Some school systems have made carbon dioxide monitors part of their official pandemic precautions, but elsewhere, parents are taking matters into their own hands, sneaking in the monitors -- which can cost $100 or more -- in their children's backpacks or pants pockets.
New York City has distributed the devices to every public school, and the British government has announced plans to do likewise.
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 monitors into mesh water-bottle pouches that are now standard on many backpacks.
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 carbon dioxide would indicate that it was well-ventilated, reducing her son's odds of catching the coronavirus.
But she quickly discovered that during lunch, carbon dioxide levels in the cafeteria rose to nearly double those recommended by the 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.
Rothwell is among a growing number of parents who are sneaking carbon dioxide 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.
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 it starts beeping if the virus is in the air," said Jose-Luis Jimenez, an aerosol scientist at the University of Colorado Boulder, who is sending a carbon dioxide monitor to school with his son. But in the absence of such a device, he said, "CO2 is something that provides an affordable and very meaningful shortcut."
Every time people exhale, they expel not just aerosols but also carbon dioxide; the worse the ventilation, the more carbon dioxide builds up in an occupied room.
"If we see the CO2 rising, then that also implies that the concentration of aerosols are rising," Huffman said. "Even just bringing a sensor for a day or two can give you a really interesting and useful window into the world of the ventilation of that space."
Jeanne Norris, who lives in the St. Louis area, said she bought her monitor after losing confidence in officials in her son's school district.
"They just hadn't been very transparent about their ventilation," she said. "They say that it's fine and that they did their own testing, but then they wouldn't share that data with me."
Norris and her husband are both science teachers, and so far their data suggests that the ventilation is excellent in both of their classrooms. But carbon dioxide levels in her son's classroom sometimes surpass 1,300 parts per million. The CDC recommends that indoor carbon dioxide levels remain below 800 parts per million.
After she collects more data, she plans to take her findings to school officials and ask them to improve the ventilation.
"I'm willing to be creative and brainstorm with them," she said.
Some parents have gotten results. When Jeremy Chrysler of Conway sent a monitor in with his 13-year-old daughter this fall, the carbon dioxide readings were a sky-high 4,000 parts per million.
He took his findings to district officials, who discovered that two components of the school's heating, ventilation and air-conditioning system were not working properly. After the units were fixed, the carbon dioxide levels plummeted.
"What my measurements showed was, hey, measuring CO2 can identify problems, and sometimes those problems are easy to fix," he said.
Although Rothwell has not persuaded her son's school to move lunch outdoors, the principal has said he is committed to improving the ventilation in the cafeteria, she said.
VARYING RESULTS
"There are some success stories," said Kimberly Prather, an atmospheric chemist at the University of California, San Diego. "Unfortunately, I've heard more parents rejected."
Graham Freeman, the father of two boys in Santa Cruz, Calif., said his request to send carbon dioxide monitors to school with his sons was denied.
Kris Munro, superintendent of Santa Cruz City Schools, said she is confident in the ventilation upgrades the district performed last winter and that it would be inappropriate to put individual students in the position of monitoring school air quality.
"It's our responsibility to assure every space is safe," she said. "Not just to have individuals coming on campus to find out: Is a specific space safe?"
Freeman has been sending the monitors into school anyway, tucked into the pockets of his sons' cargo pants. He's been pleasantly surprised by the readings, which have remained under 700 parts per million as long as the classroom doors and windows have been open.
But the monitors did capture a small spike, with carbon dioxide rising above 900 parts per million, during a lockdown drill at his son's middle school when the teacher closed the classroom door.
So his sons will continue taking the devices to school for the indefinite future. "We're going to be wearing a lot of REI cargo pants and carbon dioxide monitors in the pockets," he said.
MONITORS' BENEFITS
There are limitations to the monitoring. Some devices are more reliable than others, and the readings can be skewed by a variety of factors, including where the monitors are placed.
Children can still catch the virus in spaces with low carbon dioxide levels and good ventilation. And high-quality air filters can trap viral aerosols but have no effect on carbon dioxide levels. So in schools that have installed these filters, carbon dioxide readings alone may overestimate the risk of viral transmission.
But even in the absence of the virus, reducing indoor carbon dioxide levels can have benefits. Studies show that even moderately high levels of the gas may muddle thinking and that improving ventilation can boost performance on cognitive tasks.
Of course, many families cannot afford a $100 air quality monitor -- and they should not have to, parents and scientists said.
Chrysler, whose carbon dioxide readings prompted his Arkansas district to repair its HVAC system, is now lobbying officials to buy air quality monitors for every classroom in the district.
Pointing to Belgium, which has mandated carbon dioxide monitors in restaurants, gyms and other buildings, Jimenez said he would like for all public indoor spaces to provide permanent real-time displays of the carbon dioxide levels.
"This is something that we should do permanently in schools but also in all places where we share air," he said.