Carbon dioxide monitoring in schools

Carbon dioxide monitoring in schools
Posted on 04/28/2021
Carbon dioxide monitoring in schools

You asked - Carbon dioxide monitoring in schools

We’ve received some questions about carbon dioxide (CO2) levels and monitoring in schools. We have connected with our HVAC experts and wanted to share some information with you.

How are CO2 levels linked to the possible transmission of COVID-19?

ASHRAE, or the American Society of Heating, Refrigerating and Air-Conditioning Engineers, is a professional association seeking to advance heating, ventilation, air conditioning and refrigeration systems design and construction. This organization provides guidance and sets standards that institutions, such as schools, follow to ensure safe ventilation systems.

While Public Health Ontario notes "CO2 is not an indicator of COVID-19 transmission risk," high indoor CO2 can assist in identifying locations where there may be poor ventilation. According to ASHRAE, ensuring appropriate ventilation and filtration are the most effective measures to reduce the airborne transmission risk.

If there is a potential that ventilation rates do not meet ASHRAE recommended values due to the limitations of the existing systems, OCDSB Facilities team have increased filtration to compensate. HEPA filtration will filter out bacteria and viruses, but will not affect CO2 levels. So in poorly ventilated rooms with HEPA filtration systems, the CO2 levels may be higher, but the airborne transmission risk is low because of the HEPA filtration.

Most of our schools have CO2 monitoring performed on an ongoing basis through our Building Automation Systems. Before COVID, CO2 levels were used to balance ventilation rates throughout the day (ie: increase ventilation when CO2 levels are high and decrease ventilation rates when CO2 levels are low). 

When COVID started, we adopted the ASHRAE recommendation to remove CO2/demand-based ventilation controls. What this means is that while CO2 levels are still being monitored, CO2 levels have been disconnected from ventilation rates, which are set at higher levels to encourage the flow of fresh air through schools. 

But what about portable CO2 testing machines?

We have received questions about CO2 readings in classrooms taken by portable measuring devices.

While it’s difficult to comment on specific devices, it is generally accepted amongst HVAC professionals that CO2 metering/sensing devices are inherently inaccurate and prone to fall out of calibration easily. Even a high-quality hand held device must be professionally recalibrated at least once per year. At the OCDSB, we have invested in self-calibrated CO2 sensors in our schools to obtain the most accurate readings practical. 

How are CO2 levels set in schools?

Before COVID, we were required to comply with ASHRAE Standard 62 "Acceptable Ventilation for Indoor Air Quality". Essentially the standard provides ventilation rates required to meet the desired CO2 level of 700 parts-per-million (ppm) above ambient (outdoor) levels.

The tested ambient levels at our schools are approximately 450 ppm. This is the basis for our standard setpoint for CO2 levels in classrooms of 1100 ppm. At this ventilation rate, 95% of airborne contaminants are eliminated.

As part of their COVID measures, ASHRAE recommended increasing ventilation rates beyond this standard as much as practical for each system. We have done this and are seeing lower CO2 levels in our schools. 

The graph below is from a typical classroom air handling unit (from Vimy Ridge Public School, taken in April 2021). The 1-day trend of CO2 levels shows that during our purge cycles (before and after school) the CO2 levels are consistently at 450 ppm, or the same as the outdoor air. During classroom hours the CO2 levels rise but are kept in the 650-750 ppm range because of our increased and constant ventilation throughout the day.


Regardless, as mentioned earlier, CO2 levels recorded do not mean a heightened risk of COVID-19, with filtration support added for classrooms that may record higher CO2 levels. 

With additional funding received from the federal and provincial government, we are providing enhanced filtration for all schools, even the well-ventilated schools. With the low CO2 levels and enhanced filtration we are able to ensure a very low airborne transmission risk.

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