Airborne exposure should be acknowledged as threat

With schools reopening and businesses evaluating whether to bring employees back to their offices, it is important for all of us to have as clear an understanding as we can of the risk of contracting and spreading the SARS-CoV-2 virus that causes COVID-19.

One topic that is often not well understood is whether and how COVID-19 can spread by airborne transmission.

Airborne transmission, in this case, refers to the virus travelling more than two metres and lingering in the air, such that it can travel from an infected person to then infect someone else. Current research suggests that COVID-19 can travel by air, though health authorities have been slow to reach this conclusion.

For example, the B.C. Centre for Disease Control (BCCDC) claims that “while there is some discussion that COVID-19 can spread by staying in the air (by aerosols), there is no convincing scientific evidence to support this.”

The U.S. Centers for Disease Control and Prevention (CDC) has recently acknowledged that SARS-CoV-2 can be transmitted by airborne pathways (The Washington Post), noting “there is growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond six feet.” Unfortunately, the CDC removed this acknowledgment from its website after only a couple of days (L.A. Times, CDC).

The World Health Organization, while previously also dismissive of the potential for airborne transmission, now states, “aerosol transmission, particularly in … indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods of time with others, cannot be ruled out. More studies are urgently needed to investigate such instances and assess their significance for transmission of COVID-19.” (WHO)

In contrast to the ambiguity adopted by health authorities, researchers in several fields are urging such authorities to accept that airborne transmission of COVID-19 is a real threat that they should acknowledge and address.

A recent open letter from 239 scientists calls for health authorities to acknowledge the potential for airborne transmission of the COVID-19 virus and take steps to mitigate this potential (Morawska & Milton, 2020). In fact, there is now a substantial body of research that argues that airborne transmission of COVID-19 is indeed possible and should be addressed.

One implication of the danger of airborne transmission relates to indoor ventilation. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), a North American professional organization that is a preeminent authority on matters of building ventilation, has taken the position that “Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating, ventilating and air-conditioning systems, can reduce airborne exposures.” (ASHRAE, 2020, p2)

Given that schools will likely keep their windows and doors closed as the weather turns cold (and that many offices do not even have operable windows to open), it is clear that the ventilation systems of schools and workplaces will be a primary line of defence against airborne transmission.

Unfortunately, these systems were never designed to minimize the spread of disease, so we are asking them to perform a new function. How well will they be able to do this? Until these systems are evaluated on a building-by-building basis, we won't really know.

Given the potential for airborne transmission of the SARS-CoV-2 virus, these evaluations should begin immediately.