The first thing you notice walking the halls of Holabird Academy is the windows.

A grid above the entryway floods the main corridor with cool light, and classrooms boast nearly floor-to-ceiling views of the surrounding Baltimore neighborhood.

What you won’t notice — what anyone rarely notices in any building — is the air quality.

A state-of-the-art HVAC system hums on the top floor of this gleaming new elementary school, sucking in fresh air that’s run through filters capable of trapping air pollution. Before it reaches classrooms, the air is conditioned and passes through filters fine enough to catch virus-laden particles. Each classroom has a carbon dioxide detector that signals when fresh air is needed, delivering up to eight total air changes per hour.

More than 1 in 6 Americans breathe the air within public schools each weekday, but very few enjoy air as clean as do the students and staff of Holabird. That’s a problem.

While covid taught the world about the importance of safe indoor air, fights over masking in schools have obscured a more durable — and potentially more effective — solution. Decades of studies show that improving indoor air ventilation and filtration doesn’t just help prevent the spread of disease. It can also improve academic achievement for kids most at risk.

These effects would last long after the pandemic ends. But few school districts are adequately aware of the problem or can afford to address it — even now, halfway through the third year of the covid crisis.

“It’s really been a sleeper issue, but it isn’t,” said Claire Barnett, founder of the Healthy Schools Network. “Schools aren’t paying enough attention to it, and states aren’t paying enough attention to it.” That starts, she said, with neglected infrastructure.

The average U.S. school is more than 40 years old. Forty-one percent of school districts nationwide need HVAC upgrades in at least half of their schools, according to a 2020 Government Accountability Office report.

In many districts, especially those that serve historically marginalized groups, the majority of schools lack adequate — or even any — air conditioning. That was the case in Baltimore until recently, a result of the city’s decadelong effort to modernize its school buildings. Nationally, some districts rely on equipment that is more than 100 years old to heat or ventilate classrooms. These problems are often worse in areas with higher levels of poverty and more Black, Latino and Indigenous students.

The result is that 50 million children attend schools where the air they breathe is substandard — and sometimes downright dangerous for their health. That was true before the pandemic, but the virus has made the situation more acute.

Covid spreads more easily in stagnant air, where virus particles can build up as infected people exhale. But so do illnesses like colds and the flu. Other harmful particles, from dust to outdoor pollution like smog, can reach dangerous concentrations and contribute to chronic ailments as varied as asthma and headaches. “Children breathe more air per pound than adults do,” Barnett said. “If there are contaminants in the air, they’re going to take more of that in, and they’re taking that in during their developmental years.”

Lack of fresh air doesn’t just harm children’s health. A deep body of peer-reviewed research suggests it also impacts academic achievement. Better ventilation in a school can improve students’ performance by anywhere from a few percent to as much as 15 percent, especially when it’s coupled with proper temperature control. And there’s emerging evidence that the gains are biggest for students from poor and marginalized backgrounds.


That makes improving air quality in schools a powerful tool to improve not only children’s health but their educational attainment, especially for students most at risk of being left behind.

School air quality “has the potential to really impact kids either negatively or positively,” said Meredith McCormack, a pulmonologist at Johns Hopkins University who is studying how Baltimore’s school infrastructure improvements affects children. “When we can improve indoor air quality in schools, we’re improving it for a community of children,” she said, in ways that might help shrink health and academic achievement gaps.

Stuffy air harms student health

In a typical crowded classroom, about 3 percent of every breath a student or teacher takes was recently in someone else’s lungs. Without an influx of fresh air, kids and staff will be virtually swimming in a miasma of shared breath — and the pathogens that might be riding it — by the end of class.

Particles that might quickly dissipate outside can become concentrated indoors to potentially harmful degrees. Viruses can accumulate. Pollutants, ranging from formaldehyde in floors to mold, can reach unsafe levels. Even carbon dioxide exhaled by students can reach concentrations that dull thinking and impair decision-making. Outside, carbon dioxide concentrations are about 400 parts per million (ppm). In a crowded, poorly ventilated classroom, concentrations can surpass 1,000 ppm (which experts consider an indicator of inadequate ventilation) within an hour and reach as high as several thousand within a few hours.

