Feel like a sneeze or cough coming on? Cover it in a cloth or tissue paper, or even your sleeves, and wash your hands, admonishes the Centers for Disease Control and Prevention (CDC) — and for good reasons, too! Microbial pathogenic agents of a variety of respiratory illnesses, both viral [ranging from the common cold (rhinovirus); influenza (orthomyxovirus); parainfluenza viruses, respiratory syncytial virus (RSV), and human metapneumovirus (all paramyxoviruses); severe acute respiratory syndrome (SARS-Coronavirus)] as well as bacterial [such as those responsible for pneumonia (Streptococcus pneumoniae), whooping cough (Bordetella pertussis), and tuberculosis (Mycobacterium tuberculosis)] are often transmitted by cough, sneeze, and/or unclean hands/palms carrying these germs on their surfaces.
But why cover it? Physiologically, our body has evolved to protect the lung and respiratory passageways from injury via a series of complex and coordinated neuromuscular responses, which include cough and sneeze – both violent expirations that release turbulent flows of hot air, moistened with saliva and/or nasal secretions, containing suspended droplets of various sizes. Mathematician Lydia Bourouiba’s laboratory at MIT has been involved in investigating these process using experimental and theoretical methods of the ﬂuid dynamics; generally, large droplets (diameter >100 µm; 1 micrometer/µm = 1 millionth of a meter) 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.
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 last week by Nature Video:
Now, if you actively have the types of 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 are what comes out via the airborne and settled droplets. From the cough/sneeze of infected person A, direct short-range 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. Size of the inhaled droplet particles determine how far into the respiratory system the pathogen may be deposited. Depending on the viability of the pathogen in the ambient environmental conditions, even large settled particles may be dispersed later from fomites, such as 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.
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Therefore, Friends, Romans, and Countrypersons, for the sake of the health and well-being of the public around you —at present and in immediate future— please cover your mouth if you feel that cough or sneeze coming on, won’t you?