Opinion 

Coughing and sneezing are two effective ways to expel invading bacteria, viruses, dirt, and irritants. The function of both the cough and sneeze reflex is to clear the airways from irritating and/or dangerous compounds from the environment. For people suffering from an allergy this can be tree or grass pollen, hair from cats or dogs, or any other inhalation allergen. Both bacteria and viruses can infect the upper and lower respiratory tracts [1]. Bronchi and trachea express mechanosensitive irritant-receptors, which, when triggered signal via the nervus vagus to the medulla oblongata to initiate the cough reflex [2]. A cough starts with the inspiration of about 2.5 liters of air, after which the glottis and vocal cords contract to shut of the larynx and contain all the air in the lungs. By contraction of the rectus abdominis, the internal and external obliques, as well as the transversus abdominis and the internal intercostal muscles the air pressure in the lungs increases by +100 mm Hg. Opening of the glottis and vocal cords allow the air to escape with a speed of 800 km/hour. The flow of air will take all particles with it and thus clears the airways. In case of a viral respiratory infection, most if not all viruses will have been cleared after 5 to 6 days. The cough reflex may continue for several weeks, probably because of damaged epithelia or other mechanical triggers.

The sneeze reflex is initiated in a similar way, but now receptors in the upper respiratory tract are triggered [3]. The triggering event may also be sunlight [4] or the thought of sex or the act itself [5]. Mechanosensitive irritant-receptors in the nasopharynx stimulate, via the nervus trigeminus, the medulla oblongata. During the sneeze reflex, uvula and soft palate are depressed so that air can be expelled undisturbed via the nose (although a substantial amount of air exits via the mouth). Cleaning of the sinuses is optimized by the so-called venturi effect: directly behind the flow of air, an area of under pressure is created which acts as a vacuum cleaner for the sinuses. Sneezing releases approximately 40,000 droplets (Figure 1), with an initial speed of over 166 km/hour. When that speed would be maintained, an airway virus would travel from Amsterdam to Paris in 3 hours. The habit to keep ones hand in front of the mouth when sneezing should be discouraged. Transmission of respiratory viruses such as Rhinovirus mainly occurs via hand contact [6,7]. Use of (paper) handkerchiefs or sneezing in the elbow is to be preferred.

When someone sneezes, a common verbal response of bystanders is that they wish the sneezer a good health (such as in Germany: Gesundheit) or protection by God (in Spain: Dios te bendinga) (Figure 1). Whishing the sneezer a good health stems from the common wisdom that a sneeze could be the first sign of an upcoming infection, and he or she would need the full capacity of the immune system in order to remain healthy. The “may God protect you” wish has a completely different origin. In the past it was believed that by sneezing the soul would leave the body. Without a soul, the body would no longer be protected against devils, such as illustrated in the Jheronimus Bosch painting in (Figure 2).

Therefore, the protection of God would be required in order to keep the devil out. When we would take the devil as a metaphor for an infectious disease, both wishes would have the same meaning: sneeze to ward of an infection. 

Figure 1: Sneezing (photograph courtesy Prof. M. Golightly, Stonybrook University, New York, USA). The description of the sound produced during sneezing is indicated for the various languages (indicated by their respective national flags), as well as the most common verbal response given by bystanders.

Figure 2: Mankind Beset by Devils painted by Jheronimus Bosch in 1514 on the reverse side of the diptych Hell and Flood. Museum Boijmans van Beuningen, Rotterdam, Netherlands. https://www.wikiart.org/en/hieronymus-bosch/mankind-beset-by-devils-reverse-of-noah-panel-1504

1. Curley FJ, Irwin RS, Pratter MR, Stivers DH, Doern GV, et al. (1988) Cough and the common cold. Am Rev Respir Dis 138: 305-311.
2. Driessen AK, Farrell MJ, Mazzone SB, McGovern AE (2016) Multiple neural circuits mediating airway sensations: Recent advances in the neurobiology of the urge-to-cough. Respir Physiol Neurobiol 226: 115-120.
3. Songu M, Cingi C (2009) Sneeze reflex: facts and fiction. Ther Adv Respir Dis 3: 131-141.
4. Whitman BW, Packer RJ (1993) The photic sneeze reflex: literature review and discussion. Neurology 43: 868-871.
5. Bhutta MF, Maxwell H (2008) Sneezing induced by sexual ideation or orgasm: an under-reported phenomenon. Journal of the Royal Society of Medicine 101: 587-591.
6. L'Huillier AG, Tapparel C, Turin L, Boquete-Suter P, Thomas Y, Kaiser L (2015) Survival of rhinoviruses on human fingers. Clin Microbiol Infect 21: 381-385.
7. Winther B, McCue K, Ashe K, Rubino J, Hendley JO (2011) Rhinovirus contamination of surfaces in homes of adults with natural colds: transfer of virus to fingertips during normal daily activities. J Med Virol 83: 906-909.