There are two ways to dispel all this shared breath: ventilation, which brings fresh outdoor air in, and filtration, which removes particles from the indoor air. The American Society of Heating, Refrigerating and Air-Conditioning Engineers, or ASHRAE, sets a minimum ventilation rate of 5 liters per second per person (L/s), plus 0.6 L/s per square meter of floor area (which is roughly equal to 10 square feet).

For the average classroom with 25 people, that minimum turnover standard equates to about two full air changes per hour, said William Bahnfleth, an indoor air researcher at Penn State University. That’s far below Holabird Academy’s eight exchanges per hour.

“There’s a lot of room to raise the bar because we have standards today that are only focused on safety and satisfaction and don’t strive to attain a higher level of health and productivity for building occupants,” Bahnfleth said.

But many schools miss that baseline, even when their HVAC equipment is relatively new, because fans often get turned off or routine maintenance is left undone. In a study of over 100 recently retrofitted classrooms, over half still had problems.

By contrast, elementary schools in Georgia that improved ventilation and used high-efficiency particulate air (HEPA) filters — the gold standard for air purification — in classrooms had 48 percent lower incidence of covid, according to a study by the Centers for Disease Control and Prevention and Georgia’s health department. In Italy, classrooms that installed mechanical ventilation systems during summer 2021 cut coronavirus transmission by as much as 82 percent, researchers estimate, with the biggest reductions correlating with the highest ventilation rates.

“There are lots of other diseases out there that are transmitted the same way,” said Anisa Heming, director of the Center for Green Schools at the U.S. Green Building Council. “We’ve been talking for years about how better ventilation and filtration systems can help prevent flu outbreaks [and] reduce common colds.”

Less illness translates to fewer student absences. One study that followed 162 classrooms in California over two years estimated that boosting ventilation rates in these classrooms to the state minimum (7 L/s per person) would decrease illness-related absences by 3.4 percent and increase attendance-linked funding to schools by $33 million each year — at a cost of just $4 million extra.

Asthma is also linked to poor indoor air, and disparities in incidence — about 7 percent of white children have asthma, compared with about 15 percent of Black children — stem in part from unequal environmental exposure to pollution. Outdoor air pollution, often concentrated in poorer areas, raises the risk of developing the life-changing condition. Without adequate filtration, that outdoor pollution seeps indoors and becomes even more concentrated.

“We know that indoor air pollution has been associated with worsening of childhood asthma,” said McCormack, the Johns Hopkins pulmonologist. “Improving air quality improves lung growth and development and also reduces risk for asthma attacks.”

In Detroit, for example, researchers estimate that replacing inefficient filters with enhanced filters in schools would reduce the asthma burden by 13 percent annually, with higher benefits for more efficient filters.

“Why in the world do you want a child with asthma in a dirty building?” asked Burnett. “You don’t … healthy buildings means healthier air and healthier students.”

Better air translates to higher achievement

Stuffy rooms have long been associated with foggy thinking. A steady stream of research over the past few decades backs this up, showing that poor indoor air quality impacts cognitive abilities. Improving indoor air can help.

“Without a doubt, indoor air, in one way or another, impacts academic performance,” said Richard Shaughnessy, an indoor air quality researcher at the University of Tulsa. “A classroom that is not well-ventilated presents a less-than-ideal situation for learning.”

Classrooms with higher CO2 levels — a proxy for stale indoor air and poor ventilation — tend to be associated with slightly worse standardized test scores.

Shaughnessy and colleagues monitored CO2 levels in 100 elementary school classrooms across the U.S., comparing standardized test scores with inferred ventilation rates. Most of the classrooms had substandard ventilation rates, and for every unit (1 L/s per person) increase in airflow, the researchers estimated that the proportion of students passing the state’s core curriculum standardized tests increased by 2.9 percent for math and 2.7 percent for reading. Another analysis by Shaughnessy found math scores ticked up 0.5 percent, on average, per each 1 L/s per person increase in ventilation.

