I've got a Carbon monoxide detector hanging in my kitchen for a few years now. Lately I've noticed it reads a PPM of 0 (zero). Is this right? Doesn't the air contains around 400 ppm?

In other words, is my detector even working? I'm getting the feeling I should throw it out.

  • 9
    Maybe you are confusing carbon monoxide and carbon dioxide? The latter is normally present in air around the concentration you mention.
    – peterG
    Commented Dec 28, 2019 at 12:53
  • @peterG, Yes, you are absolutely right. I've mixed them up when I was asking Doctor Google...
    – Tim
    Commented Jan 2, 2020 at 10:45
  • Thanks everyone. I'll see if I could do a small DIY test and if not I'll buy, at least, a better brand. Just to get some peace at mind.
    – Tim
    Commented Jan 2, 2020 at 10:46
  • I’ve always tested mine by taking the CO detector out to the driveway and holding it up to the tailpipe while the car idles. This by definition will set off the alarm. If it’s a calm day, holding one detector in each hand and bringing them up slowly a few times always made me feel better about their relative sensitivity.
    – newcoder
    Commented Jun 8, 2020 at 15:31

4 Answers 4


As stated in @jwh20 's answer, the question is incorrect - 400ppm isn't a commonly occurring level - but it's a great question in that it brings up testing your alarm.

I think it's a legitimate concern. If something is important, it's important to test that thing. The self-test button is better than nothing, but it does nothing to test the sensor, it is a self test of the electronics. (And in general you can never really trust a self-testing device - if device has failed or is defective, how do you know the failure or defect doesn't affect the self-test?)

You can do a crude test by making some smoke with an incense stick or a cigarette or something and flooding the sensor with smoke. Obviously not a great test but it might at least show that the detector sees something.

Doing any better gets complicated. To test the ppm level display on the alarm's LED readout, you'd have to expose it to CO in a controlled environment and compare the alarm's readout to a calibrated CO meter. That's going to be well beyond DIY.

  • I always just held the CO detector near the tailpipe of an idling car. Worked consistently.
    – newcoder
    Commented Jun 8, 2020 at 15:27

No, 400 ppm is NOT a safe level. Here is the US Govt. CPSC information on CO:

CPSC CO Information

To quote the relevant part:

What CO level is dangerous to my health?

The health effects of CO depend on the CO concentration and length of exposure, as well as each individual's health condition. CO concentration is measured in parts per million (ppm). Most people will not experience any symptoms from prolonged exposure to CO levels of approximately 1 to 70 ppm but some heart patients might experience an increase in chest pain. As CO levels increase and remain above 70 ppm, symptoms become more noticeable and can include headache, fatigue and nausea. At sustained CO concentrations above 150 to 200 ppm, disorientation, unconsciousness, and death are possible.


Yes, zero ppm CO is quite possible, especially if your home is very drafty or has decent ventilation. If I remember correctly, anything over 30 ppm CO calls for evacuating the occupants to fresh air. If you question your CO reading contact your local fire department or someplace that services smoke alarms and fire extinguishers. My local FD responds to possible CO/CO2/natural gas calls in case oxygen is needed and carries a meter on their truck. The FD and smoke/fire equipment places were able to either test the equipment or help me narrow such a search.


Reading zero should be normal. If you can carry it to the right authority they might be able to test it, or tell you when is a good time to test everyone's at the same time.

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