As Washington schools reopen to more students, one concern at top of mind for many families and teachers is this: How are schools ventilating classrooms to reduce the chance of spreading COVID-19?

The Seattle Times Education Lab received a surge of questions over the past month about how air quality is measured in schools, the ways schools can keep air circulating and how ventilation stacks up relative to other school safety measures. To answer your questions, we spoke with public health and occupational safety experts about improving ventilation. We also asked Seattle Public Schools for a demonstration of how employees assess buildings and ventilation equipment for air flow.

How important is ventilation when it comes to protecting kids and teachers during the pandemic?

Refreshing air inside classrooms is among the most important measures schools can take against airborne diseases like COVID-19, say government health officials and occupational safety experts, and should be done in combination with other methods like wearing masks, distancing and hand-washing.

The more students and teachers in an indoor space, the riskier the scenario for spreading the virus that causes COVID-19. Bringing in new air can help lower the concentration of particles containing viruses and the risk of disease spread.

In its most recent advice to schools, the U.S. Centers for Disease Control and Prevention placed new emphasis on the importance of ventilating classrooms. The agency advises that schools bring in “as much outdoor air as possible” and ensure that air filtration and ventilation systems are working at max capacity. Since reopening buildings, the Seattle school district, for example, has cranked settings in HVAC systems to pump 100% outside air, versus the typical setting of 30%. The Washington State Health Department also has school ventilation guidelines.


How do schools typically exchange or refresh air inside classrooms? 

Even before COVID-19, ventilating classrooms was important: students and staff breathe CO2 into the air, dust accumulates and maintenance staff use cleaning agents that may contain toxic airborne chemicals. 

Schools have a few options to keep air moving.

Teachers can open classroom windows, a low-cost option that helps bring in fresh air and dilutes or disperses infectious particles. “In general, opening windows is often my first recommendation,” said Marissa Baker, assistant professor of environmental and occupational health sciences at the University of Washington. The more windows you have in a space — and the wider you can open them — the better, she said. 

Many schools have some type of built-in air exchange system, known as a heating, ventilation and air conditioning system, or HVAC. These systems vary, but usually involve ductwork and air vents that can suck stale air in, force fresh air out, or both. HVACs may be fitted with filters that capture dust or other particles from recycled or outdoor air.

Schools can also purchase stand-alone air high efficiency particulate air filters. HEPA filters capture a range of particle sizes, and are efficient at filtering those that carry the virus that causes COVID-19. But HEPA filters are costly. These filters don’t bring in new outside air — they just remove particles — and they can be noisy. In buildings where HVAC problems are significant or windows can’t be opened, Seattle schools have placed HEPA filters in classrooms and other areas. 

How do schools measure their ventilation systems? What does that process look like? 


This varies depending on the building and the standards the school district adheres to. In Seattle, the district has committed to providing 25 cubic feet of airflow per person, and measures the output of air from heating and cooling systems using a device called a balancing hood. 

Fernando Luna, a Seattle Public Schools employee responsible for getting school buildings up to par with national and local air safety guidelines, demonstrated this recently at Hamilton International Middle School. In a hallway of the school, he lined up the hood — a square tent with a black electronic panel at its base — to a vent on the ceiling where outside air pumped from the building’s heating and cooling system exits. With its tent draped over the vent’s diffuser (the exterior part of a vent that pushes air in different directions) the hood ticks like a metronome, taking 10 samples of the air flow per second. After waffling between digits, the screen reads 240 cubic feet per minute, a measurement of how much outside air is exiting the HVAC system into the building. 

Classrooms usually have up to four of these vent diffusers in the ceiling. Employees and district contractors take the measurements from each diffuser — and a couple of other data points — and use a formula to calculate how many cubic feet of airflow per person the HVAC system can deliver to a space. 

As another way to gauge fresh air levels, the district also measures CO2 levels in a room using a separate device, which looks like a walkie-talkie. It aims for the number to be below 1,000 parts per million. 

How often should HVACs exchange air inside classrooms?

Many HVACs can be adjusted to increase how often they exchange air. In classrooms, HVACs should bring in completely new air every 10-15 minutes, Baker said — or more often, if possible. “When there’s increased air exchange in a room there’s less time for viral particles to linger in one place and that reduces the risk of exposure and viral transmission for other kids in the room,” Baker said.

Some classroom windows don’t open. What options do these schools have?


Some windows can’t be opened because it’s noisy outdoors or outside air is polluted from highway traffic or airplanes. And during wildfire season, many schools have to keep windows shut. This is where HEPA filters may come in, said Elena Austin, a colleague of Baker and assistant professor of environmental and occupational health sciences at the University of Washington. 

Austin is currently working on a research project that looks at how well stand-alone HEPA filters decrease the concentration of particles inside classrooms. At five schools in South King County, Austin and her colleagues are measuring air exchange rates — or how often outdoor air replaces classroom air per hour — and whether outside pollutants make their way into classrooms. The team is testing $500 HEPA filters in 1,000-square-foot classrooms to see how efficiently they filter out particles of a variety of sizes.

Austin’s project isn’t focused on COVID-19, but it may have lessons for pandemic safety. “As we gain more information about COVID transmission, and particularly the size of the particles that may be most contributing to transmission, we can understand how efficiently the HEPA filters might remove those,” Austin said.

What do we know about how well Washington schools are equipped to ventilate classroom air?

Washington doesn’t require schools to regularly test their air quality, so we don’t have statewide data. 

The best way for families and teachers to find out about their school’s air quality is “to ask their school district facility director,” said Nancy Bernard, a public health adviser at the Washington State Department of Health, since those are the employees who are tasked with following state recommendations. But this isn’t always possible — parents and teachers are often busy, or school facilities directors might not have time to respond. Baker advises that families and teachers think of safety measures, including ventilation, holistically. “If one of them can’t be implemented,” families and teachers can be reassured if schools are taking other measures seriously, she said.


Washington’s education department, the Office of Superintendent of Public Instruction, keeps a record of the condition of certain schools’ HVAC systems. But those logs only indicate whether a system needs to be replaced or repaired — not whether the air quality is actually poor. An HVAC might “be in outdated condition,” said Randy Newman, director of school facilities and organization at OSPI, but “still be doing the job of filtering the air.”

Baker says the age of a school building doesn’t always indicate how well it’s equipped to filter classroom air. Newer buildings that are built to be energy efficient might exchange air less often than a building with an older HVAC system. Older buildings might be drafty — and thus better ventilated, with outside air leaking in through cracks in windows and doors. In both cases, Baker said, schools can take steps to improve ventilation by opening windows or using air filters.

A few districts such as Seattle, which follows guidelines set by a national panel of HVAC experts, have replaced nearly all the air filters in building HVAC systems. These filters, called MERV-13 filters, are more effective at catching particles than the filters previously in use by the district. Some educators unions have HVAC and air quality provisions in their contracts; Seattle schools, for example, are required to provide a copy of classroom HVAC reports if an educator requests them.

What do Washington health officials say about how to clean air in schools?

The Washington State Department of Health’s guidance fits closely with what the CDC recommends: Schools should open windows and use HVAC systems to bring in as much outside air as possible. DOH also recommends that schools regularly change their air filters and, when possible, upgrade to filters that trap small particles that can evade lower quality options. 

DOH strongly urges against spraying aerosolized disinfectant or any other chemicals into the air.