Ensuring a healthy indoor environment is a growing priority, and carbon dioxide (CO2) measurement is an accessible way to assess indoor air quality. This colorless, odorless gas is a natural byproduct of human respiration. Its concentration acts as a reliable proxy for how well a space is ventilated. Monitoring CO2 levels provides insight into the air exchange rate, revealing whether a space receives enough fresh air to dilute contaminants generated by people and activities inside the home, reflecting overall indoor air quality.
Why Monitoring Indoor CO2 Matters
Monitoring indoor CO2 primarily checks ventilation effectiveness, which impacts comfort and cognitive function. In poorly ventilated spaces, the CO2 people exhale accumulates, often alongside other pollutants. Elevated CO2 levels can affect occupants, even at concentrations commonly found indoors. Studies have linked higher concentrations to measurable impacts on the brain’s ability to function.
Concentrations exceeding certain thresholds can lead to noticeable effects, including decreased decision-making ability and reduced attention span. Occupants may experience fatigue, drowsiness, and difficulty focusing as CO2 levels rise. For instance, levels exceeding 1,000 parts per million (PPM) have been associated with impaired cognitive function. Measuring CO2 provides an actionable metric to ensure the air is exchanged frequently enough to maintain a healthy environment.
The Technology Behind Home CO2 Sensors
Most consumer-grade home monitors rely on Non-Dispersive Infrared (NDIR) technology to measure carbon dioxide concentration. An NDIR sensor operates on the principle that CO2 molecules absorb infrared (IR) light at a specific wavelength, around 4.26 microns. Inside the sensor, an IR light source directs light through an air sample chamber toward a detector. The more CO2 present, the more IR light is absorbed, meaning less light reaches the detector.
The sensor measures the reduction in transmitted light intensity and converts this measurement into a concentration reading, typically displayed in Parts Per Million (PPM). This method is highly specific to CO2 because other common indoor gases and even water vapor do not absorb light at the same wavelength, which helps ensure accuracy. Proper placement of the monitor is important for representative readings. The device should be placed in the “breathing zone,” three to six feet above the floor, and kept away from drafts, windows, or vents that could skew the measurement.
Many quality monitors feature Automatic Baseline Correction (ABC) to maintain long-term accuracy. The ABC function assumes the indoor CO2 level will drop to the outdoor ambient level at least once every few days, typically overnight. The sensor uses this lowest reading as a baseline for calibration, correcting for any small drift over time.
Interpreting Your CO2 Measurement Readings
Carbon dioxide measurements are expressed in PPM, or parts per million, and understanding the benchmarks allows for clear interpretation of indoor air quality. The typical outdoor CO2 concentration, which serves as the baseline, is currently around 420 PPM. In a well-ventilated space, indoor readings should remain close to this outdoor level, generally falling within the 400 to 800 PPM range.
An indoor reading consistently between 800 and 1,000 PPM suggests that the ventilation is acceptable but could be improved, often indicating a slightly stuffy environment. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends that indoor CO2 levels not exceed the outdoor concentration by more than 650 PPM, translating to a maximum of approximately 1,070 PPM. Readings climbing above 1,000 PPM signal poor air exchange and prompt immediate action.
Concentrations ranging from 1,000 to 2,000 PPM are associated with increased complaints of drowsiness, fatigue, and poor concentration among occupants. If your monitor displays levels approaching or exceeding 1,500 PPM, it indicates a significant ventilation problem that requires urgent attention. The primary response to elevated CO2 levels is to increase the introduction of fresh outdoor air, such as by opening windows or doors, or by increasing the fan speed on the HVAC system.