Infectious aerosols are collections of airborne particles containing pathogens. An aerosol particle may settle on or be inhaled by a vulnerable individual. Aerosol transmission is physiologically feasible when infectious aerosols are produced by or emanate from an infected individual, the pathogen is alive in the environment for a length of time, and the target tissues in which the pathogen begins infection are accessible to the aerosol. The biological plausibility of aerosol transmission is assessed for the Severe Acute Respiratory Syndrome coronavirus and norovirus, as well as influenza, the Ebola virus, and Mycobacterium TB.

 What are the transmission mechanisms of infectious aerosols?

Infectious aerosols are often expelled by an infected individual during respiratory activities such as coughing, speaking, singing, sneezing, and breathing. During such respiratory actions, all individuals, sick or not, emit droplets of respiratory fluid (sputum, mucus, or saliva) in a broad range of sizes. Some droplets are so enormous that they cannot stay suspended in the exhausted jet for more than a few seconds. 

Some droplets are sufficiently tiny to be classified as aerosol particles that may stay suspended in the air for a lengthy period of time. Under the circumstances other than the most humid, the smallest droplets evaporate fast, leaving behind a solid or semisolid residue of the nonvolatile components of the respiratory fluid. The respiratory droplets and aerosols of an affected individual may contain infections and be contagious.

 How to handle the dangers of infectious aerosols?

Non-engineering treatments (e.g., administrative controls, medicinal interventions, etc.) and engineering controls may lower the risk of pathogen transmission. It seems improbable that the risk of exposure to infection from diverse aerosolized pathogens can be lowered to zero. Therefore, the objective must be to pick a set of engineering and non-engineering techniques that most effectively reduces risk and waste.

Due to the diversity of variables influencing the risk of infection in any particular situation, no one set of mitigation techniques can balance evidence, efficacy, timeliness, and cost against all potential combinations of risk factors.

 In general, policymakers confront two main sets of operational conditions: normal circumstances, in which the risk level is relatively constant, and epidemic situations, in which the risk level is temporarily elevated.

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