Photograph of a 120Vold carbon monoxide detector deviceCarbon Monoxide Gas Alarm Causes
Why does the CO alarm keep going off? Response Levels for Current & Obsolete Carbon Monoxide Detectors.

  • CO ALARM CAUSES - CONTENTS: What makes a CO alarm or carbon monoxide detector go off? At what levels do carbon monoxide detector alarms sound? What are the current CO detector alarm sensitivity standards? What caused nuisance CO alarm sounding or tripping on older CO detectors. What should you do if your CO detector alarm goes off?
  • POST a QUESTION or READ FAQs about the behavior of CO detectors: what sets off carbon monoxide detectors, and is it always CO?
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Carbon monoxide CO alarm sound-off troubleshooting:

this document discusses the causes of CO alarms going off - when a carbon monoxide alarm sounds you should assume there is dangerous carbon monoxide gas (CO) present. But other things might set off some alarm and not all of them are CO hazards.

Beware that the production of dangerous carbon monoxide gas in a building is usually not constant - it can start and stop. So even if someone tests and does not find CO gas present, especially if your CO alarm has been sounding, you can NOT assume that conditions are safe in the building, and further expert visual inspection of heating equipment, chimneys, etc. are in order.

For example, simply closing the door to a boiler room where gas fired equipment is operating can cause sudden production of CO gas if there is insufficient combustion air when the door is closed. Yet when someone opens the door to inspect the area, more combustion air is provided. CO production may stop.

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What Makes a CO Detector Alarm Go Off

IF YOU SUSPECT CARBON MONOXIDE POISONING GO INTO FRESH AIR IMMEDIATELY and get others out of the building, then call your fire department or emergency services for help.

Seek prompt advice from your doctor or health/safety experts if you have any reason to be concerned about exposure to toxic gases. Carbon monoxide poisoning can be fatal but exposure at lower limits can produce flu-like symptoms and headaches that are often mistaken for ordinary illness.

Carbon monoxide detector alarms may sound for a variety of reasons, but until you have diagnosed for sure why a particular alarm has sounded, you should assume that it has detected dangerous carbon monoxide indoors and you should follow our safety advice above.

Here are some causes of Carbon Monoxide Detector alarms sounding:

All of these conditions are dangerous. Follow our safety advice above

Watch out: never ignore the sounding of a CO or carbon monoxide detector alarm. As our fire expert has pointed out:

2015/09/19 NHFirebear said:

...Older CO detectors were designed to an earlier "occupational" exposure limit and would alarm at a level where daily exposure becomes a health hazard. The UL and similar standards were amended to PROHIBIT alarms at less than a "dangerous level", i.e., that which can quickly make a person sick (under 100 ppm).

Therefore, it is NEVER acceptable to ignore a CO alarm or disconnect it to make it stop.

Indeed there was such a frequent issue with CO detector alarms sounding at very low levels that there developed a "boy who cried wolf" problem: people became habituated to ignore a carbon monoxide detector's alarm sound, just shutting it off. For example some gas fired equipment will emit a harmless "burp" of flue gases at initial start-up: a burp that could trigger the older, sensitive CO detectors.

Modern CO detectors integrate both the level of carbon monoxide and the duration of its sensing to cross an alarm sounding threshold that should never be ignored.

Carbon Monoxide Detector Methods

CO detectors use a range of detection methods depending on the detection requirements such as the response time required and the level of sensitivity required. The carbon monoxide detection technology in a highly-sensitive CO detection instrument used by a building investigator or an industrial hygienist will typically be far more sensitive than the detection methods use in a home CO detector or in a badge-type detector used to protect workers. Some CO sensor types include:

MOS: metal-oxide semiconductor chip CO detectors: circuitry on an electronic chip detects CO by lowered electrical resistance across a sensor, causing an alarm to sound.

Electrochemistry: electrodes in a chemical solution respond to CO by detecting a change in the conductivity of the solution that has in turn been affected by CO.

CO-sensitive gel: used in badge type sensors, this "biometric CO detector" gel changes color after absorbing carbon monoxide. A human observes the color and interprets the meaning based on the badge design.

