# Air exchange calculator October 14, 2022 10:46 AM   Subscribe

How do I calculate how long to air out a house after someone has been inside, for covid-prevention purposes?

I’m trying to figure out how long we need to leave the windows open after having someone come in the house (for example, a repair person). Is there a calculator or metric online that can help me figure out how long we need to air out the house to ensure there isn’t enough covid floating around to infect someone? I can’t find one via google but this seems like such a basic question. As the weather gets colder, we don’t want to leave the windows open for hours, but we do occasionally need to have people inside. I live in a four square house with a similar floor plan to this. Downstairs is about 600 sq feet, with 3-4 windows on each wall on three sides and two small windows on one side. We also have a portable air filter (the Coway AirMega) that we run on high after having someone in. We do not have an HVAC system that filters air.

Note, any answers that imply we don’t need to worry about it will be flagged as noise.
posted by john_snow to Home & Garden (10 answers total) 5 users marked this as a favorite

Best answer: The US EPA has a lot of information about COVID-19 and indoor air quality.

There is an online calculator here from the CDC. Circa 2019 and not very complex but should be a good start.

I would think if you bought a cheap box fan (1000 cubic ft/min or better) and created a positive pressure situation by pulling in outside air and cracking the other windows in the house for exhaust you will exchange the air faster than the CDC's 4-hour guideline.
posted by JoeZydeco at 10:59 AM on October 14, 2022 [2 favorites]

NIST also has some additional information, including a more detailed calculator called VIPER (Virus Particle Exposure in Residences).
posted by JoeZydeco at 11:05 AM on October 14, 2022 [3 favorites]

Response by poster: Not trying to threadsit, but I have no idea why these tools didn’t come up in my googling! The CDC tool is exactly the kind of thing I am looking for but the assumptions used in their model are too different than my home to make that particularly useful - my space is significantly smaller, and we have about 14 windows open instead of 1 - I am positive this means that less than four hours is sufficient, but how much less? Is 30 minutes enough? One hour?
posted by john_snow at 11:39 AM on October 14, 2022

If you're open to a functional approach to calculating this number for your specific house: there are a number of studies correlating CO2 ppm over time to cumulative COVID transmission risk. If you're willing to get a monitor ahead of time, you can test how long it takes to get to a stable ppm with the windows open and what that number ends up being. Then use the monitor again to know roughly when enough of the tradesperson's expelled air has been exchanged and it's safe to close the windows.
posted by michaelh at 11:50 AM on October 14, 2022 [6 favorites]

Best answer: Agreed that a CO2 meter is your best bet to get a good idea what's actually going on in your house, because testing has shown that the number of air changes per hour in houses with all or some of the windows open varies dramatically from house to house and with different weather conditions. There just isn't a going to be a good rule of thumb that isn't assuming the worst and ventilating more than you need.

But if you place a CO2 meter somewhere in the middle of the house, where you suspect the airflow is lower and then ventilate until the CO2 level reaches a predetermined level (maybe 500ppm, outside air is about 415ppm), you'll know when most of the air has been replaced. You can adjust your ppm target depending on your caution level and on the initial level before opening all the windows.

I have a Qingping Air Monitor Lite, which is less than \$100, but people also seem to like the Vitalight, which is a little cheaper and more portable or you can go all out and buy the Aranet for a few hundred bucks.
posted by ssg at 4:25 PM on October 14, 2022 [2 favorites]

A CO2 meter also will tell you if you have enough ventilation in your house when all the windows are closed, which is useful not just for infectious disease prevention but also mold prevention and general indoor air quality. Many houses in colder areas do not have enough ventilation!

If you need to improve your ventilation in general, this is well worth doing. It makes your house so much more pleasant to be in (and reduces transmission of Covid and other airborne diseases, of course). A CO2 meter is the first step to figuring this out. You can use a chart like this to estimate your ventilation rate and compare to established standards.
posted by ssg at 4:29 PM on October 14, 2022 [1 favorite]

Best answer: here is one tool, found on this info sheet, which I linked to on a site I'm working on.

Basically you need 7 air exchanges to clear airborne virus. Those cheap 20 inch box fans are good for 1,200 cubic feet per minute. Position them 2 feet in front for a window blowing out. Have windows on opposite side of the house open so you get a good flow. Then a simple calculation of cubic feet of house times 7 divided by 200 equals minutes needed. Double that if you want to be extra sure.
posted by Sophont at 8:51 PM on October 14, 2022

My local public library's "Library of Things" loans out high quality CO2 monitors. Maybe check with yours?
posted by apparently at 9:41 AM on October 16, 2022

Response by poster: Thanks all, I’ve been considering a CO2 monitor already so I think I’m just going to get one. And a box fan!
posted by john_snow at 10:30 AM on October 16, 2022

The aranet sensor was the more expensive choice but it has been amazing — no hassle, eink screen is great, and I feel more confident in its accuracy than the cheapo ones.
posted by sixswitch at 1:33 PM on October 16, 2022

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