The short guide to CO2 readings
You don't need to be a scientist to use this site. Here's what you should know.
Why CO2?
People breathe out CO2. When fresh air comes into a room, CO2 stays low. When fresh air doesn't come in, CO2 builds up, and so does whatever else is in the air people are exhaling, including germs.
CO2 itself isn't usually dangerous at the levels found indoors. We use it as a stand-in for ventilation. A low number means the air is being shared well.
What the colors mean
Under 800 ppm
Air is being shared well.
801–1200 ppm
Air is getting a little stale.
1201–2000 ppm
Not much fresh air coming in.
Over 2000 ppm
Very little fresh air. Many people may feel it.
Outdoor air sits around 420 ppm. A well-ventilated indoor room typically stays under about 800 ppm. These zones are a community convention informed by ASHRAE's 2025 Position Document on Indoor Carbon Dioxide, Health Canada's 1000 ppm long-term residential guideline, and the scale popularized by community CO₂ meters like Aranet. ASHRAE notes there is no single health-based CO₂ threshold, these numbers describe ventilation quality, not toxicity. For workplace exposure, OSHA's permissible limit is 5,000 ppm over an 8-hour shift.
How to read a reading
- Look at when it was taken, a busy Friday night is different from a quiet Tuesday morning.
- Check how full the room was. Empty rooms run lower no matter the ventilation.
- A single reading is one data point. Patterns matter more than any one number.
Taking your own reading
Use any common CO2 meter (Aranet, Vitalight, and many DIY meters work fine). Let it settle for about a minute before recording. Hold it near where people sit, away from your own breath.
This isn't medical advice
These zones help you make informed personal choices. They aren't a diagnosis of risk. Anyone making health decisions should consider their own situation and consult a professional when needed.