It is one of the most powerful involuntary actions the human body can perform. But is a big sneeze a sign of illness, pollution or something else entirely?

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.”