Do it yourself

Check your room in 5 minutes.

You can't argue with a number. Catch the dumb air in the act.

The method

Five steps

  1. Get a CO2 monitor. It costs about the same as a nice dinner, and it lasts years.
  2. Put it in the room during a normal meeting. Real people, doors closed, like always. Don't cheat by opening a window first.
  3. Watch the number climb through the meeting. Note the worst reading you see.
  4. Check it against the scale. Under 1000 is fine. Yellow or red means the room is making people dumber.
  5. Do something. Open a window, prop the door, thin out the room. And if it's a room you don't control, that's what the Fix It page is for. Fix It
Screenshot the worst reading with the time and how many people were in the room. That screenshot is your evidence.
The gear

Which monitor should you buy?

You need a real NDIR CO2 sensor, not a cheap "eCO2" estimate that guesses from other gases. That's the one rule. Here's the honest lay of the land:

The gold standard

Aranet4. Pocket-sized, e-ink screen, brutally accurate, batteries last months. What most of us actually carry. Pricey but the one to beat.

Our pick →

Best value

A solid NDIR desktop monitor (Qingping, SmartAir and similar). Two-thirds the price, plugs in, does the job on a desk.

Solid →

Tinkerer / open

AirGradient and other open-hardware builds. Great if you want to log data, self-host, and go deep.

Nerd out →
Full buyer's guide with current picks and prices is coming. We'll only ever recommend monitors we'd put in our own bag, and we'll always tell you when a link earns us a few cents, so we can keep this free.
Got a bad reading? Fix it → Read the scale again