Covid-19: How open windows and doors can help lower the risk of transmission in classrooms

Opening windows and doors to create natural ventilation is one of the best ways to reduce the risk of Covid-19 being transmitted in classrooms, Niwa air quality experts say.

The team has been studying indoor and outdoor air quality in New Zealand for about 15 years, including a decade looking at air quality in people’s homes and in schools.

“We know that some Aucklanders are feeling anxious about the return to school for some students next week. But they can feel more confident knowing that opening windows and doors to replace indoor air with fresh air from outside is very effective at cleaning air,” Niwa air quality scientist Dr Ian Longley said.

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“In a closed classroom, everyone’s breath spreads out to fill the space. If anyone is infected with Covid-19, virus particles can circulate through the air across the whole room. But build-up of contaminated air is reduced by ventilation. This just means removing the stale air inside the room and replacing it with clean air.

Year 11 to 13 students are allowed to return to school in alert level 3 areas on Tuesday (file photo).


Year 11 to 13 students are allowed to return to school in alert level 3 areas on Tuesday (file photo).

“One of the most effective ways to do this is simply to open doors and windows to create air flow across the classroom.

“This will remove air from inside and replace it with fresh air from outside, which should be free from the Covid-19 virus.”

For classrooms with all the windows or doors on one side, turning on a fan could help create air flow and bring outdoor air inside.

Niwa explains how air circulates in a classroom


Niwa explains how air circulates in a classroom

Mechanical ventilation might be needed if a classroom didn’t have windows that opened. But for most New Zealand classrooms in the warmer months, ventilation units were no more effective than natural ventilation, Longley said.

Air in a room could be cleaned by running it through a filter that removed particles, including virus particles, before recirculating the cleaned air.

Some air conditioners were fitted with filters, and there were also many types of standalone filtration units available, including portable ones that could be plugged into a standard plug.

Filtration was an option for classrooms with insufficient ventilation, for instance, where there were few or small opening windows, or where fresh air did not reach all parts of the classroom.

Air conditioners such as heat pumps typically recycled air in a room, and while they made rooms more comfortable, they did not improve ventilation.

Ventilation is one of the best ways to reduce Covid being transmitted in a classroom


Ventilation is one of the best ways to reduce Covid being transmitted in a classroom

Carbon dioxide monitors installed in some newer classrooms could give a good indication of how fresh or stuffy the air inside a room was, and the likelihood of Covid to be circulating.

“Outside CO2 levels sit at about 410 parts per million (ppm), while the current indoor standard is 1500 ppm. But international studies are showing that to reduce the risk of Covid-19 transmission, you really need to increase ventilation so that CO2 stays below about 800 ppm,” Longley said.

“Keeping CO2 levels down offers the win-win of ensuring Covid-19 is less likely to be circulating, as well as keeping students more alert to improve learning.

“While using a CO2 monitor can give valuable insights into the indoor air quality, humans are also good at sensing how fresh the air is in a room we walk into and whether there’s some air flow,” he said.

While filtration could remove virus particles, it did not reduce the amount of CO2 in a room. Even with an air filter in place, ventilation was still needed to reduce CO2 levels and prevent build-up of stuffy air.

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