Uncovered Cough And Sneeze Help Spread Nasty Disease

Kausik Datta
7 min readJun 8, 2016



Feel like a sneeze or cough coming on? Cover it in a cloth or tissue paper, or even your sleeves or the crook of your elbow, and wash your hands, admonishes the Centers for Disease Control and Prevention (CDC) — and for good reasons, too!

When we cough or sneeze, we suddenly and forcefully expel air from our lungs and upper airways through the mouth and nose. Ordinarily, this is fine. But sometimes, unbeknownst to us, our lungs and air passageways can carry a load of nasty, disease-causing creepy-crawlies of the microscopic kind (a.k.a. “microbes” or germs), whether we appear ill or in perfect health. When we cough or sneeze, these microbes are pushed out along with air into the open; when we touch our faces or rub our noses or mouths — especially immediately after coughing or sneezing — we can get a layer of germs on the surface of our hands and palms, like butter on bread, which we unwittingly pass along to any surface we touch thereafter, our own or someone else’s faces, rods and handles in public transport, door knobs, and so forth.

Many of these microbes, thusly transmitted via cough, sneeze, and/or unclean hands/palms, are responsible for a variety of respiratory illnesses of varying degrees of severity. For example, bacteria which cause pneumonia, whooping cough, and tuberculosis, and viruses responsible for the common cold and influenza belong in this category, to name but a few. In the last two decades, the latest additions to this fearsome list are from a family of viruses, known as ‘coronavirus’ — the triumvirate of SARS-Coronavirus (responsible for Sudden Acute Respiratory Syndrome) of 2003, the MERS-Coronavirus (responsible for Middle East Respiratory Syndrome) of 2012, and now the novel coronavirus of 2019, christened ‘2019-nCov’ or ‘COVID-19’, currently ravaging through the globe.

THIS is the reason why public health agencies at the local, regional, national and international levels, as well as medical professionals, are urging people to practice good respiratory hygiene, which means covering the mouth and nose with a tissue, cloth, your sleeves or a bent elbow, when a cough or sneeze comes along. The used tissue should then be immediately disposed of. Two additional and urgent recommendations are to wash your hands frequently and thoroughly with soap and water (or an alcohol-based hand sanitizer, if soap and water are not immediately available), and to consciously avoid touching your eyes, nose and mouth. These are practical means to cut down the transmission of these germs, including COVID-19, from person to person.

Credit: Image created by Minnesota Department of Health; distributed via CDC

What Happens During a Cough or Sneeze?

Cough and sneeze are both violent expirations that release turbulent flows of hot air, moistened with saliva and/or nasal secretions, and contain suspended droplets of various sizes. Physiologically, they occur as a result of a series of complex and coordinated neuromuscular responses, when something tickles or irritates our lung and respiratory passageways; these mechanisms are how our body has evolved to protect respiratory tracts from injury. At the Massachusetts Institute of Technology (MIT), mathematician Lydia Bourouiba’s laboratory has been involved in investigating these processes using experimental and theoretical methods of the fluid dynamics; generally, large droplets (diameter >100 µm; 1 micrometer or µm = 1 thousandth of a millimeter) quickly settle down on the ground and other surfaces, whereas smaller droplets undergo evaporation, depending upon ambient temperature and humidity, and remain suspended as a cloud of air. These small droplets also provide a joy-ride to the assortment of respiratory microbes to outside the body and allow them to hang out in that noxious air-cloud (a.k.a. an ‘aerosol’).

An earlier fluid dynamics study showed that different people have different cough flow characteristics, but in general, a cough starts with a short inhalation, followed by an explosive exhalation during which air, which may amount to greater than 2 liters in volume, is expelled with an extremely accelerated flow rate (up to 9 liters in one second), taking saliva with it, and generating millions of aerosolized droplets of 0.1–10 µm in size. The Bourouiba lab has done additional work to characterize sneezes, which produces a denser cloud than does a cough, with larger droplets and slightly different trajectories. See the following video of their work on the fluid dynamics of sneeze, published by Nature Video:

Now, if you are unfortunate enough to already have the bacterial or viral respiratory infections mentioned above, or are simply carrying those germs in your nasal passageway without being ill yourself (because you have strong immunity, perhaps as a result of being vaccinated), the germs inhabit the airborne and settled droplets. From the cough/sneeze of infected person A, direct short-range (about 1 meter or 3 feet) transmission involves direct spraying of the infected large and small aerosol droplets onto eyes, nose, or lips of person B in close proximity, whereas indirect short and long-range transmission occurs when person B or a remote person C inhales small infected droplets, which can float around in the ambient air for a long time even after person A has disappeared from the scene.

