Question: I am sure you have addressed indoor transmission however I have a friend with a specific concern. We would welcome your perspective. She works in a 15 story busy trade mart (in Dallas TX) and may be spending 10 hour days in her showroom for a week in late August. There is central air conditioning throughout the building with no outside air entering the building. She is a 66 year old who is following mask, shield and sanitizer guidelines as best possible but is wanting your opinion on how dangerous the AC system is in this setting.

Answer: Thanks for bringing up airborne transmission, which we haven’t really discussed much! I would be very uncomfortable with the situation you’ve outlined. First, Dallas is an extreme hot spot right now with cases and hospitalizations quickly rising. Unless something dramatic happens in short order, Dallas will not be safe in late August for a trade show or the like. Second, because no outdoor air can enter the building — you can’t even open a window — and the ventilation system is unlikely to be supplemented with airborne infection controls (described below; ask your building maintenance folks what they are doing), I personally think it’s not a safe set-up. Though the scientific debate still continues as to what constitutes sufficient evidence for airborne transmission, what we’ve seen so far leads me to believe that the type of indoor setting you’re describing is highly risky. Read on for more details and thanks for following all of the other public health rules — they are so important and you are doing yourself and your community a service!

Transmission Routes

We know that SARS-CoV-2 spreads from person-to-person mostly from respiratory droplets that are expelled when coughing, sneezing, and talking (hence the masking rule). These droplets generally have a range of only 6 feet and hang in the air for just a short time (hence the 6+ feet rule). There’s also some evidence supporting fomite transmission (e.g. transmission from contaminated surfaces, see Q&A for 3/12) (hence the hand washing rule). Another route of transmission is looking increasingly likely — airborne transmission — transmission from tiny droplets that can hang in the air for prolonged periods and accumulate in poorly ventilated places to such a degree that they spread infection to others.

Role of Airborne Transmission

Increasing evidence from super-spreader events (see Q&A of 6/3) as well as lab-based studies of the virus’s “aerosol fitness” show that airborne transmission is both theoretically possible and a plausible explanation for a number of super-spreader events. In response to mounting evidence, 239 scientists wrote to WHO last week asking WHO to revise its guidance. And on 9 July, WHO issued an updated brief on the transmission of SARS-CoV-2 concluding that further study is needed on “the role and extent of airborne transmission outside of healthcare facilities, and in particular in close settings with poor ventilation”. The scientific debate on the role of airborne transmission rages on.

New Public Health Rules if Airborne Transmission is True Threat (quoted from letter of 239 scientists)

• Provide sufficient and effective ventilation (supply clean outdoor air, minimize recirculating air) particularly in public buildings, workplace environments, schools, hospitals, and aged care homes.
• Supplement general ventilation with airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights.
• Avoid overcrowding, particularly in public transport and public buildings.