People breathe out CO2, so in a closed room it climbs as long as someone is there. That makes CO2 the simplest gauge of ventilation: if it is high, the air in the room has not been replaced, and whatever else is in it (moisture, odors, chemical vapors, airborne germs) has built up too.
The sensor matters more than the brand
Buy a monitor with an NDIR (non-dispersive infrared) sensor. It measures CO2 directly. Many cheap "air quality" devices show "eCO2", which is calculated from a VOC sensor and can read high or low by hundreds of ppm. The spec sheet or listing usually says which one it has.
What the numbers mean
- About 420 ppm: outdoor air.
- Under about 800 ppm: a well-ventilated room.
- About 1,000 ppm and up: a common rule of thumb for "open a window or turn on ventilation". It is a ventilation marker, not a health limit.
- 5,000 ppm: the workplace exposure limit for an 8-hour day. Homes rarely get near it.
Does CO2 make you think worse?
A widely cited 2016 Harvard study (Allen and colleagues) found lower decision-making scores in office workers at higher CO2 and VOC levels. It was a small lab study, about 24 people over several days, and later studies testing CO2 alone have had mixed results. The safer takeaway: high CO2 means poor ventilation, and better ventilation is worth having for many reasons.
Bringing it down
- Crack a window or door, especially in bedrooms overnight.
- Run bathroom and kitchen exhaust fans, and use your HVAC fresh-air setting if it has one.
- A purifier does not lower CO2. Only fresh air does.