Outdoor air sits around 420 ppm CO2 in 2026 (and rising about 2.5 ppm per year). Indoors, every person in a room exhales roughly a kilogram of CO2 per day; in a sealed space, levels climb fast. Two adults in a closed bedroom can push CO2 past 2,000 ppm overnight.
CO2 readings can help assess ventilation, but a reading alone does not establish the cause of a headache or reduced concentration. Satish et al. (2012) reported differences in decision-making performance under controlled conditions; findings do not predict an individual response. Exposure level and duration matter. The NIOSH occupational guidance lists separate time-weighted, short-term, and immediately-dangerous exposure limits. Its 40,000 ppm IDLH value is not a boundary below which all exposure is safe, nor a household ventilation target.
CO2 is a useful contextual indicator of occupancy-related ventilation: when it climbs while people are present, fresh-air delivery may not be keeping pace. It does not track every other indoor pollutant or prove overall air quality. ASHRAE's technical guidance explains that, under typical office assumptions, a steady-state level about 700 ppm above outdoor air corresponds to roughly 15 cfm of outdoor air per person. That is a conditional indoor-outdoor relationship, not a universal 1,000 or 1,100 ppm limit.
What to do: open a window, open the door, run an HRV/ERV, or just leave the room for ten minutes. CO2 drops quickly with any of those. The sensor uses the Sensirion SEN66's integrated CO2 channel, which measures CO2 via photoacoustic infrared spectroscopy (Sensirion's "PASens" technology) and self-calibrates against outdoor baseline air whenever the room is ventilated.
References
- ASHRAE Standard 62.1 - Ventilation for Acceptable Indoor Air Quality www.ashrae.org
- Harvard T.H. Chan - CO₂ and cognitive function healthybuildings.hsph.harvard.edu
- NIOSH Pocket Guide - Carbon dioxide www.cdc.gov
- Satish et al., LBNL - Is CO₂ an indoor pollutant? doi.org
