In many discussions online, the topic of monitoring Carbon Dioxide (CO2) in indoor spaces has become prominent. At CETEC, where I work, we routinely engage in this practice. In this post, I aim to shed light on why measuring CO2 levels is crucial, what it reveals, and the associated limitations.

Understanding Carbon Dioxide (CO2): CO2 is a gas naturally present in the air, with an outdoor concentration of about 420ppm. In everyday indoor environments like offices and homes, it mainly stems from human and animal respiration. When spaces lack proper ventilation, the CO2 concentration can rise, impacting occupants with effects ranging from reduced cognitive function to sleepiness.

CO2 as a Ventilation Proxy: Given that people produce CO2 and we know the outdoor concentration, we use CO2 as a proxy to assess indoor ventilation levels. If indoor CO2 levels match those outdoors when many people are present, it indicates good ventilation. Elevated levels suggest insufficient ventilation, a crucial factor in preventing pollutant buildup and the spread of diseases like SARS-CoV-2. For instance, if outdoor CO2 is 415ppm and indoors it's 800ppm, about 1% of the air you breathe would be someone else's exhaled breath in a typical office.

Why Carry CO2 Monitors: People now carry CO2 monitors to gauge ventilation levels in different environments. However, there are limitations to this approach:

  1. Dependency on CO2 Source: You can only determine ventilation levels if there are people in the space. Secondly there may be sources other than people which may affect the result.
  2. Limited Air Quality Insight: CO2 is solely a proxy for ventilation and doesn't indicate air quality. For instance, a space with HEPA-filtered air may have high CO2 levels but low risk for infections like COVID.
  3. Sensor limitations: sensors may experience drift over time, or may have biases leading to incorrect results.
  4. Equilibrium: it may take some time for CO2 to stabilise and reach equilibrium.

While CO2 measurements offer insights into fresh air circulation, they don't consider recirculated and filtered air, potentially leading to misconceptions about ventilation adequacy. Nevertheless, CO2 remains a valuable tool for estimating the influx of fresh air. Elevated levels in offices or homes may signify increased pollutant buildup and infectious disease risk, while in a plane, it might hint at limited external ventilation without necessarily implying higher risk.

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