Recent years have brought a sharper focus on the quality of air in our homes, offices, and schools, stemming from a better understanding of how indoor environments relate to our health. While air is invisible, we can use certain indicators to gain valuable insight into the relative safety of the spaces we occupy.
Carbon Dioxide as a Ventilation Proxy
Every time a person exhales, they release carbon dioxide into the surrounding air. In an indoor setting, this exhaled CO2 can accumulate if the space is not adequately ventilated. This accumulation serves as a useful indicator for how much fresh, outdoor air is circulating. When an individual with a respiratory virus is in the room, they are also exhaling virus-containing particles, known as aerosols, along with CO2.
If the air is stagnant, both carbon dioxide and infectious aerosols will linger and become more concentrated. A rising CO2 level therefore suggests a low rate of air exchange and a higher proportion of “rebreathed” air. The CO2 itself is not the hazard; it is an easily measured proxy that signals poor ventilation and an increased probability of encountering infectious particles.
Interpreting CO2 Monitor Readings
A carbon dioxide monitor is a device that measures the concentration of CO2 gas in the air, in parts per million (ppm). Most portable monitors available to the public use non-dispersive infrared (NDIR) sensors. These sensors work by passing infrared light through a sample of air; the amount of light absorbed at a specific wavelength reveals the quantity of CO2 molecules present. This provides a direct and immediate reading of the CO2 level in a room.
Understanding the readings involves comparing the indoor level to the baseline concentration of CO2 in outdoor air, which is around 420 ppm. A well-ventilated indoor space will have readings that are not significantly higher than this baseline. A CO2 level below 800 ppm is considered indicative of good air exchange with the outdoors, suggesting that any airborne contaminants are being effectively diluted.
As ventilation decreases, CO2 levels begin to rise. Readings between 800 and 1,000 ppm suggest that ventilation is moderate but could be improved. When concentrations climb above 1,000 ppm, it is a clear sign of poor ventilation. Levels exceeding 1,500 ppm warrant immediate action to improve airflow. These ppm values serve as practical guidelines to assess the relative risk of a given indoor environment.
Limitations of CO2 Monitoring for Virus Risk
While CO2 monitoring is a useful tool, it is not a foolproof method for assessing viral risk, and its limitations must be understood. A low CO2 reading does not automatically mean a space is free from risk, nor does a high reading confirm the presence of a virus. The measurement simply reflects the amount of exhaled air in a room, not the definite presence of infectious particles. The risk is dependent on an infected individual being in the space.
One significant limitation arises in spaces that use air filtration systems, such as those with high-efficiency particulate air (HEPA) filters. These filters are effective at capturing and removing tiny viral particles from the air. However, they do not remove the smaller gas molecules of carbon dioxide. Consequently, a room could have a high CO2 level, indicating poor fresh air exchange, but still have a relatively low concentration of viral aerosols because the air is being effectively cleaned.
The size and occupancy of a space also affect the reliability of CO2 readings. In very large areas like airport terminals or arenas, the CO2 exhaled by occupants can become so diluted that levels remain low even with minimal ventilation. This can create a false sense of security, as infectious aerosols could still accumulate in localized areas. CO2 monitoring is not a useful measure for outdoor environments, where exhaled gases dissipate almost instantly.