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.
There is no acute toxicity at these levels. CO2 becomes dangerous only at the extreme end (>40,000 ppm, well above anything a household sensor would ever see). But there is a well-documented cognitive cost. Satish et al. (LBNL, 2012) found decision-making performance dropped meaningfully at 1,000 ppm and substantially at 2,500 ppm. A Harvard T.H. Chan follow-up confirmed it across nine cognitive domains.
CO2 is also the most useful proxy indoor metric: when CO2 climbs, ventilation is failing, which means every other indoor pollutant (VOCs, allergens, viruses) is also accumulating. ASHRAE 62.1 targets a fresh-air supply rate that keeps a typical occupied room under about 1,100 ppm. If yours sits higher, you have a ventilation problem.
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
