We recently tested ventilation rates for nine classrooms in three buildings of different ages. Sure enough, as with most schools, the mechanical ventilation systems were bringing in only a fraction of the air required.

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But when we opened windows, even just six inches, we consistently saw air-exchange rates above the “excellent” target of five air changes per hour, with some rooms getting much more. (The benefits of open windows extend to buses, too. When we measured air changes in a bus doing its regular route, we found the number of air changes per hour was 20 to 40!)

So, what can we do with this information? First, open windows and doors. It’s the simplest and quickest way to increase the air-exchange rates. But be mindful that the amount of air that comes inside will depend on outdoor winds and temperature gradients. Many schools that rely on natural ventilation have a central stack that draws air in and out, acting as a giant vacuum of sorts. They should make sure these are running. Adding a box fan or two in the windows might also help.