The most common form of transmission of bacterial of viral infection from one individual to another is through the respiratory system.
When an infected person coughs or sneezes, tiny particles containing droplet nuclei are expelled into the air.
These particles are about 1 to 5 microns in diameter. Droplet nuclei can remain suspended in the air for several hours, depending on the environment. The most effective droplet nuclei tend to have a diameter of 5 microns. Droplet nuclei are generated during talking, coughing and sneezing. One cough can generate 3,000 droplet nuclei. Talking for 5 minutes can generate 3,000 droplet nuclei and singing can generate 3,000 droplet nuclei in one minute. Sneezing generates the most droplet nuclei by far (tens of thousands), which can spread to individuals up to 3 metres away.
Air filters commonly applied in health care air conditioning systems have a very high efficiency on removing airborne droplet nuclei. The minimum standard of care for areas of a facility where infected individuals are cared for would have MERV 7 prefiltration and MERV 14 final filtration. Some of these areas would also have an additional stage of HEPA filters.
The initial efficiency of a MERV 14 filter on 1-5 micron size particles is well over 95%.