Why CO2 Monitors & Indoor Ventilation May Be A Key To Curb Covid Spread
By Annie Vainshtein
Nearly a year into the pandemic, we’ve learned that ventilation in indoor spaces is critical in preventing the spread of COVID-19.
Along with mask-wearing, social distancing and hand hygiene, ensuring clean indoor air is an important piece of the pandemic-fighting toolbox — but the one that’s been paid the least amount of attention, experts say.
Since the pandemic’s early months, most public health agencies and experts have stated that the coronavirus spreads primarily through respiratory droplets. These particles larger than 5 microns in diameter travel “ballistically” and are pulled to the ground by gravity within a range of around 3 to 5 feet.
Our pandemic messaging reflects the science: 6 feet apart can help keep you safe.
But in recent months, researchers have urged officials to acknowledge that the virus is emitted in another way — through much smaller particles that can float around in the air and travel far beyond 6 feet.
These droplets have “superpowers” because they can remain suspended in the air for prolonged periods of time, said Peter Chin-Hong, an infectious disease expert at UCSF.
Linsey Marr, an engineering professor at Virginia Tech who specializes in the airborne transmission of viruses, likes to think of the virus in another way. “It behaves like cigarette smoke,” she said. “If you can imagine that you’re in the room with a cigarette smoker, the smoke can build up in the air. But if the room is really well-ventilated, then the smoke will be removed.”
Several studies so far have bolstered the case for ventilation in our indoor spaces. One limited study of a transmission event in a restaurant in China found that people had gotten coronavirus more than 6 feet away from each other; one diner was 20 feet away from the infected source. Another study found a woman contracted the coronavirus on a flight while wearing a N95 mask — because she took her mask off in the bathroom.
Chin-Hong said he’s been surprised by some patients in his practice who were infected with the virus even when they were socially distanced indoors and wearing masks.
It came down to the ventilation, he said.
So why has the messaging around it been so vague? Why are there no protocols in place?
One limitation, Chin-Hong said, is that ventilation is an amorphous, ill-defined concept. “Ventilation itself is a continuous variable, it’s not dichotomous,” he said. “It’s very hard for a human to go into a situation and say, this is high risk, this is medium risk, this is low risk. We’re very good at dichotomizing: outdoors, indoors, more than 10 people, less than three households.”
Ventilation is more complicated than just whether a place seems stuffy or hot, and intuitively, it can be hard to gauge or qualify any given space. “You need an app for that,” Chin-Hong said. Newer buildings often have better ventilation systems, and places like malls are much better than, for example, a small Victorian flat that’s been converted into a boutique store. Windows and open doors make a tremendous difference in boosting the ventilation, he added.
How CO2 monitors can help
Carbon dioxide monitors may be part of the solution.
CO2 is an “easy-to-measure proxy” for calculating the exchange of fresh air indoors, according to Shelly Miller, a professor of mechanical engineering at the University of Colorado Boulder. In a recent article, she explained that when you exhale, you release CO2 into the air. And because breathing, coughing and talking are the primary ways coronavirus spreads, CO2 levels can indicate whether enough fresh air is entering a room to counter potentially infectious exhalations.
On Twitter, a small but mighty community of people have taken to buying and comparing the statistics of portable readers that measure the levels of CO2 in parts per million to analyze the relative ventilation of a space.
A collaborative FAQ about aerosol transmission from a collective of researchers and professors around the world, including Miller, gave a good example: The level of CO2 outdoors was 445, while in a car with two people and a child with the windows closed (and recirculated air) it was 4,685. In the same car, with the same people and windows closed (but a ventilation system with outdoor air), it read 795.
Miller wrote about a tuberculosis outbreak at Taipei University, noting that the school’s rooms had CO2 levels above 3,000 ppm. When engineers improved the ventilation levels to under 600 ppm, the outbreak stopped, with the increased ventilation accounting for 97% of the drop in transmission.
Miller recommended trying to keep the CO2 levels in indoor spaces below 600 ppm to reduce the chances of coronavirus transmission. The collaborative FAQ states that the level of ventilation in an indoor space is very good at 400-500 ppm, but readings around 800 ppm start to get risky because 1% of the air has already been breathed by someone.
The FAQ acknowledges that specifying a general range for “safer” CO2 levels is a “compromise” to make it understandable and feasible, with the understanding that the higher the number, the greater the risk.
Portable CO2 readers can be one way to assess the levels of ventilation in places we frequent a lot, like grocery stores or workplaces, but they have limitations. They’re almost like a thermometer, taking the ventilation of any given place at that place in time.
But they also can’t replace behaviors. “You can have the best CO2 reader but then be in a break room and take your mask off to eat,” Chin-Hong said.
Practical steps to reduce risk
So what can we do?
The first thing is windows. “Opening a window is incredibly effective, it’s a no-cost solution,” said Krystal Pollitt, an assistant professor of epidemiology and chemical engineering at Yale University who also contributed to the FAQ. If you’re in your home and have people over for essential reasons, having a window open even if people are wearing masks will make a big difference, she said, as it allows for the exchange of air and improves circulation.
For a costly solution, think about a HEPA filter, Marr says. Such air cleaners definitely help, but you’ll want to make sure they have a high enough clean delivery rate and that it is at least the right size for your room; larger is even better.
If you’re curious about a particular indoor space, you might invest in a CO2 monitor. It can be helpful for making decisions about particular retail stores or nonessential spaces — if the reading is too high, you can leave. If you work indoors, you can also ask your facilities manager for the building’s ventilation information, Chin-Hong said. And definitely don’t take your mask off if you’re inside somewhere beyond your home.
Information about ventilation can play a huge role in giving nuances to indoor spaces — some that have been open nearly all of the pandemic, and others that have been shut. Experts on the FAQ recommended that measuring and displaying CO2 become common practice in all public places.
“As this pandemic subsides, this would allow a more rational re-opening of indoor spaces, with limits on occupancy based on the amount of re-breathed air,” they wrote. “This appears much preferable to arbitrary limits such as ‘restaurants can open at 50% capacity,’ when we know that different restaurants may have very different degrees of ventilation.”