Sneezing may give a lot more away than the fact that someone has a cold. It could mean he or she is thinking lurid thoughts about sex, say scientists.

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Researchers have found evidence that in certain people sneezing can be triggered by sexual fantasy.

The news could lead to raised eyebrows on the bus or train next time a passenger snorts into a tissue.

Both men and women are susceptible to the problem, which may be inherited. It is thought to be closely linked to the way sunlight can make some people sneeze.

Dr Mahmood Bhutta, an ear, nose and throat specialist at the John Radcliffe Hospital in Oxford, launched the study after seeing a patient who suffered "uncontrollable" sneezing fits every time he had a sexual thought.

"We thought this unusual and performed a literature search of the topic," he wrote in the Journal of the Royal Society of Medicine.

Together with colleague Dr Harold Maxwell, a retired consultant psychiatrist, Dr Bhutta scoured internet chatrooms looking for examples of conversations about sneezing and sex.

Typing the words "sex, sneeze OR sneezing" into Google produced a surprising number of hits. Seventeen people of both sexes reported sneezing immediately they thought about sex, and three had the same experience after orgasm.

"Although internet reports do not give us an accurate incidence of these phenomena, our findings do suggest that it is much more common than recognised," wrote Dr Bhutta and Dr Maxwell.

Sneezing usually occurs in response to nasal irritation, triggering a reflex that expels air at speeds of around 150 kilometres an hour.

Eyebrow plucking can also provoke sneezing by stimulating the trigeminal nerve, which produces sensations in the face.

But other more puzzling sneezing triggers are also known, said the researchers.

One was the "photic sneeze reflex" - an apparently inherited sneezing reaction to looking at bright sunlight, which affects almost a quarter of the population.

More rarely, there were cases of people from the same family sneezing after meals.

Dr Bhutta said he believed sneezing when thinking about sex probably ran in families too, but was less likely to be discussed by children and parents.

He added: "It certainly seems odd, but I think this reflex demonstrates evolutionary relics in the wiring of a part of the nervous system called the autonomic nervous system.

"This is the part beyond our control, and which controls things like our heart rate and the amount of light let in by our pupils. Sometimes the signals in this system get crossed, and I think this may be why some people sneeze when they think about sex."