Indoor air monitoring goes to school

The science of indoor air quality used to be of interest only to researchers and people working in hazardous spaces. Then the COVID-19 pandemic swept across the world. Seemingly overnight, people outside those niche communities began worrying over the quality of indoor air. Schools were a focal point: kids needed to return to class, and virus-free air was fundamental to their safety.

In Colorado and Boston, collaborations between scientists and school districts that helped get students safely back to school at the height of the pandemic have continued and expanded. The indoor air monitoring programs that begun during the pandemic are now ensuring that kids headed back this fall are breathing clean air in homeroom.

“The little kids were the first ones to go back to school, and they were the last to get vaccinated.” says Mark Hernandez, a professor of environmental engineering at the University of Colorado Boulder (CU Boulder). Young children don’t always wear masks properly or reliably, so when schools reopened, administrators had to deploy a different intervention to keep them from becoming ill.

It took time—and intense advocacy from scientists—for health officials to publicly accept that COVID-19 is transmitted primarily through the air (Clin. Infect. Dis. 2020, DOI: 10.1093/cid/ciaa939). When that fact was finally acknowledged, the US Centers for Disease Control and Prevention (CDC) recommended increasing ventilation to help prevent the virus’s spread.

But it took years for the agency to offer specific guidance. In fact, it wasn’t until the day after the US ended its COVID-19 public health emergency in May 2023 that the CDC provided a precise ventilation target of five air changes per hour in any occupied space. For schools to meet this target, the air in every classroom would need to be completely refreshed every 12 min. In Denver, Hernandez says, “none of these schools could do that.”

Some schools lacked a central ventilation system. Others had systems too old to generate the air flow needed to achieve that air-exchange rate. Opening a window might work on warm days but risks exposing kids to outdoor air pollution from vehicles, nearby industrial plants, or wildfires.

Retrofitting or updating ventilation systems wasn’t feasible as Colorado schools began reopening toward the end of 2020. Local permitting laws made it difficult to modify the facade of older public buildings, including schools, but such changes are often necessary to install new ventilation equipment. Even if the alterations were allowed, there was no time for major overhauls with children already returning to classrooms.

Modeling studies conducted before the pandemic suggested that mobile, filter-based air cleaners could minimize the spread of disease, Hernandez says, but there was little real-world data to support the models. Hoping to protect kids while generating field data for other scientists to build off, he procured nearly $1 million in funding from three companies—Carrier, Intel, and Ryan Innovation Group—to devise a protocol to assess the effectivesness of air purifiers in elementary school classrooms.

“The air purifiers are like a seat belt”—a stopgap solution that could protect kids from COVID-19, Hernandez says. The purifiers are essentially a high-efficiency particulate air (HEPA) filter and fan and about the size of a suitcase.

To determine where the air purifiers would be most useful, the team started with a ventilation survey. It measured the air-exchange rate, temperature and humidity, and concentration of particulate matter in the classrooms of 20 Denver schools, about 10% of the entire district. With the blessing of teachers and school administrators, Hernandez deployed air purifiers to 75 classrooms deemed to have the poorest ventilation. After the purifiers were installed, he and his team needed to answer a vital question: Do they work?

Measuring indoor air quality

While setting up the air purifiers, Hernandez’s team also installed multiple air quality monitors in each classroom. Each device sent real-time data to a dashboard accessible to school officials and the CU Boulder team. With a click, Hernandez could see how temperature, humidity, and the concentration of carbon dioxide, volatile organic compounds (VOCs), and particulate matter fluctuated in the rooms.

Such measurements convey critical information about room comfort and air quality. Most people are familiar with the importance of temperature and humidity in keeping a room comfortable—being too hot, cold, damp, or dry can be hard to bear.

The effects of carbon dioxide might be a little less obvious. Some research suggests that higher concentrations of the gas cause people to become sluggish (Building and Envir. 2023, DOI: 10.1016/j.buildenv.2023.110331), but the measurement is primarily used by researchers to quantify ventilation. It also acts as a proxy for the concentration of other gases: high CO₂ concentration indicates limited air exchange between the indoors and outdoors, which signals that more-hazardous pollutants may be building up in the enclosed space.

Measuring VOCs more directly reveals the concentration of such pollutants. There are many sources of VOCs. Off-gassing from furniture, carpets, cleaning agents, and people vaping electronic cigarettes or dab pens can cause spikes in concentration of VOCs.

The monitors measure two sizes of particulate matter—PM₁₀ and PM_2.5, which include particles smaller than 10 µm and 2.5 µm, respectively. Exposure to particles in both size ranges has been linked to increased asthma attacks, as well as heart and lung disease.

Hernandez says that as he analyzed the data collected by the sensors, it was clear that the mobile air cleaners worked to remove particles. “They have the same effect as ventilation,” he says. “But they just remove particles—they don’t bring in fresh air.” In other words, mobile air purifiers cannot replace ventilation to lower CO₂ and VOC levels. Even so, the effective air-exchange rate in the classrooms when using the purifiers was above the CDC recommendation.

But do filters prevent disease?

Removing particles should minimize airborne pathogens and ultimately lower infection rates for respiratory illness. To get a sense if the portable air cleaners were really reducing disease rates, Hernandez and his team plugged the data from their 20-school pilot study into a COVID-19 risk estimation model published while the pandemic was raging (Risk Anal. 2021, DOI: 10.1111/risa.13844). The researchers found that their intervention could halve the chance of catching a respiratory illness.

“The average kid in the average classroom would have a 50% less chance of getting sick if we did this engineering intervention,” Hernandez says.

