Sneeze in Sunlight

4 March 2015

On the 20th December 2013, the 68th Session of the United Nations General Assembly proclaimed that 2015 would be the "International Year of Light and Light-Based Technologies". The short version of that mouthful is "IYL 2015".

Light still has many mysteries. One of its biological mysteries is why some people sneeze when they go out from the dark into the sunlight.

(Actually, it also happens with artificial light, not just sunlight. The colour of the light doesn't matter - just the change in brightness from dark to light.)

The official name is the "Photic Sneeze Reflex" - but some medicos with a sense of humour called it the "ACHOO Syndrome". "ACHOO" is the sound you make when you sneeze, and so with a bit of imagination, the medicos came up with the "A" in "ACHOO" standing for "Autosomal Dominant".

And yes, the Photic Sneeze is genetically dominant - if one parent has it, then any of their children have a 50% chance of also having it.

The "C" stands for "Compelling", which is not quite accurate (but at least it starts with the letter "c"). Roughly one quarter of us have some degree of this strange reflex of sneezing in the light - but it varies enormously in how it affects us. A very small percentage of people will sneeze the same number of times, as regular as clockwork, whenever they go into the light. But at the other end of the spectrum are those people who might be affected only if they are just on the point of sneezing, with the inside of their nose tickling away like crazy - and then, when they stare into a very bright light, they'll have their sneeze. So "C" for "Compelling" is more artistic than accurate.

Next in the word "ACHOO" is the letter "H". This stands for Sun, from the Greek word for "Sun", "Helios".

Finally, we have the two "O"s at the end of "ACHOO". The first "O" stands for "Ophthalmic", coming from the Greek word "ophthalmos" meaning "eye", while the second "O" stands for "outburst", which could be very loosely thought of as a "sneeze".

A sneeze is not just an accident. It's a carefully co-ordinated series of reflex actions involving structures in your head and chest, and lots of nerves and muscles firing off at exactly the right time. The fact that it's a reflex means that you don't have a lot of control over it. We have found a "Sneeze Centre" in the brain of some animals, and we suspect that humans have one - but we haven't proved this yet.

A sneeze has two parts.

First, there's the inspiratory part, where the air is sucked into your lungs, following some kind of stimulus. This stimulus can be fine dust or irritants inside your nose, cold air, the plucking of your eyebrows, rhinitis, an anaesthetic injected into your skin near the eyes, sexual excitement or orgasm, a full meal and yes, stepping out into the bright light.

In the second part of the Sneezing Process, the vocal cords snap shut, the air pressure builds up inside your lungs and then the air bursts out of your nose and mouth, blowing out nasal debris and irritants.

So what makes people sneeze in bright light? At this stage, we simply don't know.

One theory says that the nerves that set off a sneeze travel close to the nerves that carry visual information, and that there's some kind of leaking of information or cross-talk - and so light sets off a sneeze. Another theory says that it's related to one of your major Nervous Systems - the Parasympathetic Nervous System. We know that the Parasympathetic Nervous System makes your pupils smaller when light lands on your eyes. It also is involved in triggering sneezes. Again, maybe there's some cross-talk from one nerve to another. And quite separately from either of these theories, some recent research shows that when Photic Sneezers see stuff, their visual cortex responds in a more excited manner than non-Photic Sneezers.

But overall, we still don't know why about 3% of us sneeze in the Sun.

We do know that Photic Sneezing can be hazardous. Imagine that you are a strong Photic Sneezer driving a car through the alternating light-and-dark generated by a line of trees at the roadside. In this case, you could wear a hat or polarizing sunglasses to shield your eyes.

I don't know if Law Enforcement officers would accept the Photic Reflex as a valid excuse for driving into somebody's car. Saying "I'm sorry Officer, but I have the ACHOO Syndrome" probably won't cut it.

© Karl S. Kruszelnicki Pty Ltd 2015

Dr Karl Kruszelnicki
Dr Karl Kruszelnicki

Dr Karl Kruszelnicki is the Julius Sumner Miller Fellow at the University of Sydney and is best known as a popular science communicator and one half of the comedic science duo, the Sleek Geeks, with Adam Spencer. Learn more about Dr Karl.