Coronavirus is airborne. Here’s how to know if you’re breathing other people’s breath.

With its five wall-length windows, Nick Crandall’s restaurant, Railroad Pub & Pizza, can bring in a lot of outside air. In late December, though, Washington state regulators said the restaurant could not qualify as “outdoor” dining, and would have to close because of heightened coronavirus restrictions.

So Crandall went to Facebook to protest, giving a video tour of the Burlington, Wash., pub and its vast, garage-door-style windows. “I’m just kind of curious on what the science is for outdoor dining, how much airflow you need to do,” he said. He took aim at the state’s Democratic governor, Jay Inslee, suggesting he use “common sense.” The video was viewed over 73,000 times.

It may sound like yet another politicized, Trump-era battle over coronavirus restrictions - yet this one ended in something that looks less like polarization and more like compromise.After Crandall and others complained and took to the media, state regulators introduced a new policy, which appears to be one of the first of its kind, allowing certain restaurants to count as “open air” dining even if they have four walls. In a new pandemic trend, these establishments can open up large windows or doors and actively measure levels of carbon dioxide, the gas we all exhale when breathing, as a key indicator of how much fresh air is circulating.

Now Crandall’s restaurant is open again - with a CO2 monitor whose reading he tries to keep under 450 parts per million, only slightly higher than levels in the outside air, per state policy. Thanks to the human burning of fossil fuels, outdoor levels currently average around 415 parts per million, and are steadily rising.

It’s part of a new wave as scientists, citizens and businesses including gyms, restaurants and bars try to quantify the airborne coronavirus risk in hopes of staying open. Sales of handheld carbon dioxide monitors have boomed, so much that one popular model, the $250 Aranet4, sold out rapidly, requiring its Latvia-based manufacturer, SAF Tehnika, to dramatically ramp up production.

“We did not expect to, you know, have this increase so exponential,” said Toms Rekšņa, marketing director for the Aranet, speaking from the country’s capital, Riga.

The trend is also catching on fast with a number of coronavirus activists - or citizen scientists - who tweet out their readings in different locations and use the hashtag #covidco2. In Australia, a group of “CO2Guerillas” have been documenting measurements in grocery stores, doctor’s offices, and businesses, often displaying very high levels of carbon dioxide. In Japan, the use of monitors is also catching on, including on a massive screen recently at a concert venue.

The impetus for measuring carbon dioxide is simple: An increasingly powerful body of evidence suggests the coronavirus is airborne, capable of traveling distances well beyond six feet in tiny aerosols released when infected people talk, shout, sing or just breathe. But there’s currently no sensor that can monitor, in real time, whether these infectious aerosols are floating around us when we’re indoors.

But carbon dioxide can, in some ways, act as a proxy. People exhale it when they breathe, and the gas builds up in indoor spaces that aren’t well ventilated, reaching concentrations far above the baseline level of outside air.

“It gives you some insight into ventilation, which is really hard to figure out otherwise,” explains Linsey Marr, an aerosol expert at Virginia Tech. “Even building owners and managers often don’t know much about the ventilation. The person who knows is the person who installed it, and they are usually long gone.”

Marr is a medical adviser to the network of CrossFit gyms - installing indoor monitors is now part of their coronavirus guidelines, at her urging.

Longtime experts on indoor air say the heightened attention to ventilation is very valuable, and that carbon dioxide measurements can definitely be useful. Yet amid the grass-roots frenzy to find the next gadget that can confer a safety edge during the pandemic, some worry about misunderstandings.

“It is a piece of information, not a smoking gun,” said indoor air expert Jeffrey Siegel of the University of Toronto. “If you have a long period of measurement in a space with a sensor that you know how to interpret, then it means something different than if you bring a sensor inside, read a few minutes of data, and say, ‘Oh, my God, the ventilation doesn’t work.’”

When scientists want to measure carbon dioxide to a very high level of accuracy, they use sophisticated lab equipment. It can cost thousands of dollars. That is not what citizen activists are generally using during the pandemic.

Rather, a variety of handheld or mountable sensors, costing around $100 and up, have become popular. Experts recommend devices that use a technology called non-dispersive infrared sensing (NDIR), a well-known technique based on the same basic physical principles that drive the so-called greenhouse effect. Whether in the atmosphere or a small chamber within your sensor, carbon dioxide absorbs a type of radiation with a wavelength longer than that of visible light, often dubbed infrared or heat radiation.

At the scale of the Earth, greenhouse gases like carbon dioxide not only absorb this type of radiation but also re-emit it, keeping some of the heat within the planet’s system, rather than letting it escape to space. Within the far smaller sensor, infrared gets beamed from one side of a chamber to the other, and carbon dioxide concentrations are calculated based on how much radiation arrives at the other end without being absorbed by the gas.

In theory, at least, such measurements can give a good sense of how many humans have been exhaling into a space, and how much their breath is lingering.

“The reason CO2 measurements are important is that they can give an indication of how much air you are breathing that is coming out of other people’s respiratory systems,” says Richard Corsi, an indoor air quality expert at Portland State University who has been taking carbon dioxide measurements for years with many different instruments.

