For years, I thought I had an almost magical "special ability" – every time I looked up at the hot, dazzling sun or bright light for a few seconds, I would trigger a burst of sneezes that made me feel an irrepressible curiosity about it.
When I was a child, I would sneeze when I watched the sun. Source: Generated by the author with DALL-E3
However, it wasn't until more than 30 years later that one day I stumbled upon a startling fact, when my colleague said he wanted to sneeze, he turned his head to look at the sun outside the window, and then sneezed. At that moment, I realized that this was not a "special function" of my own person, but a more general physiological phenomenon.
So the question is, what is the principle of sneezing when you see light? Let's start with sneezing.
How is a sneeze generated?
When our nasal passages, throat, or other respiratory tracts are irritated by dust, pollen, viruses, bacteria, or other foreign substances, sensory nerve cells in the body are activated. These cells send signals to the brain through the Trigeminal nerve, especially to Medulla Oblongata. The brain is a very important neural structure that controls many basic functions of the autonomic nervous system, including breathing, heartbeat, and digestion.
Nerve cells are activated. Source: Generated by the author with DALL-E3
After analysis and processing by the brain, these signals are then sent through the vagus nerve and phrenic nerve to activate the relevant muscle groups. To clear the irritant more effectively, the body starts with a deep breath, which is dominated by the diaphragm and pectoral muscles. Then, in an instant, air is ejected from the lungs through the nose and mouth with strong force.
This series of complex but highly coordinated movements constitutes the sneeze as we know it.
Why does looking at the sun help with sneezing?
Looking at the sunny sky, you can't help but sneeze loudly? If yes, then you're not alone. This phenomenon is known as the "photosensitive sneeze reflex" (Autosomal dominant compelling helio-ophthalmic outburst) and is also known by a more specialized name: obsessive-compulsive autosomal dominant photoculpatory syndrome. According to statistics, 18%~35% of the world's population triggers a sneeze reaction when exposed to bright light.
Source: Pexels
At present, the exact mechanism of the photosensitive sneeze reflex is still controversial, but the following theories are widely accepted:
1 Cross-stimulus theory
This theory suggests that the eye's response to light intersects with the neural pathways that trigger sneezing. The signaling pathways of the optic and nasal nerves in humans may be close to or cross each other in the brain. When bright light stimulates the eye, the signal may partially "leak" into the pathways of the nasal nerve, triggering the sneeze reflex.
2 Parasympathetic generalization
This theory implies that when a single stimulus (e.g., a certain stimulus in the eye) activates multiple fibers of the parasympathetic nerve, the fibers may collectively send signals. The result of this generalization may be a response that is not specific to the original stimulus species, such as sneezing. In other words, even if the original stimulus is a substance or light in the eye, the neural activity it produces may affect other parts of the body (e.g., the nasal cavity), causing reactions such as sneezing.
3 Increased light sensitivity
Sometimes light does not cause a person to sneeze, but plucking an eyebrow or hair and causing direct stimulation of the trigeminal nerve may cause the eye nerve to become more sensitive to light, and then sneeze. In many people with light-reflex sneezing, this direct stimulation can accelerate the resulting photosensitive sneezing.
Is sneezing hereditary when you see light?
Photosensitive sneeze reflex syndrome is a dominant genetic trait. But it has long been thought to have a latent but unclear genetic basis. A study as early as 1985 looked at a family and suggested that the syndrome was likely to be inherited in an autosomal dominant pattern. This means that if one of a person's parents has this trait, there is a 50% chance that the person will inherit this optical sneeze reflex. However, the sample size of this pedigree study is limited (there is only one family), so the underlying inheritance pattern of this syndrome may remain inconclusive.
In a 2019 study published in the journal Scientific Reports, researchers from Sichuan University conducted an association analysis between 419093 genetic variants and the photosensitive sneezing reflex syndrome phenotype in 3,417 Chinese samples, and found associated underlying genetic markers (single nucleotide polymorphisms, SNPs), demonstrating the polygenic and non-ethnic specificity behind the photosensitive sneezing reflex phenotype.
Published in Scientific Reports The latest study of ACHOO syndrome. Source: Ref. [2]
While this phenotype is not harmful enough to constitute a disease, it can be dangerous when susceptible people enter a fairly bright environment from dark or dim light. For example, drivers exiting tunnels, pilots flying, patients undergoing surgery, miners, etc.
Why do you sneeze two or three times in a row?
When people encounter respiratory irritants, sneezing two or three times in a row is the body's natural removal mechanism, a process that is mainly to ensure that the irritants are completely expelled.
First, the first sneeze may break or loosen the irritant, preparing the way for subsequent sneeze clearance. Subsequently, the second sneeze usually brings the irritant to the front of the nose, closer to the outlet. Finally, a third sneeze helps to completely expel the irritant.
Source: Pexels
However, if the first sneeze is strong enough, the irritant may be successfully removed, in which case additional sneezing is no longer necessary. However, if the first sneeze does not completely remove the irritant, the body will try to remove the irritant from the airways by sneezing continuously. This natural response mechanism ensures that our respiratory tract remains clean when exposed to external stimuli, thus maintaining the normal function of the respiratory system.
How fast/far do droplets travel in a sneeze?
The sneeze reflex is seen as a natural protective mechanism of the human body when the body reacts naturally to changes in the external environment, such as strong light, smoke, irritating gases, or chemicals, it helps the body identify and respond to factors that may cause irritation or harm to the respiratory tract.
Sneezing quickly and effectively removes foreign irritants such as dust, pollen, bacteria, and viruses that enter the nasal passages or upper respiratory tract. Through the force of the sneeze, these irritants can be expelled quickly, reducing their irritation to the mucous membranes of the respiratory tract and the risk of possible infection. So how powerful is the sneezee?
Speed: Studies have shown that a sneeze can reach a maximum speed of 160 kilometers per hour;
The power of the sneeze: a full-body movement that covers the throat, chest, diaphragm and abdomen;
Number of particles: A sneeze can produce up to 40,000 droplets;
Spray Radius: A sneeze can have a spray radius of up to 1.5 meters.
You see, that's why wearing a mask protects against a lot of infectious diseases.
epilogue
The sneeze response is a complex physiological phenomenon that is influenced by a variety of factors. This article explores in detail the different types of sneeze reactions, their causes, and the research associated with them.
However, there are still many unsolved mysteries and directions that need further research in this field. Future research may shed more depth into the mechanisms of the sneeze response, or explore more effective treatments.
bibliography
[1] Breitenbach, Ray A., et al. "The photic sneeze reflex as a risk factor to combat pilots." Military medicine 158.12 (1993): 806-809.
[2] Wang, Mengqiao, et al. "A genome-wide association study on photic sneeze reflex in the Chinese population." Scientific reports 9.1 (2019): 4993.
[3] Forrester, J. M. "Sneezing on exposure to bright light as an inherited response." Human heredity 35.2 (1985): 113-114.
[4] Ahn, Eric S., et al. "Sneezing reflex associated with intravenous sedation and periocular anesthetic injection." American journal of ophthalmology 146.1 (2008): 31-35.
[5] Abramson, David C. "Sudden unexpected sneezing during the insertion of peribulbar block under propofol sedation." Canadian journal of anaesthesia 42 (1995): 740-743.
Planning and production
Author丨Denovo Science Team
Audit丨Shi Jindong, Chief Physician of the Department of Respiratory Medicine, Shanghai Fifth People's Hospital
Planning丨Ding Kun
Editor丨Bai Li
Source丨Popular Science China