Ventilation and CO2 Sensors

What a CO2 monitor is really measuring

Ventilation is the amount of air moving in and out of a building. EPA describes it as replacing indoor air with fresh outdoor air. [1] A home CO2 monitor measures carbon dioxide gas, which is used in school ventilation standards as a stand-in: those standards are often based on how far indoor human-generated CO2 sits above outdoor levels. [2]

So the monitor reports one gas (a direct measurement with some error). Whether that means "good air" is an interpretation. A CO2 monitor does not measure carbon monoxide, which is a different toxic gas. [3] It also does not measure particles from cooking or fumes from products, which EPA names as common indoor sources. [4]

How accurate are low-cost CO2 sensors?

• 23 sensors tested together (2026). Twenty-three low-cost infrared sensors had strong linearity, but the average error before correction was 18.3 ppm, and 0.45% of readings fell outside the makers' stated accuracy. After a correction method the error fell to 7.4 ppm at outdoor-level concentrations. [2] The sensors varied from one another, which matters when you compare two rooms with two units.
• One sensor type, calibrated (2024). For one specific sensor type (SenseAir K30) in a purpose-built instrument, calibration against standard gases brought errors down to a few ppm, and the authors suggested recalibrating at least every three months to stay within 3 ppm. [5] That applies to that instrument, not to every consumer monitor.
• Review (2026). Across 24 studies, performance varied with humidity and temperature, and sensor aging was not well addressed. [6]
• Standards. A test method for evaluating indoor CO2 sensors exists (ASTM D8624-25), according to EPA. [7] We did not read whether any consumer model has been tested to it.

Does ventilation affect health?

• Symptoms and illness (2011 review). A multidisciplinary review of studies through 2005 found higher office ventilation, up to about 25 liters per second per person, was associated with fewer sick-building-syndrome symptoms. Limited data suggested more respiratory infections, asthma symptoms and short-term sick leave at lower ventilation. [8] These are mostly observational associations. Label: promising but limited.
• Thinking tasks (2020 review). A review of 37 experiments found CO2 at levels common indoors affected only high-level decision-making in non-specialized people. Lower ventilation, with other pollutants accumulating, could slow some functions without reducing accuracy. Test methods and exposure doses varied a lot. [9] Label: mixed.
• Viruses. EPA says that increasing ventilation is not sufficient on its own for preventing the spread of respiratory viruses. [1]

We do not give a single "good" CO2 number. The sources above do not establish one that applies to every room, person and task.

Practical points from EPA

Opening windows and doors, running exhaust fans, or running a window air conditioner with the vent open can raise outdoor-air ventilation. [1] Avoid bringing in outdoor air when outdoor pollution or humidity is high, or in extreme conditions. [1] Be careful where outdoor sources such as smoke are nearby. [10] In many homes, forced-air heating and cooling do not bring fresh air in mechanically. [10] Check outdoor air pollution first (EPA points to AirNow). [1]

Related guides

• Indoor air quality and consumer sensors
• Air purifiers: claims and evidence
• Environmental comfort, exposure measurement and health outcomes