Carbon Monoxide Levels & Standards

While everyone agrees that high carbon monoxide levels are very dangerous, there is no consensus about how much is too much. There is also scant attention paid to low carbon monoxide levels.  We will attempt to shed some light on this topic and give you enough information so that you can make an informed decision about how much is too much for yourself and your family.

Carbon monoxide is a gas that mixes with air and is breathed into your lungs. The amount of CO present in the air is measured in parts per million (ppm).  By comparison, 1% is equal to 10,000 ppm.  It doesn’t take many ppm of CO to start causing real problems. Let’s see what the organizations which are charged with protecting us have to say about carbon monoxide levels.

What Are the Established Carbon Monoxide Levels of Exposure?

Today, we have many organizations that set standards and exposure limits for a broad range of things, including toxic OSHA NIOSH ACGIH Logoschemicals. In the USA, the Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health (NIOSH) set federal standards for exposure to carbon monoxide in the workplace, as does the non-profit American Conference of Governmental Industrial Hygienists (ACGIH), whose Threshold Limit Values do not carry the weight of law. However, as the Illinois Department of Public Health points out in their Guidelines for Indoor Air Quality, “… these occupational standards are NOT appropriate for the home environment. People spend more time in their homes and more susceptible persons, such as children and the elderly, may be present.” The upshot of this is that the acceptable level of CO in the home should be lower than that for workplaces.

The US Environmental Protection Agency (EPA) has established the National Ambient Air EPA ASHRAE logosQuality Standards for Carbon Monoxide, but this is generally interpreted as applicable only to outdoor air.  They have no other standards for indoor carbon monoxide levels. The only US organization that specifically addresses the carbon monoxide level inside residential spaces is the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) in their Standard 62.2-2013, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.

Outside of the US, the World Health Organization (WHO) has established guidelines for WHO logoindoor air quality for several pollutants, including carbon monoxide. In their publication “WHO Guidelines for Indoor Air Quality for Selected Pollutants”, they recognize the dangers posed by chronic low-level exposure to carbon monoxide, and their recommended exposure levels are much lower than those found in the US.


What Are Tolerable Carbon Monoxide Levels of Exposure?

The amount of carbon monoxide that is tolerable varies greatly from individual to individual, but in all cases, the amount is a function of the concentration of CO you’re exposed to and the amount of time you are exposed. Higher concentrations poison you faster than lower concentrations. All of the guidelines and standards take this into account by stating a CO concentration as an average over a stated period of time.

For example, the OSHA Permissible Exposure Limit (PEL) is a time-weighted average (TWA) of 50 ppm over an 8-hour period.  This corresponds to a normal workday in the US, as this is an occupational standard. This means that the carbon monoxide level may go up and down throughout the period, but the overall average should not exceed 50 ppm over an 8 hour period. So let’s see what the various organizations have to say about carbon monoxide exposures.

Carbon Monoxide Exposure Levels Exposure Levels for Carbon Monoxide

There are some interesting points in the above table. First, there seems to be some agreement about the carbon monoxide level in indoor air over an 8-hour period. There appears to be no agreement about what constitutes an acceptable level in the workplace, with the OSHA limit twice that of the ACGIH. Third, the WHO is the only organization that defines a limit for a 24-hour exposure, which is a very common occurrence with house-bound people, such as the very young and the very old. As with everything, you must decide what is best for you, but these are the standards that currently exist.


Will My Detector Protect Me At These Carbon Monoxide Levels?

That depends, but in most cases, the answer is NO! The reason lies in the standards that UL Listed logomost carbon monoxide detectors are built to comply with. Those standards are the Underwriters’ Laboratories UL 2034 in the US, and the Canadian Standards Association CSA 6.19-01 in Canada. There are 40 US states that now mandate that UL-listed CO detectors be installed in new homes, and some jurisdictions in the rest of the states CSA logohave this same mandate (created by requiring NFPA 720). In Canada, only the Yukon and the province of Ontario mandate CSA-listed CO detectors. The UL standard and the CSA standard were developed jointly and contain identical requirements. So, let’s see what a UL listed CO detector is going to do.

UL2034/CSA6.19-01 Standards Explained

These standards were developed in the early 1990s, and the early commercial detectors that came on the market produced a lot of false alarms. As a result, the standards were significantly relaxed to minimize the false alarms. Unfortunately, as we will see, the current standard only protects the healthiest adults among us, and does not protect us when the CO level in our homes exceeds the exposure limits we looked at earlier.

So when will a UL/CSA-listed carbon monoxide detector alert us to dangerous levels of CO being present? Here’s the summary:

  • 0 – 29 ppm  The detector must remain silent. If it has a digital display, it
    must show a zero reading. It may show the actual reading, but
    only if the user presses a button.
  • 30 – 69 ppm   If the carbon monoxide level remains in this range for 30 days, the audible alarm may sound. If a digital display is present, it should show the actual CO level as long as it is 30 ppm or higher.
  • 70 – 149 ppm   If the carbon monoxide level remains in this range for 1 to 4 hours, the alarm must sound.
  • 150 – 399 ppm  The alarm must sound if the carbon monoxide level remains in this range for 10 to 50 minutes.
  • 400 ppm +   The alarm must sound if and only if the carbon monoxide level remains at this level for four minutes and no more than 15 minutes.

Bottom Line: Your UL/CSA-listed carbon monoxide detector will not sound an alarm until you have been exposed to far more carbon monoxide than all of the exposure limits established by the various organizations listed previously.

Here’s a carbon monoxide levels chart to graphically illustrate the discrepancy …

esposure levels chart

The Bottom Line: Limited Protection

The above graph shows that for the ASHRAE, WHO, and ACGIH exposure limits, a UL/CSA detector will never sound an alarm.  For the NIOSH and OSHA 8-hour limits, it will take at least 30 days for a UL/CSA detector to sound an alarm, if it ever does (it’s not required).  The only exposure limit that might be supported is the NIOSH Short Term Exposure Limit, but that would only be true if the CO level did not exceed 200 ppm and the detector sounded an alarm within 15 minutes.  As you can see, there is an appalling disparity between exposure limits and alarm standards.


Can I Get More Protection Than a UL CO Detector Provides?

Unfortunately, many states and provinces and other code jurisdictions require one or more UL/CSA-listed carbon monoxide detectors. If you live in one of these places, you must install the approved devices. However, you still have options.

The first, and cheapest, option is to buy a UL/CSA-listed CO detector that has a digital Digital Display 4display and will show you the actual CO concentration when you press a button. Not all UL/CSA detectors with digital displays will do this, so follow our recommendations elsewhere on which detector to buy. This solution is far from ideal, but it is by far the cheapest way to know when you are being exposed to low levels of carbon monoxide.

The second option, although costing more, is by far the better one. This option requires you to install the required number of UL/CSA detectors, the cheapest you can find. Then, buy one or more low-level carbon monoxide detectors. These devices continuously display the carbon monoxide level (from as low as 10 ppm) and will Digital Display LLsound an audible alarm at much lower levels than the UL/CSA-listed devices. These devices are designed to protect your health, not to comply with the limp UL/CSA standards. As a result, you cannot use these exclusively if your local codes require “approved” devices. But if you want the best protection against CO poisoning for yourself and your family, you really ought to have at least one of these low-level CO detectors nearby.