A critical figure, Corsi says, is the “rebreathe fraction,” which refers to the percentage of the air you breathe in that others in the same indoor space recently breathed out. For instance, he has calculated that when the indoor concentration of carbon dioxide reaches 800 parts per million, then each time you breathe in, one percent of the air you inhale has come from the exhalations of others. During a pandemic, that’s an alarming thought.

But carbon dioxide concentrations of 1,000, 2,000, or even 4,000 can be found in poorly ventilated indoor spaces, where people simply aren’t aware how much of the gas has built up.

“Many teachers are reporting, they say they have 2,000, 5,000, and then curves of it going up in classes,” said Jose-Luis Jimenez, an aerosol science expert at the University of Colorado at Boulder who has promoted the use of the sensors and has been involved in testing a number of them for accuracy.

It is very good news that the pandemic has raised our consciousness about the quality of air within buildings, a subject that has been neglected for decades, say longtime experts such as Corsi and the University of Toronto’s Siegel. And in their field, carbon dioxide has long been used as a proxy for how well ventilated a space is by outside air.

But at the same time, these scientists worry that carbon dioxide measurements can be misinterpreted or even, in some cases, give a false sense of security.

Siegel warns, for instance, that handheld devices can require calibration, can sometimes be confounded by other greenhouse gases (such as water vapor), and can drift in their measurements as time passes. That doesn’t make measurements useless, he said - but it does mean that you have to have some experience with your instrument, and should be measuring consistently over time.

“The more engagement with indoor air, the better everything is,” he said. “But the problem is, good indoor air or bad indoor air is not defined by a spot measurement of CO2 with a low-cost sensor, without appropriate interpretation.”

Corsi, meanwhile, cautions that even if very low or very high carbon dioxide concentrations may appear easy to interpret, many readings will fall into more of a gray area, somewhere between around 700 to 1000 parts per million.

Are you safe in such a space? The answer is, it depends. For instance, Corsi notes, a space with 25 people in it and a CO2 measurement of 700 parts per million is far better ventilated than one with three people in it and the same measurement.

Moreover, he adds, if a room has a portable HEPA air filter, or a good HVAC system with similarly strong filters (properly installed), then your risk will be lower even though carbon dioxide levels may seem a tad high. Carbon dioxide, a tiny molecule, passes right through these filters, even though the larger aerosols containing viruses can be caught by them.

“I think a single point measurement of CO2 can tell you something in the extremes, but when you get into this middle, typical area, there’s a lot of nuance,” Corsi said. Ideally, he thinks, there should be an app that would help people interpret CO2 levels by inputting other information, such as the number of people in a space and how much time they plan on spending there.

Rekšņa, marketing director for the Aranet, says the device’s start-up guide tells users how to calibrate it, and after that it is accurate to within about 50 parts per million (which would certainly be enough to distinguish low concentrations from high ones).

“We have dedicated technical support for the whole business ecosystem where we try to explain these things,” he said. “We have recently launched an Aranet forum as well. So we do try to inform the consumers as much as possible.”

One more thing to keep in mind: Just as has happened with outdoor air all across the Earth, humans can fill the air indoors with carbon dioxide by burning wood or fossil fuels, such as in fireplaces and gas stoves. In these cases, concentrations can spike for reasons that have nothing to do with our breathing.

In other words, CO2 measurements can be useful and informative, but have to be understood in context.

Still, the baseline principle is hard to dispute: If carbon dioxide levels are very low in a business, or office, or grocery store, or wherever - then your coronavirus transmission risk is probably also low, at least from people who aren’t very close to you. (The risk will be lower still if people are also masked and wearing their masks properly.)

What’s unfolding in Washington state right now may be a case study of how well - or how poorly - the technique can be employed through a concerted policy effort involving state regulators and individual businesses.

“There’s been a number of studies that have used CO2 levels as kind of a risk proxy for covid,” said Sheri Sawyer, a policy adviser to the state’s governor, Inslee, who was centrally involved in issuing the new guidance on “open air” dining. The document is a joint product of the state’s health department and its Department of Labor and Industries.

“And we thought that made great sense for businesses to use that as a tool for what their risk is for covid transmission.”

“It’s kind of uncharted territory,” Sawyer continued. “But certainly, given what businesses are going through, we think it’s a worthy endeavor to try to figure these answers out.”

The new Washington state policy is one factor that is driving “a tremendous spike in demand for these products,” said Travis Lenander, the CEO of CO2Meter.com, which manufactures the devices used in Crandall’s restaurant.

Indeed, Sawyer says the state is now receiving a large number of questions from businesses about how to use carbon dioxide monitors, and suggests that this is probably the beginning of a lengthy dialogue between the state and its restaurants and other venues. The state has just added an FAQ document to further help restaurants learn to use their monitors, adjust them when needed and set up spaces with good airflow.

The large response so far is “a sign businesses are seeing this as a way they can open up,” she said.

That’s certainly how Crandall, in Burlington, feels. “Everybody is trying to figure out how to open their doors, and spending all this money on heaters, and trying to figure out these CO2 detectors,” said Crandall, who has also installed one at his other business, the Train Wreck Bar & Grill.

“You’ve got all these employees that you want to get hired back, and get your system up rolling again, and hopefully you can start paying your bills and get ahead again.”