Airborne Transmission

Air is not an appropriate growth medium for any pathogen. A pathogen in the air had to come from someone or somewhere—food, water, soil, an animal, or a human. In the air a pathogen is riding a magic carpet—a dried mucus carpet; a dry fecal flake or dust particle, and can be inhaled into human lungs. Some pathogens are hardier than others. Mycobacterium tuberculosis can remain desiccated for months after expulsion from an infected individual and remain viable, infecting new individuals once inhaled. Coughing, sneezing, talking, and laughing release moist droplets into the air. With a sneeze, secretions from the nose and mouth propelled from behind clenched teeth can be ejected with the force and speed of a bullet sailing through air at the rate of 50-100 meters per second (m/s). Figure 3.1 demonstrates the explosiveness of a sneeze, with eyes closed, teeth clenched, lips open, and an aerosol forcefully ejected. A sneeze can eject as many as 106 droplets from 10 microns to a millimeter in diameter. And note how far from the source droplets are ejected. The larger, heavier droplets fall colse to the source and are less of a hazard.

Figure 3.1. High-speed photograph of an aerosol generated by an unstifled sneeze. Note the distance the smallest droplets traveled from point of ejection. Also note the sizes of droplets, which consist of virus and bacteria-laden saliva and mucus. If inhaled by a susceptible person, a new infection can occur. (Figure adapted from The Major Ascana. The Female Picture Galleries Deck 2, VIII.)

Figure 3.1. High-speed photograph of an aerosol generated by an unstifled sneeze. Note the distance the smallest droplets traveled from point of ejection. Also note the sizes of droplets, which consist of virus and bacteria-laden saliva and mucus. If inhaled by a susceptible person, a new infection can occur. (Figure adapted from The Major Ascana. The Female Picture Galleries Deck 2, VIII.)

The smaller, lighter particles evaporate rapidly, leaving behind droplet nuclei which can remain suspended indefinitely, with the consequent accumulation of large numbers of potentially infective organisms. Respiratory transmission via droplet nuclei is the preferred means of transmission of most human infectious diseases—a highly efficient route for passing organisms among crowds, and a highly efficient means for pathogens to locate new venues, which they must do to survive.

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