When Lizzie Rothwell, an architect in Philadelphia, sent her son to third grade this fall, she stocked his blue L.L. Bean backpack with pencils, wide-ruled paper — and a portable carbon dioxide monitor.
The device gave her a quick way to assess how much fresh air was flowing through the school. Low levels of CO2 would indicate that it was well-ventilated, reducing her son’s odds of catching the coronavirus.
But she quickly discovered that during lunch, CO2 levels in the cafeteria rose to nearly double those recommended by Centers for Disease Control and Prevention. She shared what she’d learned with the principal and asked if students could eat outside instead.
“He expressed surprise that I had any data at all,” she said.
Ms. Rothwell is one of a growing number of parents who are sneaking CO2 monitors into schools in a clandestine effort to make sure their children’s classrooms are safe. Aranet, which makes a monitor popular with parents, says orders have doubled since the new school year began.
Some school systems have made the monitors part of their official pandemic precautions. New York City has distributed the devices to every public school, and the British government has announced plans to do likewise.
But elsewhere, parents are taking matters into their own hands, sneaking in the monitors — which can cost a hundred dollars or more — in their children’s backpacks or pants pockets.
Although the devices, which can be set to take readings every few minutes, work best when exposed to the open air, they can generate informative data as long as they are not completely sealed away, said Dr. Alex Huffman, an aerosol scientist at the University of Denver who has sent the monitors to school with his children. (He recommended leaving backpacks or pants pockets unzipped, or tucking the monitor into the mesh water-bottle pouch that is now standard on many backpacks.)
Many of these parents have forged a community on Twitter, where they are using the hashtag #CovidCO2 to trade tips about how to smuggle the monitors into the classroom, how to interpret the data they are collecting and how to approach the school with their findings.
Some school officials have frowned upon these guerrilla air-monitoring efforts, but parents say the devices have armed them with data to advocate for their children.
“It’s possible that the school district may not be all that happy with this because I think it gives us a window into the fact that they may not actually be treating ventilation as seriously as they should be,” Dr. Huffman said.
A window into indoor air
The coronavirus spreads through tiny, airborne droplets known as aerosols. Improving indoor ventilation reduces the concentration of these aerosols and the risk of infection in an indoor space, but there is no easy way for members of the public to measure the ventilation rate — let alone the accumulation of viral aerosols — in shared spaces.
“Ideally there’d be some machine that cost $100 and…