There is growing consensus that one of the primary ways the novel coronavirus spreads is through the air. That makes it risky to put a lot of people in a poorly ventilated space. As schools, offices, and businesses reopen, facilities managers are looking at one particular metric to gauge whether there’s an elevated risk of coronavirus transmission: air changes per hour (ACH).
Air changes per hour (also known as “outdoor air changes per hour”) is pretty easy to understand—it’s the rate at which the air in a space is completely recycled. The higher the ACH, the more frequently air is cycled through, reducing the risk that a person in that space will inhale viral particles and get infected.
So far there’s no official recommendation on the ideal ACH to dramatically reduce the risk of transmitting Covid-19. That’s because it depends in part on numbers we don’t know yet, like how many viral particles an infected person spreads, or how many can make an exposed person sick, says Bill Bahnfleth, a professor of architectural engineering at Pennsylvania State University. Experts are still studying this (pdf).
For now, Bahnfleth says, most experts suggest at least 3 ACH, and ideally 6 ACH, though these numbers haven’t been officially adopted by organizations that set ventilation guidelines, such as The American Society of Heating, Refrigerating and Air-Conditioning Engineers.
Here’s the estimated ACH in spaces where you might find yourself:
- Home: 0.35 ACH
- Home with one window open: 0.8-1.3 ACH
- School: 3 ACH is the minimum recommended; 1.5 ACH is the reality for most schools
- Office: 6-8 ACH recommended
- New York City subway: 18 ACH
- Parked car with a window open: 6.5 ACH
- Car moving up to 72 mph with ventilation system off: less than 6.6 ACH
- Airplane: 10-20 ACH
- Laboratory: 4-8 ACH
- Patient room at a dentist’s office: 6-12 ACH
- Medical facility with patients affected by airborne diseases: 6 ACH minimum, 12 ACH recommended for new constructions
In practice, however, Bahnfleth notes that “actual outdoor air flow rates in most buildings are lower than the total equivalent air change rates being recommended for Covid-19 airborne risk management.” In other words, you can’t rely on the numbers above as a perfect metric to gauge your risk of catching Covid-19.
Figuring out air changes per hour in your space is possible, if a little complicated. You’ll need to know the volume of air in your space (width x length x height of the ceilings), and the amount of outdoor air flowing in (measured in cubic feet per minute). That last metric is something you can find out if you have an air purifier or an HVAC system. So:
Air changes per hour = cubic feet per minute x 60 / room volume
Or you can just use this handy calculator.
ACH is not the only metric that can give you a sense of whether a space is well ventilated, Bahnfleth says. The outdoor air flow rate (pdf)—how fast outdoor air is moving into a space—is also important to consider.
Plus, ACH has limitations to its usefulness. In houses, the amount of contaminated air varies based on how many people are home; in spaces like auditoriums with high ceilings, only the bottom space is occupied, so “if air change rate is used at all, it should probably be based on a reasonable estimate of the occupied volume—for example a zone extending to 10 feet above floor level,” Bahnfleth says.
“This really underscores the need for recommendations for changes to building operations to be implemented by competent professionals,” he adds.
“Once the virus escapes into the air inside a building, you have two options: Bring in fresh air from outside or remove the virus from the air inside,” Shelly Miller, a professor of mechanical engineering at the University of Colorado Boulder, wrote in The Conversation.
Open a window or door. “We’ve got this big crisis and you’re telling me to open up a window? Yes, I’m telling you to open up the window,” Anthony Fauci, the US’s top infectious disease expert, said during an August panel. As the numbers above indicate, opening a window—even just a crack, but ideally all the way—can increase ACH.
Bahnfleth cautions that “unless there is somewhere for air to be exhausted, opening a single window may not result in much air flow.” Opening multiple windows can help; so can pairing open windows with exhaust fans.
Improve your air filtration. A filter’s performance can be calculated as equivalent to ACH. “For example, if room air is recirculated through a filter with a flow rate corresponding to 2 ACH and the filter removes 50% of the particles in the air that may contain viruses, it can be said to provide one equivalent air change per hour of clean air,” Bahnfleth notes.
Spaces with central air can benefit from improved systems, such as higher-quality mechanical filters—MERV 13 or higher, whatever the system can tolerate—and they don’t have to be fancy. Spaces without central air can benefit from portable air purifiers, or even a mechanical filter attached to a box fan. Humidifiers might also be helpful.
Improved ventilation, though, isn’t going to stop the spread of the coronavirus on its own. Wearing masks, washing hands, disinfecting surfaces, and maintaining physical distance are all important tools in the fight against Covid-19.