As students and teachers prepared to return to class this week, public health officials tried to clear the air about classroom air quality during a virtual media event on Jan. 14.
Air quality readers which measure carbon dioxide levels had been expected to be delivered to 90,000 Quebec classrooms by the end of 2021; as of Jan. 14, 68 per cent of the promised devices had been delivered or were awaiting delivery, according to assistant deputy minister of education Caroline Imbeau, who spoke at the briefing. Additionally, about 400 air exchangers had been delivered for classrooms without sufficient ventilation. “Anyone can apply for an air exchanger; the school service centre can send a request and we’ll look at it,” Imbeau added.
Dr. Yves Jalbert, assistant director general of the Directorate of Public Health Protection, stated that the air quality measures would be part of a multilayered COVID prevention strategy, including the wearing of masks “everywhere and all the time” except when eating, playing a musical instrument or on excessively hot days; the encouragement of vaccination; surveillance of symptoms; and expanded use of rapid tests. “We don’t have particular concerns about air quality in classrooms; if the other measures are correctly applied, we don’t think air quality is a major factor,” he said.
The ministry of education has thus far resisted calls from unions including the Quebec Provincial Association of Teachers (QPAT) to expand the use of air exchangers and air purifiers in classrooms. Jalbert says the ministry of health “entirely supports” their position.
Ali Bahloul, an air quality and ventilation expert at the Institut Robert-Sauvé en santé et sécurité au travail and member of a working group consulted by the government on the issue, explained the differences between air exchangers and air purifiers. “An air exchanger brings fresh air from outside and takes out [existing] air. An air purifier filters air within a space – it doesn’t bring in outside air; it filters and recirculates existing air. It can be useful when placed near an infected person, but it loses its usefulness if it’s further away.”
“Air purifiers don’t prevent transmission of COVID-19 through close contact, and when poorly installed, they are potentially risky,” said Bahloul, citing a report published by the group of experts last year. “They can generate air currents which are favourable to droplet dispersal…and undermine a building’s ventilation system.”
The great purifier debate
“Purifiers are one of the greatest tools we have,” counters Nancy Delagrave, a physicist and scientific co-ordinator of the Collectif COVID-Stop. Filters “are made from the same material as [high-quality] masks; they trap the particles and the air that comes out is very pure. Adding a purifier is like opening a window.”
Jeffrey Siegel is a ventilation expert and a professor in the Department of Physical and Environmental Science at the University of Toronto. He says ventilation and air circulation “are very important” for controlling COVID-19 transmission. “There is no substitute for masks and vaccination… every [preventative measure] is imperfect on its own, but ventilation is important.”
“Bringing in outdoor air is good, and that’s what an air exchanger does, but it’s not always easy to have [quality] systems,” says Siegel. He says an air purifier with a HEPA filter “can be useful” in a classroom if it is sized correctly.
He calls Bahloul’s statement that poorly installed purifiers can lead to increased risk a “weird criticism.”
“In the scheme of things I worry about with purifiers, that’s not even in the top ten,” he says. “Purifiers bring in air through the top and circulate it out through the sides, and that can resuspend particles that are on the floor…but if you elevate the purifier off the ground, that [concern] is gone.”
Siegel adds that homemade air purifiers known as Corsi-Rosenthal boxes – the devices that went viral on social media after a teenage aspiring engineer in Ontario began building them for his neighbours – can be a “very nice option” in the absence of a commercial filter. Originally conceived to filter wildfire smoke, they are ideally made with a commercial box fan, cardboard and four off-the-shelf air conditioner filters. Bahloul, for his part, says the boxes are well suited to homes, but that standardized, certified devices are preferable in schools and workplaces.
“In a perfect world, we would have good central ventilation systems in schools without the noise and trip hazard of purifiers, but installing good central ventilation takes time and money and effort,” Siegel says. “Let’s use purifiers as a stepping stone.”