have cilia, or tiny hairs, in our noses that sense and move on their own. When these cilia trap an irritant, they signal nerve endings to trigger a sneeze as a response to remove it.”

The causes of irritation leading to sneezing are varied, including allergens, viruses like the common cold or flu, and irritants such as dust or pepper. Yet, sneezing isn’t always prompted by these factors. The trigeminal nerve, responsible for various sensory and motor functions in the face, can be stimulated by cold air or even eyebrow plucking, sending unnecessary sneeze signals to the nerve endings.

One of the more puzzling sneezing triggers is exposure to bright light, humorously named the autosomal-dominant compelling helio-ophthalmic outburst, or Achoo syndrome, in the 1980s. While the exact cause remains unclear, it is known to be hereditary and congenital.

“It’s fundamentally a protective mechanism,” Cruickshank notes. “The initial immune response aims to prevent harmful substances from entering the body, and the next step is expelling potentially damaging particles. Though it can be excessive in dusty environments or with allergens, it’s a sensible response evident in many mammalian species.”

“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.”

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.