indoor air quality, ventilation systems, indoor spaces, schools, hvac system, co2, iaq, co2 sensors, air quality, ventilation rates, co2 levels

    CO2 Doesn’t Give Us the Whole Story About Air Quality in Schools – But It Tells Us a Lot

    By Michael Cavanaugh, vice president and general manager of building management solutions, Honeywell

    Concerns about COVID-19 transmission in schools have increased focus on carbon dioxide level as a critical proxy for indoor air quality (IAQ) – and with good reason. CO2 concentrations indicate how well a building space is ventilated relative to the number of occupants – a salient factor in gauging viral transmissibility.[i]

    Evidence shows that the virus can travel beyond six feet in tiny aerosol droplets released when infected people talk, shout, sing or just breathe.[ii] As CDC infection control guidelines confirm, diseases can spread faster in stale, compromised air, as the last two years have painfully demonstrated.[iii] As people exhale, CO2 levels increase in indoor spaces that aren’t well ventilated, often reaching concentrations far above the baseline level of outside air.

    While there’s currently no sensor that can detect airborne pathogens in real time, CO2 concentration can serve as a proxy. The sensor technology used to measure it is more sophisticated and more affordable than it was a few years ago, thanks in part to standards set by California’s Title 24 indoor air quality code, the most rigorous in the nation.

    Even before the pandemic, however, U.S. schools were facing an IAQ crisis. Title 24, enacted in 1999, gained new impetus from a 2020 report from the California Energy Commission, which found that 85% of 104 HVACs installed in California public schools between 2013 and 2016 did not meet Title 24 ventilation standards.[iv] Unacceptably high CO2 levels were detected in these classrooms, which can diminish students’ cognitive abilities as well as increase viral transmissibility.

    The problem is compounded in schools nationwide by the advanced age of their ventilation equipment. A 2020 U.S. GAO survey found that a third of American school districts – roughly 36,000 – have HVAC systems in their facilities that need to be updated or replaced.[v] Harvard’s T.H. Chan School of Public Health found that lower ventilation rates and increased concentrations of fine particulate matter (PM2.5) correlate with slower response times and reduced accuracy on cognitive tests.[vi] (There are sensors available that detect PM2.5, but CO2 can serve as a proxy for it as well if budgetary limitations dictate.) Conversely, multiple studies have documented a correlation between increased ventilation rates and better academic performance.[vii]

    While CO2 sensors don’t tell the whole IAQ story, they can detect whether a ventilation system is keeping carbon dioxide within acceptable limits based on occupancy levels in a space. If CO2 concentrations are acceptable at lower occupant densities but exceed the high threshold as density increases, it can mean the HVAC system is not adequately performing to meet current occupancy levels.

    Shortfalls can also stem from ventilation settings. Schools often rely on local codes to set baseline levels, but such codes don’t typically address all conditions – for example, current weather or occupant density in a particular classroom. Think of a school cafeteria, which can have drastic swings in occupancy, or a classroom that may have 10 students one period and 30 the next. Regardless of baseline settings, a CO2 sensor will alert staff if there’s a need to adjust the ventilation to increase fresh air ventilation.  

    CO2 sensors, however, also lay the groundwork for more dynamic ventilation, which leverages advanced building controls to put the fresh air where it’s needed when it’s needed. If a single, blanket level of ventilation is applied throughout an entire facility, it will be inadequate for some spaces and unnecessary for others. Excessive ventilation can waste energy and run up utility bills if the HVAC system has to work overtime to heat or cool and dehumidify an unnecessary volume of outdoor air.

    Honeywell’s newly released CO2 sensors – suitable for use under California’s Title 24 – have the ability to alert occupants to an unacceptable level of CO2, giving school superintendents the needed information to adjust ventilation rates. While perhaps not an ideal solution, as it is prone to human inefficiency, it’s a step in the right direction.

    CO2 sensors can have greater impact if they are integrated into an HVAC or building management system to help enable dynamic, condition-based ventilation that responds in real time based on sensor data. Using highly accurate non-dispersion infrared technology, the new wall-mounted, self-calibrating Honeywell sensor series also allow programmable CO2 threshold setting in each classroom or building space. This not only provides a healthier IAQ solution for schools but also measurably reduces utility bills, effectively balancing energy efficiency with classroom comfort.


    [i] Washington Post, The coronavirus is airborne. Here’s how to know if you’re breathing other people’s breath, Chris Mooney, February 10, 2021. [Accessed March 2, 2022]

    [ii] Proceedings of the National Academy of Sciences, Mechanistic transmission modeling of COVID-19 on the Diamond Princess cruise ship demonstrates the importance of aerosol transmission, Parham Azimi et al., February 3, 2021. [Accessed March 2, 2022]

    [iii] Centers for Disease Control and Prevention, Infection control: how infections spread. [Accessed March 2, 2022]

    [iv] California Energy Commission, Improving ventilation and indoor environmental quality in California schools, July 24, 2020. [Accessed March 2, 2022]

    [v] U.S. General Accountability Office, “K-12 education: School districts frequently identified multiple building systems needing updates or replacement,” June 4, 2020. [Accessed March 2, 2022]

    [vi] Harvard University, T.H. Chan School of Public Health, “Schools for health: foundations for student success, Erika Eitland and Joseph Allen, 2017. [Accessed March 2, 2022]

    [vii] Harvard University, T.H. Chan School of Public Health, “Schools for health: foundations for student success, Erika Eitland and Joseph Allen, 2017. [Accessed March 2, 2022]