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Displaying CO2 details also gives people more information with which to assess risks and adjust their behavior. Consider, for example, a diner walking into a restaurant, seeing a CO2 reading of 1,300 parts per million (ppm)—far above the 800 ppm level considered to represent good ventilation—and deciding to eat somewhere else.

It is, in fact, possible to smell whether C02 levels are too high in a space you’ve just entered, said Li. Such spaces often feel stuffy or filled with odors. “If [the smell] is not so good, then you go away,” he said. But for those with less acutely tuned olfactory senses, CO2 monitors can help.

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Belgium presents a test case for indoor air quality

One government is betting that requiring CO2 monitoring by law will have big public health payoffs. Belgium will soon become the first country in the world to mandate continuous monitoring of CO2 levels in public indoor spaces.

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Last December, Belgium enacted a law regulating indoor air quality in enclosed spaces accessible to the public. The law states that, within a year of enactment, all operators of publicly accessible indoor spaces—restaurants, gyms, bars, and offices—must use an air quality monitor such as a CO2 meter.

“It is important to incorporate the experience, knowledge and expertise on indoor air quality and respiratory diseases gained during the Covid-19 pandemic,” notes the explanatory memorandum (pdf, link in Dutch) to the law.

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A bar in Brussels, Belgium, in 2021
A bar in Brussels, Belgium, in 2021
Photo: Yves Herman (Reuters)

And by 2025, operators of Belgium’s public indoor spaces will have to alert visitors to air quality levels. Just as food safety inspectors award visible certificates to restaurants, these spaces must display certification labels that show an official has determined them to have met the required standards.
The Belgian law currently sets two different (and, as of now, non-mandatory) indoor air quality targets that buildings should aim for: less than 900 parts per million (ppm) of CO2 in a room, meaning that CO2 comprises less than 0.09% of the room’s air; and less than 1,200 ppm of CO2.

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And it’s not just at the national level where CO2 monitoring can become part of standard practice. This coming September, the public school board in the Canadian city of Waterloo will pilot a program to track CO2 levels in classrooms.

“It’s really the best tool we have,” Fox said of CO2 monitoring. “It’s not the perfect tool, but it’s the best tool.”

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Part I: How covid could turn out to be like cholera—in a good way

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