The results of the modeling study were certainly promising. But it is not possible to verify the model without knowing how many students actually fell sick in each classroom. Now, in a study called the Clean Air for Schools Project, Hernandez is collaborating with epidemiologists and social scientists to determine if the model translates to real life.

Launched in 2022 with funding from the CDC, the project seeks to correlate air quality in classrooms with the number of student absences due to respiratory illness. The study’s rollout was done in collaboration with the Colorado Department of Public Health and Environment, which provided portable HEPA air purifiers to classrooms across the state. Hernandez and his team installed air quality monitors to measure how well the filters were performing. By the end of 2023, 369 schools were enrolled in the project and agreed to send the researchers anonymous data on student absences. Many more schools received filters but opted out of monitoring.

As Hernandez and his team tease out long-term trends in air quality and absenteeism, real-time monitoring will also undoubtedly prove useful for school staffers. In one school, Hernandez says, turning on the ventilation system in the morning released a massive number of particles. The culprit was a dirty filter. After it was replaced, the ventilation system once again removed PM from classrooms. “There are all of these unanticipated, positive consequences when building managers have this real-time data,” Hernandez says.

Monitoring in Boston schools

Colorado schools are not the only ones monitoring indoor air quality. Across the country, schools have installed monitors in classrooms to help ensure that students are breathing clean air. Zach Carson, a representative from Attune, which is providing their air quality sensors to Colorado schools, says the company is partnering with school districts in 12 other states. Colorado is currently the company’s only statewide partnership, he adds.

When Boston began reopening schools, it took advantage of federal Elementary and Secondary Schools Emergency Relief (ESSER) funds to deploy portable air cleaners with HEPA filters and install more than 4,400 air quality sensors from SGS Galson in the city’s 125 schools—enough to monitor every classroom, main office, and nurse’s office.

Like Colorado’s sensors, those in Boston record temperature, humidity, CO₂, and particulate matter. They also measure carbon monoxide—a deadly gas that can leak from cracked heat exchangers—but not VOCs. The live data from every classroom in Boston is available on a public dashboard.

When Patricia Fabian, an associate professor of environmental health at Boston University, read about the Boston sensor program, she saw an opportunity for collaboration. A former master’s student in Hernandez’s lab, Fabian has training in indoor air and environmental engineering and now leads her own group researching built environment sustainability. She says she reached out to Katherine Walsh, the leader of the Boston school district’s Sustainability, Energy, and Environment Program, and asked, “How can we help?”

The answer: data analysis. Fabian says she and her team are taking the school district’s massive air quality dataset and finding trends that they share with administrators “to try to inform the indoor air quality management paths for the schools.” Her group also installed sensors on school roofs to help validate the data collected from the indoor sensors.

Fabian declines to discuss trends at specific schools, saying that “it’s important to share with the community first before sharing with the world.” But generally the variability between classrooms was the most surprising finding. Rooms in the same building could have vastly different temperatures and particle concentrations.

The plan is to use the data to maximize the impact of interventions, such as deploying portable air cleaners, by targeting the classrooms and schools that have the worst indoor air quality metrics. And the variability in metrics such as temperature between classrooms can be a boon. Rooms that are consistently cooler can be used as cooling rooms during heat waves, especially in schools with limited air conditioning.

Regulating indoor air

With hundreds of schools now actively monitoring indoor air quality, it is perhaps surprising that it took a global pandemic to get air sensors into classrooms. The onus of monitoring and regulating the air pollutants inside buildings has always fallen to building managers. The US Environmental Protection Agency “doesn’t have broad responsibility for monitoring indoor air or ensuring its quality in the same way we are authorized for ambient air,” says Vito Ilacqua, acting director of the agency’s Center for Scientific Analysis. In other words, there is no Clean Air Act for indoor spaces.

Instead, Ilacqua says, the EPA’s primary role in protecting indoor air quality is to lend guidance and technical assistance to those seeking to improve their buildings’ air quality. “One of the biggest limitations in us giving guidance has been a lack of large-scale studies, particularly indoor air quality epidemiology,” he says. Studies like those performed by Hernandez and Fabian could prove an important step in this direction. Recently, the agency also announced that it will provide $34 million in funding to five proposed initiatives for improving indoor air.

This hasn’t stopped scientists from advocating for more indoor air legislation. In fact, a team of scientists, policymakers, and legal experts recently drafted the Model Clean Indoor Air Act as an example to lawmakers interested in legislation aimed at improving indoor air quality and standardizing guidance nationwide.

In July, US House Reps. Paul Tonko (D-NY) and Brian Fitzpatrick (R-PA) introduced the Indoor Air Quality and Healthy Schools Act. Although that bill was not based on the Model Clean Indoor Air Act, a member of Tonko’s staff says it was drafted with input from some of the model bill’s authors.

Even in the absence of legislation, schools can currently access federal funds to improve their ventilation systems and install air quality monitors. Fabian points to unspent ESSER grants, the Inflation Reduction Act, and the White House’s Clean Air in Buildings Challenge as potential funding sources. “This is a moment where people can make a difference in improving indoor environmental quality in schools,” she says, adding that such changes can ensure that schools are healthy and equitable places for children to learn and grow.

Although school boards and administrators might balk at the sticker price of overhauling subpar ventilation systems, Hernandez argues that the benefits are worth it in the long run. Using mobile HEPA filter air purifiers in classrooms is a lower-cost interim solution, he says, and that either way, installing indoor air quality monitors should be included in cost calculations. After all, Hernandez adds, “you can’t manage what you don’t measure.”