Visualization of direct and indirect short range and long range transmission via aerosol droplets

Size of the inhaled droplet particles determines how far into the respiratory system the microbes may be deposited. Depending on their viability in the ambient environmental conditions, even large settled particles may be dispersed later from inanimate surfaces (a.k.a. fomites), such as metal, glass, plastic, cloth materials like hospital bed covers. Understandably, especially vulnerable are those whose immunity is inadequate — due perhaps to HIV infection, cancers of various kinds, or solid organ transplants.

Many viruses have the ability to hang around in aerosolized droplets, for hours to even days; these include the measles virus, the cold and flu viruses, and the MERS-coronavirus. Rhinoviruses, responsible for the common cold, are able to survive for extended periods of time on fingers when the concentration is high. In settled droplets on fomites, the coronaviruses are known to persist for up to 9 days. It is important to note that currently there is no evidence for aerosol transmission of the new coronavirus in the block, COVID-19, but precautions are important because of the observations with its cousins.

And to think that this important mechanism of disease transmission can be effectively broken with the simple contrivance of covering your cough or sneeze, isn’t that amazing?

What’s Up with all the Handwashing, then?

I’m glad you asked. You are aware that the another proactive action the public health agencies are urgently recommending is plain and simple washing of hands, preferably with good, old soap and water — no need for them fancy antibacterial soaps, which, the FDA found, were not better than ordinary soap. However, there is a technique to effective handwashing. After wetting your hands, you have to lather with the soap all parts of your hands, front and back, between fingers and under the nails, and scrub for at least 20–30 seconds.

This is not arbitrary. Our skin releases a lot oily, greasy substances, to which germs of various kinds glom on. Components of soap, by their chemical nature, act as emulsifiers which cut the grease and allow the water to lift and carry off the germs when you rinse. For the same reason, soap works well against viruses, which are covered by a layer made from protein and lipid (a.k.a. fat) molecules; soap emulsifies and dissolves the lipids, breaking apart the viruses. Win-win. And, of course, the more you rub, the more it cleans — but you surely don’t want to chafe your skin, so 20–30 seconds seem to be optimal.

On the off chance you don’t have immediate access to soap and water, you can alternatively clean your hands with a hand sanitizer with at least 60% or greater proportion of alcohol, which — when used in appropriate volume and given time to dry (about the same 20–30 seconds) — can kill most viruses and many bacteria. However, be aware that hand sanitizers may not work efficiently on heavily dirty or grimy hands.

Space… man!

We, unfortunately, still have no clue as how exactly COVID-19 spreads from person to person. From an abundance of caution — and in view of the observations of the transmissibility of other respiratory microbes via aerosols — the public health agencies are advising you put some space (about 3–6 feet) between yourself and someone who, you note, is visibly coughing or sneezing. But this admonition is not absolute, mind you, because whether a person falls ill from this coronavirus may depend on their level of immunity — and an apparently healthy person may carry and transmit the virus without showing symptoms.

Some Quick Numbers

With the outbreak entering its third month, in the parts of the world affected by COVID-19, current numbers from China and elsewhere show a case fatality rate of 2–4 patients per 100 infected; the SARS and MERS coronavirus epidemics had much, much worse numbers, but COVID-19 is still developing. The numbers also show that about 8 out of every 10 infected person have a mild form of the disease and recover from it. However, what is remarkable about this outbreak is that the groups maximally vulnerable to this virus appear to be the elderly (nearly 1 in 4 of diagnosed Chinese patients of 60+ years succumbed to it) and the patients with underlying, pre-existing health conditions, such as cardiovascular disease or diabetes.

Source: Max Roser and Hannah Ritchie (2020) — “Coronavirus Disease (COVID-19)”. Published online at OurWorldInData.org. Retrieved from: ‘https://ourworldindata.org/coronavirus' [Online Resource]

Therefore, Friends, Romans, and Countrypersons, for the sake of the health and well-being of you and yours, as well as the public around you — at present and in immediate future — please cover your mouth if you feel that cough or sneeze coming on, and don’t forget to wash your hands. There is no cure, nor vaccine, for coronavirus COVID-19 yet. Remember the wise and timeless words from Benjamin Franklin, written in 1736 — “An Ounce of Prevention Is Worth A Pound of Cure.

This is an updated and expanded version of the essay originally published at https://inscientioveritas.org on June 8, 2016.



Kausik Datta

Wannabe storyteller in science. Graduate of John Hopkins Science Writing MA program.