School & District Management

What the CDC Guidelines Don’t Say About Classroom Ventilation and COVID-19 Spread

By Sarah D. Sparks — February 17, 2021 6 min read
New York City Schools Chancellor Richard Carranza, left, feels for airflow from a ventilation unit inside a classroom at Bronx Collaborative High School, during an August visit with Mayor Bill de Blasio, right, to review health safeguards in that city's schools.

Is opening a door or window enough ventilation to prevent the spread of COVID-19 in schools?

Guidance from the Centers for Disease Control and Prevention released last week provided significantly more detail on how schools should approach issues like community spread and mask use when deciding how to reopen schools safely during the pandemic. But critics say the new recommendations downplay the importance of improving indoor air quality and ventilation to prevent the spread of the airborne virus.

The CDC guidance encourages schools to improve ventilation as part of their overall cleaning strategy, and particularly points to opening doors and windows to “increase circulation of outdoor air to increase the delivery of clean air and dilute potential contaminants.”

In separate tips on ventilation, the agency describes this as a cost-free way to reduce the spread of the virus, along with inspecting and maintaining local exhaust ventilation; repositioning outdoor air dampers; and disabling demand-controlled ventilation, a common energy-saving system that reduces the rate at which outdoor air is pulled into a building.

“The guidance addresses near-field (close contact) inhalation dose with masks and distancing. That’s good,” said Richard Corsi, the dean of engineering and computer science at Portland State University, an expert on indoor air quality, via a tweet. “Ventilation is given lip service with little guidance. Incredibly disappointing. The lack of understanding of ventilation or its importance (or perhaps just disregard) is wholly obvious.”

And 13 scientists this week, in a letter to President Joe Biden’s administration, are calling for a greater focus on limiting airborne transmission in schools, meatpacking plants, prisons, and other indoor settings.

Safe air circulation is crucial

Early in the pandemic, public health officials believed the coronavirus was spread mainly through droplets—virus-laden liquids larger than 5 microns across. The current rules on social distancing come in part from studies that find a cough or sneeze can project these droplets several feet away, where they may hang in the air for several minutes before landing on surfaces.

But more recent studies have found that COVID-19 spreads through much smaller aerosolized particles, which both travel farther and hang in the air indefinitely unless blown away. That makes keeping air circulating—safely—much more important.

Studies find that opening windows and doors can help replace the concentration of air inside more quickly—but how quickly depends on whether there are multiple openings available to provide better air flow.

One new study of the coronavirus in buildings found that because of the viruses’ transmissibility and tendency to hang in the air over time, administrators may not be able to rely on the normal rate of air flow from their ventilation systems to clear virus particles from the air. And a separate new study of Dutch schools found aerosols built up steadily in school gyms even when they had ventilation, but the combination of increased ventilation and the use of mobile air filters cut the concentration of aerosols in the rooms by 80 to 90 percent.

However, the CDC’s other recommendations to bolster ventilation beyond opening doors and windows have costs that can add up quickly for schools: The agency estimated fans at $100 each; $500 each for portable HEPA filters; and $1,500 to provide ultraviolet germicidal irradiation to counter the coronavirus in ducts above rooms with limited ventilation.

A government study released this summer found more than 2 out of 5 U.S. districts need to update or replace the heating, air conditioning, and ventilation systems in at least half of their schools. The report found some 36,000 schools had outdated HVAC systems or those in need of repair or replacement—making it by far the most common infrastructure problem in schools. Federal researchers estimated high-poverty school districts spent on average $300 less per student on capital projects like HVAC upkeep and replacement than did low-poverty districts, $719 per student versus $1,016 per student.

Basic maintenance of ventilation systems is essential

But Kanecia Zimmerman, an associate professor of pediatrics at Duke University Medical Center and co-head of a National Institutes of Health project studying safe school reopening, argues basic maintenance can go a long way.

“What is most helpful is that ventilation systems work as they were designed,’ said Zimmerman in a briefing. “Overhauling ventilation systems, HEPA filters, those types of things have not been demonstrated to result in reduced transmission. Certainly, we think they could potentially work based on the fact that they are filtering air and things of that nature, but there is no evidence that reduced transmission has occurred because we had these things in place. So, the most important thing would be to have ventilation systems that actually work.”

Yet what “works” when it comes to getting rid of aerosolized virus can seem to run counter to what schools might otherwise want in their building’s HVAC system. Take, for example, one new study of COVID-19 transmission rates in New York City schools as they started to reopen. Researchers tracked COVID-19 transmission rates in more than 100 classrooms across the city as the seasons changed. Transmission rates rose as schools switched over from air conditioning—which often incorporates more natural ventilation— to heating. Newer and better resourced schools were also more likely to have higher transmission rates.

Why? Those schools were more likely to be weather-proofed and energy efficient. That’s great for maintaining a stable temperature and cost-savings, but it means indoor air doesn’t move as much and doesn’t get exchanged with fresh air as often—leaving virus particles to build up.

“Surprisingly,” the researchers noted, “schools located in older buildings and lower-income neighborhoods had lower transmission probabilities, likely due to the greater outdoor airflow associated with an older, nonrenovated buildings that allow air to leak in (in other words, drafty buildings).

Likewise, the study also found all else being equal, the little more than a third of schools that had mechanical ventilation, such as fans or central heating and air conditioning systems, had lower rates of transmission overall. The study noted that all but 18 percent of the schools studied had either no windows or windows that were broken and would not open, making it difficult or impossible to get natural ventilation.

Recent studies that have found proper ventilation to help prevent school outbreaks when community coronavirus levels are high, but Zimmerman cautioned that it cannot work without being part of a comprehensive mitigation plan.

“We know that even in settings of poor ventilation, masking and distancing can be very helpful,” Zimmerman said. “Ventilation is potentially one part” of school mitigation strategies.

Improving indoor air quality may do more than just help schools tamp down on COVID-19 outbreaks, though. One study last year found improving air quality also boosted reading and math achievement, particularly for disadvantaged students living in areas with outdoor pollution.

A version of this article appeared in the February 24, 2021 edition of Education Week as What the CDC Guidelines Don’t Say About Classroom Ventilation and Spreading COVID-19


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