Am I getting enough ventilation to reduce COVID risks? Short answer: probably not.
Unfortunately, the long answer is very technical. But here it goes…
Although it is now widely recognized that COVID-19 can spread via the airborne route and that ventilation is extremely important, we still don’t know how much ventilation is enough. In this blog post, I’ll quickly address two common questions 1) What is the ventilation target? and 2) How do I figure out if I’m meeting that target?
What is the ventilation target?
We don’t have ventilation standards specific to COVID-19. For that, we would need to know what airborne concentrations are more likely to cause disease transmission along, with the virus generation rate of infected individuals. There are some estimates people are using, but I don’t think they are established enough to be the basis for “COVID Ventilation Rates”.
We can use ventilation rates from ASHRAE, which were first published in the 1970s and are now regularly updated, but those aren’t designed specifically for airborne infectious diseases. They were based on providing enough air exchange to reduce complaints from “bioeffluents” – aka body odor. Early studies that informed these standards found that a ventilation rate of around 15 cubic feet per minute (cfm) per person was enough to make the substantial majority of people happy with the air, at least from an odor perspective. Later on in the early 2000’s, ASHRAE split the ventilation rate into two categories: ventilation to dilute contaminants from humans and ventilation to dilute contaminants from the building itself. But the old ASHRAE rule of thumb rate of 15 cfm/person rate may be a good starting point for COVID.
Others are recommending a certain amount of air changes per hour (ACH) for rooms, which is a different metric than cfm/person. ACH does not consider how many people may be in the space and could lead to over-ventilating buildings that have low occupancy. For example, a team from Harvard has published the following targets in their document 5 step guide to checking ventilation rates in classrooms:
- Ideal: 6 ACH
- Excellent: 5-6 ACH
- Good: 4-5 ACH
- Bare minimum: 3-4 ACH
- Low: <3 ACH
Please note that these air exchange rates are designed for school classrooms, which typically have a high occupant density.
How do I figure out if I’m meeting that target?
Measuring ventilation is usually the role of a professional such as an indoor air quality consultant or test & balance contractor.
Option 1: Monitor carbon dioxide levels when the space is fully occupied
If your target is 15 cfm/person in a space, you should see the indoor CO2 levels not exceeding 700ppm above the outdoor background amount (which is around 400 ppm globally but fluctuates geographically). We like to monitor CO2 using calibrated NDIR sensors. These measurements only work when the space is fully occupied.
Option 2: Measure the decay of a tracer gas
If you have a target for a certain number of air changes per hour, you can have a study conducted where a tracer gas is released into the space and the rate at which it decays is measured. As you might imagine, the tracer gas concentration will drop quickly in a well-ventilated indoor environment. The rate of decay is used to calculate the ACH. The nice thing about this option is that it does not require the space to be fully occupied. So this test can be done in an office, school, or church that is not currently being occupied.
There are other methods for measuring outdoor air ventilation and air exchange in buildings, but these two are the ones we most often perform.
How do typical buildings stack up?
I have evaluated countless buildings through the years and the vast majority do not meet ASHRAE standards consistently. Some buildings will meet the standards only on mild days. Others only meet standards in private offices but not in conference rooms. Why the lack of ventilation? Ventilation costs money and can make the building more uncomfortable when the system wasn’t designed property,
For commercial buildings that have complex ventilation systems, I would recommend hiring a consultant to help evaluate your building. Indoor Science can help you establish a target ventilation rate and review your system to see if those targets are being met. Reach out to us for more information on our COVID-related services.