Some studies have gone beyond just observing ventilation rates and student performance to carry out actual experiments. When researchers in England experimentally increased ventilation rates from about 1 L/s per person to 8 L/s per person across eight elementary schools, they found students performed 2 to 15 percent better on a series of cognitive exams testing reaction time, memory and word recognition. But the effect held only when fresh outdoor air was brought in (as opposed to recirculating air).

And after nearly every building in a Texas school district got ventilation upgrades in the early 2000s, passing rates for standardized tests rose by 2 to 3 percent, an effect similar to reducing class sizes by 10 to 13 students, according to the study.

There’s growing evidence that the quality of indoor air may have greater impacts on the most vulnerable students, said Sheryl Magzamen, a respiratory epidemiologist at Colorado State University. “If kids have other health and learning challenges, small changes in their environment can potentially make a big difference in their performance.”

And the boost for kids at the bottom can make a much bigger difference than improving the performance of kids at the top.

In a study of a racially diverse school district in the Denver suburbs, Magzamen and her colleagues found that improvements in indoor air quality and other environmental factors, like thermal comfort and noise, were associated with an improvement of 1.4 percent in math for students in the 10th percentile on state standardized tests, compared with no improvement for students at the 90th percentile of math scores. A similar improvement in environmental quality was associated with a 1.2 percent increase for Latinx students, compared with no improvement for white students.

On an individual level, it’s a small improvement, Magzamen said: “But when we think about the distribution of the curve, if we can shift that entire curve up, that may mean more students going on to the next grade.”

Nationwide, nearly 90 percent of white students graduate high school in four years, compared with about 80 percent for Black and Hispanic students. Some states exhibit even greater disparities.

The benefits of improved indoor air quality Magzamen and other researchers have seen are real and significant, she said, but they’re small compared with other factors, like parental education level or socioeconomic status. “We can’t expect new schools to solve all the poverty and equity challenges we have in our community,” she said. “It’s just one part of the learning and health equation, especially for more vulnerable populations.”

Quicker fixes

Many schools with indoor air quality problems simply aren’t aware of them.

“Oftentimes, there’s just a lack of information or knowledge about these issues,” said Mansel Nelson, an indoor air expert with the Institute for Tribal Environmental Professionals who travels across the country helping tribal schools improve their ventilation. Most schools he visits fall short of the bare minimum ventilation levels, he said — even newly constructed schools.

“I remember one brand-new school where the maintenance director had turned off the outside air because it cost money to heat air, and everyone in the building was sick by the end of the day because of all the degassing from the new materials,” he said. The director simply didn’t realize ventilation was an issue, Nelson added: “Once I was able to convince the superintendent that something needed to be done, they opened the outside air vents and everyone felt better.”

Nelson sees increasing knowledge and awareness about indoor air quality as the first step toward improving it, though money always plays a role too. He often recommends schools use CO2 monitors to identify problems, but the devices work only in occupied rooms and can cost several hundred dollars.

“No one complains about ventilation,” said Joey Fox, an HVAC engineer who works for a school board in Ontario, Canada. That makes proper monitoring crucial, he said, and can help building managers change airflow patterns if possible. Newer systems, like Holabird Academy’s, can even automate airflow based on CO2 levels. In part as a response to covid, some states, including California, Nevada and Maryland, are considering or plan to require regular CO2 monitoring in schools.

Fixing the problems discovered by monitoring can mean anything from opening windows to a full-blown HVAC retrofit.

“For ventilation, the easiest thing you could do is open up windows,” said Fox. “That’s free, from a construction perspective, but could mean an increase in heating or cooling bills.”

But in many schools, especially those built during the energy-conscious 1970s, the windows can’t open. At some older public schools in Baltimore, for instance, maintenance crews had to Jerry-rig the windows to open a bit more widely, allowing more air into classrooms.

Opening windows isn’t feasible in areas with lots of air pollution, which constitutes a good chunk of schools. One in 11 public schools lies within 500 feet of a major roadway, according to a recent analysis by the Center for Public Integrity, impacting roughly 4.4 million kids.

During the pandemic, many schools have opted to place portable HEPA filters in classrooms, which suck in air and filter out particulate matter, including viruses. Baltimore City Public Schools put one of these in every classroom of schools with older HVAC systems that can’t deliver optimal ventilation. In bigger spaces, like cafeterias and auditoriums, schools placed bigger “air scrubbers” that filter larger volumes.

