The COVID report with David Blake: Aerosol spread of the coronavirus

Thursday, Oct. 15

The guidance on public health for the coronavirus has recently changed to include aerosol spread. That means small bits of water from an infected person’s lungs can, if their concentration is high enough, infect others. It also means we have a new set of weapons to fight the coronavirus, mainly those of indoor air quality.

A Spin class studio in Canada was following all the rules. Six feet between bikes. Masks on until the class began. Over a period of one week, one asymptomatic person at spin class led to 44 other infections directly in spin class, and 61 total cases including secondary (next generation exposures). The coronavirus is characterized by super-spreader events like these.

Let’s take a look at how this occurred. During exercise, your ventilation can increase 5-6X over at rest. For that infected person, that means 5-6 times more infected particles were being placed into the air. The others in the class were also breathing heavily, which means they had a greater chance to inhale those particles. The other critical factor was the indoor air quality.

What do we mean by indoor air quality?

Well, your indoor air should be exchanged with outside air at some rate that is adequate for the number of people present. It should keep CO2 levels under 1000 parts per million (ppm). At that level, the CO2 levels in your bloodstream will alter your pH and give you a respiratory acidosis that measurably impairs cognition.

And you are probably not taking bicarbs to balance your pH. Nor should you. That is the first variable of indoor air quality: CO2 levels. In any industrial setting, reasonable controls keep CO2 under 1000 when occupancy is maximal.

The other way to think about the CO2 levels is in terms of rebreathed air. When you breathe, you inhale fresh air and rebreathed air. Exhaled air is about 3.8 percent CO2, or 38,000 ppm.

Outside air in Augusta is 410 ppm CO2. The difference between outside air and our 1000 ppm limit is 590 ppm. That means one part of the air you inhale in 65 is rebreathed. You are breathing in 64 parts fresh air, and one-part air that someone else in the room recently exhaled.

The threshold for being infectious for tuberculosis or COVID-19 is at CO2 levels of 600 ppm, which is one-part rebreathed air in 200.

Normal air quality is maintained under 1000 ppm CO2. Aggressive ventilation to fight infectious aerosol diseases can maintain CO2 at 600 ppm. There may be cheaper ways to get good indoor air quality.

HEPA filters, at a rating of MERV-13, can also clean the air. The CO2 levels could be 800-900 ppm and non-infectious and fine if the air were filtered. Ultraviolet germicidal irradiation can also be applied in air ducts to kill infectious particles. If one of those air disinfecting strategies were used, indoor air quality can be high with CO2 levels between 600 and 1000 ppm.

How can you check your indoor air quality?

A CO2 meter, and your air exchange rate, are the easiest way. USG laboratories maintain six air exchanges per hour, and all USG buildings and rooms should have CO2 under 1000, and we have Environmental Health and Services both to ensure that occurs, and that it can be checked.

In our homes, however, there is typically zero intentional ventilation. We simply recycle the same air over and over, and the bathroom fans, or air leaks, provide our only ventilation except for open doors and windows.

You can see how this is problematic when Thanksgiving comes, and it is cold outside, and 20 people congregate in the house for dinner. Easy solutions include running bathroom fans, and opening windows and doors when practical.

One new key to fighting the coronavirus is indoor air quality. Most of us have no control over it. But you should know that many people, especially talking or exercising, in a small space, will likely be under-ventilated and a prime candidate for spreading the coronavirus infections.

If you have a choice, please gather outside, and not inside. If you don’t have a choice, please wear a mask and see if you can open windows or doors on two sides of the venue. Ventilation will fight the coronavirus, and each of us can make small changes to make our environments better ventilated.