As schools and other public facilities open across the nation and researchers better understand how COVID-19 operates, scientists from the Harvard T.H. Chan School of Public Health suggest that fenestration can serve as a key defense.

In a press conference, Joseph Allen, assistant professor of exposure assessment science and director of Harvard’s Healthy Buildings Program, said opening windows amid fall can add enough fresh air to classrooms to help disperse coronavirus particles. Allen directs the school’s Healthy Buildings program, as well as serves as a certified industrial hygienist (a field that anticipates, recognizes and controls hazards in the workplace)—all of which qualifies him to “say what good plans look like and maybe what not so good plans look like,” he said. When asked if he feels that kids should return to school, “I think kids should go back,” he said. “Today’s headlines will be about the occasional case in a school,” he added. “Next year’s headlines are going to be about the consequences of virtual drop out, stories of abuse and neglect, [and] lack of access to food. I mean, there are stories already from talking to teachers that they don’t know where their students went after March.”

But before students return to school premises, it should be under two conditions, he suggested: low community spread and “it can’t be schools as usual.”

Risk reduction strategies, such as hand washing and universal masking are musts, but also ventilation and filtration, he said. At the same time, he and other experts acknowledge that many existing heating, ventilation and air conditioning (HVAC) systems are woefully underequipped or ill-tuned for providing adequate protection from the virus.

When asked if there are concerns that those systems could act to spread the disease, “Airborne spread [and] airborne transmission is definitely happening,” he said. “We’ve been saying it since early February. I think people are just coming around to that now more widely. So that’s a good thing that people are starting to recognize, as every piece of evidence since February has supported this hypothesis.”

In cases where HVAC systems fail to incorporate outdoor air and proper filtration, air merely recirculates within a room and building, he said, adding, “That’s going to lead to a buildup of indoor pollutants, be it chemical pollutants that off gas from your carpet or, in this case, the buildup of viral particles. If someone is infectious and just breathing, it’ll just build up over time.”

The end result for any system, he said, includes diluting the virus through ventilation, cleaning it out of the air via filtration, “or it’s deposited in occupants’ lungs.” To support this notion, he cited numerous documented cases of spread.

Most of the particles we emit and we talk … or just breathe, they are 10 microns or smaller,” he said. “These particles travel beyond six feet and stay aloft for 30 minutes or more.” Recent testing, he said, found viral particles 16 feet from a patient. Other studies found viral ribonucleic acid (RNA), “in places that can only be reached through the air like in ducts,” he added.

Building ventilation systems are designed around minimal requirements and energy efficiency, he explained, and not infectious disease control. Updating those systems to provide adequate protection will take time and resources—if it happens at all, in some cases. In the meantime, “Start opening up windows,” he said.

Allen and other researchers conducted an experiment showing that anywhere from five to 10 air changes per hour are achievable simply by opening doors and windows.

We went out to a couple of schools, measured ventilation rates, even through just opening windows,” he said. “We took dry ice [and] built up the carbon dioxide concentrations. You watched the decay curve and you can estimate how many air changes per hour of clean air you get.”

According to the study’s findings, opening windows even as little as 6 inches is sufficient, but when confronted with the issue of window opening control devices and child safety, “You just have to get them open a little bit,” he said.


  1. It’s very easy for a scientist to tell teacher’s and schools to open windows. Need I remind them that due to all the mass shootings in schools, security has changed dramatically and most windows can’t be opened as per school safety policy or they have been sealed shut. Doors to classrooms have to always be closed and locked for the same reasons, so creating proper ventilation is also a challenge under the prescribed scenario.
    I find the flip comment “start opening up windows” to be off the mark by Professor Allen.

  2. Actually, we are seeing more (automated) operable windows in educational institutions.
    The benefits vs traditional HVAC is substantial.
    Regarding active shooter scenarios, this control is worked into the Building Management System, but this is hardly an issue. Automated windows in these environments only need to open a few inches and are usually over 8 ft high so risk is extremely low.

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