“These are a quick and effective fix,” especially for covid mitigation, Fox said. But stand-alone units aren’t always operated correctly. “They often aren’t running at the right speed, or [they’re] pushed into corners where they can’t mix air properly,” he said. They’re also expensive to operate, and each unit requires routine maintenance and cleaning. (Heming knows of one district that’s devoted a full-time staff member just to maintaining its portable air cleaners).

Another potentially easy fix is to upgrade the HVAC’s system’s filters. Currently, ASHRAE standards call for MERV 8 filters in schools. These catch particles as small as 3 microns — about the width of spider’s silk — but they “do very little to stop particles that deposit in the respiratory system, including aerosols that may have pathogens,” said Bahnfleth. MERV 13 filters efficiently snag particles as small as, or even smaller than, 0.3 microns — including most smoke, bacteria and virus particles — making the air much cleaner, he said, which is especially important in areas with more outdoor air pollution.

Additionally, many HVAC systems can be adjusted to bring in more outside air without major upgrades, Fox said. Upping ventilation rates does cost money, but on the order of a few dollars to $10 per person, which is a fraction of a percent of what’s typically spent on public education.

But it requires regular maintenance and attention to preserve the gains from any upgrades. “HVAC systems are complex pieces of equipment,” said Rengie Chan, an indoor air researcher at the Lawrence Berkeley National Laboratory. Sensors, fan belts and motors can fall out of calibration over time. “Upkeep needs to happen, but many schools don’t have the money or workforce,” for adequate maintenance, she said — especially rural districts or those in disadvantaged areas.

More funding is needed for larger overhauls

For buildings that need more than tinkering with existing systems, the cost of improving air quality can be steep.

Lakewood Elementary School in Baltimore is a two-story brick building nearly 60 years old. For most of those years, its 100 or so students relied solely on barely cracked-open windows for ventilation. Installing a central HVAC system, like that at Holabird, would’ve cost too much, so the district took a middle-road approach, installing vertical packaging units on outdoor-facing walls in each classroom. At about $60,000 each, these large cabinet-sized systems filter and condition air pumped in from outside.

Schools like Lakewood that want more substantial HVAC overhauls may need to spend millions of dollars, said Heming of the Green Building Council. “That’s a heavy lift for districts to do,” she said, and nationally the U.S. isn’t spending nearly enough to keep up.

Each year, districts fall $85 billion short of what’s needed to operate, maintain and upgrade facilities, according to the 2021 State of our Schools report from a trio of nonprofit groups focused on infrastructure quality. And what is being spent is not equally distributed across districts.

Low-poverty districts (where less than a third of students receive free or reduced-price lunch) spend on average $200,000 more on annual maintenance and operations than medium- or high-poverty districts. High-poverty districts spend 37 percent less on major upgrades than low-poverty districts.

“Rural schools and poor schools are in the most dire condition,” Heming said. “These communities just don’t have the tax base to do major upgrades on their schools,” she said, leaving infrastructure to fall behind in many districts.

A major covid relief bill enacted last year, the American Rescue Plan, included $122 billion for schools. Some districts have applied for and used some of this funding for more substantial ventilation upgrades. But it amounts to a vanishingly small slice of the overall school relief pot; so far, only about $10 billion of the money has been spent on facilities, far short of what’s needed.

But pulling off such renovations requires more than funds.

Baltimore City Public Schools has been working for over a decade to bring improvements, and the district still isn’t finished. Funding and coordinating these projects across a district with nearly 150 different schools is a major challenge, said Lynette Washington, chief operating officer of Baltimore City Schools. “We have to pool resources from a number of different places, develop plans for each school and make procurement contracts.” That takes time.

“Planning for these is actually quite complicated,” Heming said. “Poorer districts don’t have those plans because they’ve never expected to have those funds in the first place.”

Still, districts across the country are starting to draft plans as the pandemic has focused national attention on indoor air like never before. “I’m not happy that we’re in a situation where people are thinking about it, but I’m happy we’re in a situation where people are thinking about it,” Heming said. “There’s just a huge level of need.”