Study Finds No Correlation Between School-acquired COVID and CO2 Concentrations

In a recent Indoor Environments study, no clear association was observed between the incidence of school-acquired COVID-19 and CO2 concentrations in classrooms. The study aimed to evaluate whether COVID-19 transmission was higher in classrooms with less ventilation, using CO2 as a proxy.  

This study was conducted in Montreal schools by analyzing CO2 level variations by ventilation system type and measuring the association between transmission and either the maximum mid-class CO2 levels or the rise in CO2 from the start of class to the mid-class peak. Even though data were aggregated at the building level, the results revealed significant differences between ventilation system types and maximal mid-class CO2 concentrations. 

On average, mid-class concentrations were lower in schools using mechanical ventilation, compared to natural ventilation, suggesting that mechanical systems in the winter season may generally provide better ventilation, assuming a comparable density of occupants. Natural ventilation practices in schools without mechanical ventilation systems, however, have not been documented. Mechanical ventilation systems offer better control of air movement, especially in winter, within occupied spaces. They regulate the supply of fresh air and the extraction of stale air, ensuring better air renewal and reducing CO2 levels. 

The study found that hybrid ventilation systems showed maximal mid-class CO2 concentrations that fell between those of mechanical and natural systems. However, in winter, the CO2 difference between mid-class and pre-class levels suggests that schools with hybrid systems did not perform better than those with natural ventilation and may even be less effective. 

The study’s findings also suggest that most school-acquired cases were likely not misclassified as the findings do not suggest that the observed ventilation levels in elementary and high schools in Montreal influenced COVID-19 transmission. 

Between September 2020 and June 2021, a total of 12,227 COVID-19 cases were declared in Montreal’s schools, of which 9,958 (81%) were among students and 2,269 (19 %) among staff members. Of all declared cases, 59% were symptomatic and 32% were related to a school outbreak. A total of 952 outbreaks (range from twoto 39 cases) were investigated.  

The study examined the key characteristics of 384 public school buildings on Montreal Island, comprising 311 elementary schools, 69 high schools, and 4 combined elementary and high schools. These buildings accommodate, on average, 453 students and 140 staff members. Natural ventilation was the most common system, followed by mechanical H, hybrid, other mechanical system, and mechanical 100% fresh air. 

The average indoor CO2 concentrations in 2021 were 1135 ppm, with a median of 1050 ppm. Buildings with natural ventilation showed higher CO2 concentrations than those with mechanical systems (mechanical H systems, 100 % fresh air or other), with hybrid systems falling in between.