Catherine Renouf on the different types of sneezes and their scientific causes

Filling up the pepper grinder makes all of us sneeze occasionally, and nobody sees that as being particularly unusual. However, when I go outside into bright daylight or turn a corner in the car and look into the sun, I experience what is called ‘sun sneezing’; something that does not happen to us all. These two types of sneeze are caused by completely different stimuli, and neither is entirely understood. Yet theories about these causes of sneezing do exist, and another even more unusual cause has been suggested.

Sneezing is a reflex triggered by the stimulation of nerve endings inside the mucous membrane of the nose. The sneeze is intended to expel the irritant from the nose, removing the stimulus and the potential foreign body. Peppercorns contain a chemical called piperine which is not very soluble in water, but when dissolved in ethanol it tastes, unsurprisingly, a lot like pepper. Piperine has been found to inhibit some drug metabolism and increase the absorption of curcumin, selenium, vitamin B and beta-carotene. Topical application of piperine combined with UVB light exposure simulates pigmentation in the skin, and so its use in the treatment of vitiligo has been proposed. Historically pepper has been used for an almost endless list of conditions from gangrene to heart disease, and sunburn to toothache. From the 5th century onwards salves containing pepper were applied directly to the eye to treat eye conditions but, unsurprisingly, this often made the problems much worse. Piperine produces the pungency of pepper by activating the heat and acidity sensing ion channels of pain-sensing nerve cells. This explains why getting pepper into your nose causes discomfort and leads to sneezing, and everyone’s nerve endings react to piperine in the same way.

Sun sneezes, however, are not experienced by everyone. Between 18 and 35% of the population experiences uncontrollable bursts of 1-10 sneezes when they are suddenly exposed to bright lights. A study suggests that 67% of ‘sufferers’ are women, and 94% are caucasian.¹ Sun sneezing, also called the ‘photic sneeze reflex’ and ‘ACHOO syndrome’ (or Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome) may be genetic, but this is not known for sure. The condition was first mentioned by Aristotle during the 4th century BCE. It seems to be the case that some people’s trigeminal nerve, which is responsible for sneezing, and optic nerve, responsible for transmitting visual impulses to the brain, are associated in some way. Overstimulation of the optic nerve triggers the trigeminal nerve to impulse, causing uncontrolled sneezing.

The photic sneeze reflex is not dangerous except in certain occupations, and particularly for pilots. In aerial combat an attack of the sun sneezes when a pilot turns into the sunlight would stop them from having fine control over their reactions, and when landing on an aircraft carrier the reflection of the sun from the sea is very likely to trigger a photic sneeze. Furthermore, patients who have the photic sneeze reflex should warn doctors if they are about to have eye surgery, as the local anaesthetic injection often causes them to sneeze.

Sun sneezing is not a problem for most people, and in fact some are grateful for this reflex as they can usually cause a reluctant sneeze to happen just by looking at the light. One imagines that they are usually a little less happy with it, however, when a sneezing fit attacks while driving the car or landing a fighter jet. However, an even more absurd and inexplicable cause of uncontrolled sneezing is the “snatiation reflex”, where a person sneezes between 3 and 15 times after eating enough to completely fill their stomach. The only way for such a person to avoid a sneezing fit is to never eat until completely full. It seems that sneezing can have a greater impact on some people’s day to day lives, and even careers, than we might expect.

 

Catherine Renouf

Image credit- mcfarlando

¹ Semes, L. P.; Amos, J. F. and Waterbor, J. W. J. Am. Optom. Assoc. (1995), 66, 6, 372-7