Safety Suggestions: Install Carbon Monoxide Detectors in addition to Smoke Detectors

Carbon monoxide detectors are inexpensive and readily available, both as a battery-operated unit and as a unit that plugs into an electrical outlet in the home.

No home should be without this safety protection, and homes with gas-fired equipment (natural gas or LP propane), space heaters, or other sources of risk should be extra cautious. Smoke detectors do not protect against carbon monoxide poisoning, and the opposite is also true. Carbon monoxide detectors do not warn of smoke or fire.

What Level of CO Causes a Carbon Monoxide Alarm to Go Off ?

Underwriters Laboratories Inc. Standard UL2034 requires residential CO Alarms to sound when exposed to levels of CO and exposure times as described below. The Carbon Monoxide level is measured in parts per million (ppm) of CO over time (in minutes).

Table 1: Current UL2034 Standard Requirements for Carbon Monoxide Detector Alarm Response

Carbon Monoxide Level in PPM [2] Number of Minutes Until Alarm Must Sound Comments
400 ppm of CO Averaged or responding to CO at this level detected between 4 and 15 minutes Average CO level for less than 15 minutes
200 ppm of CO Averaged up to 35 minutes Average CO level for less than 35 minutes
150 ppm of CO between 10 and 50 minutes  
100 PPM of CO Averaged for 90 minutes Average CO level for less than 90 minutes
70 ppm of CO between 60 and 240 minutes  
30 ppm of CO NO response for 30 days of exposure at 30 ppm Avoids nuisance sounding of the detector. See Table 2: Obsolete CO Detector Limits


[1] We have combined CO sensitivity levels from several sources for this table. Most sources cite the 100, 200, and 400 ppm limits for CO given in the table above. Others such as BRK (cited below) give a more detailed explanation of how the CO detector is designed.

[2] Approximately 10% COHb exposure at levels of 10% to 95% Relative Humidity (RH).

[3] Some companies produce CO detectors that will respond, for example by a blinking light, at lower CO levels down to 10-25 ppm as "peak limits", that is, the detector will respond to a brief surge of CO. Responses at this level may cause nuisance calls as areas of high vehicle traffic and use of some gas fired heating equipment can certainly cause brief surges of CO at these levels.


  • CO EXPOSURE LIMITS at Includes tables of permissible carbon monoxide exposure levels, air quality index AQI levels, PELs, TLVs, and effects.
  • Wikipedia 2015
  • BRK Electronics, "What Levels of CO Cause an Alarm", BRK Brands, Inc. 3901 Liberty Street Road Aurora, Illinois 60504 Tel: (630) 851-7330, The company has offices in Australia, Canada, Columbia, Europe, Malaysia, Mexico, New Zealand, Nigeria, Peru, and the United States - retrieved 20 Sept 2015, original source:
  • Rossol, Monona, "Data Sheet: Carbon Monoxide & CO Detectors", Monona Rossol, Health and Safety Director United Scenic Artists, Local USA 829, IATSE 181 Thompson St., # 23 - New York, NY 10012-2586 212-777-0062 E-mail: February 4, 1998 revised 1/6/05, 12/18/07, 4/6/08
    - retrieved 20 Sept 2015 original source:

Obsolete CO Detector Limits

Table 2: Obsolete CO Detector Limits & Response Times

Carbon Monoxide Level in PPM Number of Minutes Until Alarm Must Sound Comments
100 ppm 16 Minutes CO Level is averaged over the response period
60 ppm 28 Minutes
16 ppm 30 Days


Examples of these now-obsolete CO detectors that responded in this range were the FirstAlert7® and the Nighthawk7® Carbon Monoxide Detectors.


