Though Omicron cases are at lower levels than they were at the peak in January, I’m still wary of eating inside restaurants or gathering in other enclosed spaces. Can a portable carbon dioxide (CO2) monitor help assure me that the space is adequately ventilated and thus relatively safe from Covid-19?

It can be another tool to add to your Covid-protection arsenal but not something to completely bet on. A good-quality CO2 monitor quickly assesses how much air is flowing through a space, which can be of some relevance since poor ventilation is a risk factor for Covid-19 transmission (as well as for other respiratory diseases such as influenza and tuberculosis).

When we breathe, we exhale carbon dioxide—about 35,000 to 50,000 parts per million (ppm) per breath for adults. The more people there are in an enclosed area and the higher the activity level, the more CO2 is produced. If there is insufficient air exchange, CO2 builds up. And since the main mode of Covid transmission is via exhaled aerosolized particles, CO2 levels can be used as a surrogate marker for how many coronavirus particles might be in the air.

Researchers have long been studying the use of CO2 monitors for assessing air quality and ventilation as a factor in disease transmission risk. Since the pandemic, some school systems around the world have installed relatively low-cost CO2-sensing devices to monitor the air, and many workplaces have also been using them to identify poor ventilation conditions.

CO2 monitors are trending on social media, too: People using hashtags such as #COVIDCO2 often provide CO2 readings at various locations, while in Japan, for example, some businesses are posting real-time CO2 levels in stores and restaurants.

But here are some concerns about relying on a CO2 monitor: First, it’s unclear what CO2 threshold should be used—threshold levels have generally been developed for comfort, not infection control, and it’s unknown what cutoff would reasonably protect against Covid transmission. After all, the monitors don’t measure levels of coronavirus (or other respiratory viruses) in the air but rather can just tell you if ventilation should be improved.

That said, the World Health Organization supports using CO2 levels to assess Covid safety risk in public buildings around the world, and several government agencies, including the Centers for Disease Control and Prevention and the Minnesota Department of Health, have set an indoor CO2 level of 800 ppm as a threshold, above which some action should be taken to improve ventilation. The Washington State Department of Health was more stringent at one point, requiring that seated restaurant occupants not stay at a table more than 15 minutes at an exposure level of over 450 ppm (this policy is no longer in force but remains as guidance). By comparison, average outdoor CO2 levels hover around 400 ppm (excluding areas of high vehicular traffic and industry). Health Canada has set a level of 1,000 ppm but for drowsiness and performance criteria. The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends that action be taken at 1,000 to 1,200 ppm CO2.

Second, there is not necessarily a direct relationship between CO2 levels and Covid risk, since many factors (such as mask wearing) can affect each of them independently—and no number on a monitor can be considered a “safe” number. That is, being in a room with a high concentration of CO2 does not mean you will become infected with Covid; on the flip side, being exposed to low CO2 levels does not mean you can’t become infected.

Then there is a quality issue. Small handheld or desktop CO2 monitors, which cost about $100 and up, are widely available, but their quality may vary. You can find reviews of some here.

BOTTOM LINE: Using a CO2 monitor inside a restaurant or other enclosed space you are sharing with other people may provide more information about your Covid risk. If you get one, be sure to follow the manufacturer’s directions, including how to properly calibrate it. But don’t let such a device give you a false sense of security, and don’t use it as a replacement for other Covid precautions, including being vaccinated, wearing a high-quality face mask and socially distancing when appropriate, and avoiding large gatherings where you are unsure of people’s vaccination status.

If you can’t dine outside, we advise looking for restaurants with the following features (the more the better): a mechanical ventilation system that maximizes fresh air circulation and minimizes recirculation, open windows or doors (for natural ventilation), HEPA air-cleaning or filtration devices, and well-spaced tables. You could also consider dining off-hours when there are fewer people and steering clear of establishments when they are crowded. If the indoor space feels stuffy or stale, it’s probably a good idea to move on no matter what a CO2 monitor may read.