Hi, Everyone,
Throughout the pandemic, I’ve talked about a simple formula for calculating the risk of getting the SARS-CoV-2 virus:
Risk of infection = volume of exposure + time exposed
The higher the volume of viral particles you are exposed to, and the longer you are exposed to them, the higher your risk of infection.
This formula helps to understand why the three major preventative measures—social distancing, masks, and hygiene—are effective.
But there’s a fourth intervention that is at least as important as distancing, masks, and hygiene when it comes to preventing COVID-19, but that doesn’t get as much attention: ventilation.
We know that the risk of getting the coronavirus is much lower outdoors than indoors—and this is largely due to ventilation (though sunlight may also play a role). In an outdoor environment, exhaled viral particles disperse quickly into the air, thus reducing the volume of exposure.
In indoor environments—especially those that are poorly ventilated—the viral particles don’t disperse, so the volume of exposure is much higher.
As fall and winter approach, and we begin to spend more time indoors, it’s critical that we consider the ventilation of the indoor spaces we’ll inhabit.
When viral particles escape into indoor air, we have two options for reducing exposure:
- Bring fresh air in from outside
- Remove the virus from indoor air
Let’s talk about bringing fresh air in from outside first.
In modern commercial buildings, HVAC systems will often bring air from outside into the building. The building manager can control the settings for how much outside air is brought in, versus how much indoor air is recirculated.
In buildings that don’t have such HVAC systems, including most homes, the best way to bring outside air inside is to open the windows. Of course, in some climates this is not practical at certain times of the year (e.g. mid-winter in Minneapolis or mid-summer in Atlanta).
The more fresh, outside air the better. Most building scientists recommend at least six changes of air per hour in a 10x10 room with four people in it. However, some recent studies suggest that nine changes per hour is better during a pandemic.
How do you know if the room you’re in has enough air exchange? One way is to measure CO2 levels.
A well-ventilated room will have CO2 levels of 800 ppm or below. A study at Taipei University in Taiwan looked at how indoor ventilation affected a tuberculosis outbreak. Many of the classrooms had poor ventilation, with CO2 levels of 3,000 ppm or higher. When ventilation was improved to where CO2 levels dropped below 600 ppm, the outbreak was completely stopped.
Unfortunately, many schools and workplaces in the United States (and elsewhere) don’t have adequate ventilation. If you’re considering whether to return to work, or whether to send your kids back to school, you might want to measure the CO2 levels in the indoor spaces.
You can buy a CO2 monitor online for about $100. Some, like the Awair Indoor Air Quality Monitor, also monitor relative humidity, PM2.5, and other VOCs.
If CO2 levels are higher than 800 ppm, the room needs more ventilation. This can be done by increasing the amount of outside air the HVAC system brings in (if there is such a system) or opening windows.
The other option is to remove the virus from the indoor air.
The best way to do this is with a high-quality HEPA air filter. For example, the IQAir HealthPro Plus filters particles as small as 0.003 microns—which are smaller than viruses. It’s important to choose effective air filters and size them appropriately for the room.
The California Air Resources Board has a list of air cleaners that are certified as safe and effective, so that’s a good place to start.
I’d suggest measuring the CO2 levels in your home as well, and then taking action to increase ventilation if necessary. You can buy air filters for each room, but you can also buy whole-house air filters that connect to your HVAC system (like the Perfect 16 from IQAir). Depending on the climate you live in, you can also simply keep the windows open more often.
Ventilation is every bit as important as distancing, masks, and hygiene, and this will be even more true as we spend more time indoors this fall and winter.
I hope this helps to keep you and your loved ones safe!
In health,
Chris