  • DATA SHEET CARBON MONOXIDE & CO DETECTORS Monona Rossol, Health and Safety Director United Scenic Artists, Local USA 829, IATSE 181 Thompson St., # 23 - New York, NY 10012-2586 212-777-0062 E-mail: February 4, 1998 revised 1/6/05, 12/18/07, 4/6/08
    - retrieved 20 Sept 2015 original source:


Response Levels of CO Detectors vs. Human Breathing

Reader Question: my doctor says my breath is triggering the CO monitor

21 July 2015 Kari M said:

After several months of seeing a new doctor, my regular visits showed my blood oxygen to be so low that it often wouldn't register or would be in the low 80% range. I also pointed out blue nail beds and toenail beds and pitting edema of my calves, ankles and feet. I have never smoked and am 53 years old.

She asked it I had a carbon monoxide detector in my home. I said I did, but why would that matter. She claimed that if I am breathing in my own carbon dioxide (that I exhale) it would cause the CO monitor to go off. This does not make sense to me as the two are the result of different ways of eliminating oxygen, resulting in different chemical change. Can you clarify? Thanks. Kari

Reply: baloney: human breath will not set off a CO detector


I'm not sure who "she" is, but human breath should not make a home carbon monoxide detector alarm sound. However there are numerous studies reporting elevated levels of CO in the breath of people suffering from a variety of illnesses including asthma.

And we may be confusing CO2 (carbon dioxide) with CO (carbon monoxide).

Exhaled human breath is mostly nitrogen - 78.04% and oxygen (13.06-16%) and contains about 4-5% (by volume) more CO2 than we breathed-in and less Oxygen than we breathed in.

Exhaled breath contains about 4.8-5% CO2. Note that this is carbon dioxide (CO2) - not carbon monoxide (CO). Those are different gases.

The remaining about 1% of gases in human breath are insignificant, such as Argon and other gases.

What is the level of carbon monoxide in human breath?

However smokers may indeed have carbon monoxide (CO) in their breath at levels of about 16.4 ppm. (0.00164 percent) and Zayasu et als reported elevated levels of CO in asthmatic patients (Zayasu 1997). In our references just below you'll see elevated CO levels reported for patients with other illnesses as well. For perspective, the level of carbon monoxide in cigarette smoke itself can be in the range of 500 - 1000 ppm (Rossol, 2005).

As we explained earlier in this article, the response point of a carbon monoxide detector is not a fixed number since most units will respond faster to higher levels (immediately to high levels) and may take many minutes to respond to a low level of CO in the area being sensed or monitored in a building.

The response time and response-level of a carbon monoxide detector used as a building safety alarm varies by model. But in general, at 400 ppm of CO a Carbon monoxide detector will respond in minutes and at 30 ppm or below the CO detector is designed not to respond for 30 days.

Since the level of CO in human breath would be expected to be well below 30 PPM, and since CO Alarms are generally designed not to respond to constant CO levels at (or below) 30 ppm over a duration of 30 days, human breath should not cause a CO detector alarm to sound.

You can see that there is a large difference between the maximum CO that a human might exhale and the response level of CO to which a CO detector will respond. In fact Home CO detectors made to current UL Standards (in the US) or Canadian Standards in Canada (CSA) are prohibited from showing CO levels below 30 ppm on digital displays - perhaps in part because of the cost, misleading effects, and impact of nuisance calls.

Therefore to whoever "she" is, I can only say about the claim that the CO2 (carbon dioxide) level in your breath is triggering a CO (carbon monoxide) detector alarm is nonsense. I must hope that this was a misunderstanding between you and your doctor and that you will discuss your concerns with her again.

It is possible to detect lower levels of carbon monoxide in buildings, but not using a home CO detector alarm. Separately, a hand-held portable gas detector or CO detection lab or field investigation instrument used by building environmental test experts, some home inspectors, some industrial hygienists, (Bachrach Instruments, Draeger Instruments, Fluke, Industrial Scientific, KWJ Engineering, Honeywell, RKI Instruments, Sensorcom, Quantum Group, and Zellweger) can respond to CO levels in air down to just a few parts per million (PPM).

IF YOU SUSPECT CARBON MONOXIDE POISONING GO INTO FRESH AIR IMMEDIATELY and get others out of the building, then call your fire department or emergency services for help.

References on CO detection levels & components of human breath

Carbon Monoxide Gas Articles


Continue reading at CO EXPOSURE LIMITS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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