Found this a useful article. Ventilation & indoor air is important.
Why Aren’t We Talking More About Ventilation? – The Atlantic
https://www.theatlantic.com/health/archive/2020/07/why-arent-we-talking-more-about-airborne-transmission/614737/
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In particular, the size of infectious particles really matters, because that determines how they travel—are they big enough to be quickly pulled down by gravity or are they small enough to float around? Since the beginning of the pandemic, the World Health Organization has considered the primary mode of COVID-19 transmission to be respiratory droplets. These droplets are defined as particles bigger than 5 to 10 microns in diameter, and WHO guidelines say that once they are sprayed out of someone’s mouth, they travel
ballistically and fall to the ground within close range of the infected person. For the WHO, that range is about three feet; for the Centers for Disease Control and Prevention, which also considers droplets to be the primary mode of transmission, it’s six feet. The dominance of a ballistic-droplet mode of transmission in this pandemic would mean that we should focus mostly on staying out of droplets’ range, especially to prevent them from falling on our unprotected mouth, nose, and eyes—hence the social-distancing guidelines. …
Many scientists believe that the virus is emitted from our mouths also in much smaller particles, which are infectious but also tiny enough that they can remain suspended in the air, float around, be pushed by air currents, and accumulate in enclosed spaces—because of their small size, they are not as subject to gravity’s downward pull. Don Milton, a medical doctor and an environmental-health professor at the University of Maryland, compares larger droplets “to the spray from a Windex dispenser” and the smaller, airborne particles (aerosols) “to the mist from an ultrasonic humidifier.”
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Plus, this debate has a long history: From the mid-19th century into the 20th century, infectious-disease specialists fought a long and hard-won battle against “miasma” theories of disease that posited that filth and noxious odors, instead of germs, were responsible for disease. In a seminal 1910 book, the public-health pioneer Charles Chapin distinguished “spray borne” diseases (WHO’s droplets that maximally travel only a few feet) from “dust borne” ones—spread by aerosols, or airborne transmission. He concluded that most pathogens were either “spray-borne” or spread through contact, and worried that an over-reliance on “air-borne” theories would needlessly scare the public or cause them to neglect hand-washing. More than a century later, there are still echoes of those concerns.
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But that’s not all. The super-spreader–event triad seems to rely on three V’s: venue, ventilation, and vocalization. Most super-spreader events occur at an indoor venue, especially a poorly ventilated one (meaning air is not being exchanged, diluted, or filtered), where lots of people are talking, chanting, or singing. Some examples of where super-spreader events have taken place are restaurants, bars, clubs, choir practices, weddings, funerals, cruise ships, nursing homes, prisons, and meatpacking plants.
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However, to date, there is also no evidence of truly long-range transmission of COVID-19, or any pattern of spread like that of measles. Screaming “it’s airborne!” can give the wrong impression to an already weary and panicked public, …Cowling told me that it’s better to call these “short-range aerosols,” as that communicates the nature of the threat more accurately: Most of these particles are concentrated around the infected person, but, under the right circumstances, they can accumulate and get around.
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However, in the community, accepting aerosol risks would mean that people around COVID-19 patients at home or anyone high-risk, such as the immunocompromised, should at least be provided with higher-grade masks such as N95s, which do a better job of keeping aerosols out. …
When windows cannot be opened, classrooms could run portable HEPA filters, which are capable of trapping viruses this small, and which sell for as little as a few hundred dollars. Marr advises schools to measure airflow rates in each classroom, upgrade filters in the HVAC system to MERV 13 or higher (these are air filter grades), and aspire to meet or exceed ASHRAE (the professional society that provides HVAC guidance and standards) standards. Jimenez told me that many building-wide air-conditioning systems have a setting for how much air they take in from outside, and that it is usually minimized to be energy-efficient.
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