Carbon monoxide, primarily as a by-product of combustion, is a colorless, odorless gas that may cause illness or death at sufficient concentrations. NFPA standard 720:2009 calls for CO detection in most buildings including schools, hotels, nursing homes, hospitals and commercial occupancies equipped with fuel-fired appliances or attached garages; not just residences as found in earlier NFPA standards. 

The NFPA recommendation for greater use of CO detectors is driven in part by advancements in technology that allow for more reliable, longer life CO detectors at much less expense — many high quality, single station CO detectors i.e. plug into an electrical outlet, sell for around $60; and, some detectors can be bought for much less.

On the surface, a best management practice is to install CO detectors that meet the ANSI/UL 2034 standard at various areas at your worksite, such as shipping docks where propane fork-trucks are used, near welding stations, or other occupied areas where CO may be expected. 

The logic seems to make sense: if UL 2034 detectors are good enough to protect a baby in the bedroom, the detector should be good enough to protect a healthy working adult on the shop floor. But is the logic sound? 

OELs and UL 2034 detection levels

Most EHS pros seek to conform to one of the following occupational exposure limits for CO:

  • ACGIH® TLV® at 25 ppm 8-hr. TWA;
  • MAK Pregnancy Risk Classification B (i.e. probable damage) at 30 ppm 8-hr. TWA;
  • OSHA PEL at 50 ppm 8-hr. TWA; and,
  • NIOSH REL Ceiling at 200 ppm.
  • CO detectors that meet the UL 2034 standard have measurement and alarm criteria that include the detector:
  • Cannot show digital readings below 30 ppm;
  • Do not sound an alarm until CO has been present at 30 ppm for 30 days;
  • Must ignore CO levels of 70 ppm unless present for a minimum of one hour; and,
  • Must ignore CO at 400 ppm unless present for at least four minutes.

In summary: OELs include chronic health effects for CO while UL 2034 only responds to the acute health effects. 


Many people in the general public may know that carbon monoxide is a colorless, odorless gas that may cause illness or death at sufficient concentrations.  Few people in the general public, however, understand acceptable concentration levels for CO and how CO may quickly evolve and dissipate. My wife is a good example.

False alarm

In 2003, U.S. fire departments responded to 40,900 non-fire CO incidents. By 2010, the number jumped to 80,100. Looking toward the future, with the increased number of CO detectors being required by local and state building codes, UL 2034 detection and alarm criteria will limit the number of calls to fire departments, emergency organizations, and other responders where CO exposures are not high enough to cause acute illness.  

In summary: to limit false alarms, UL 2034 sacrifices awareness of chronic health effects for CO, as demonstrated by the detector ignoring OELs.

Worksite detection for CO

Because CO is colorless, odorless, and toxic, NFPA standard 720:2009 appears reasonable for all occupied buildings where CO may exist. My choice for a single station CO detector (not UL 2034 compliant) at a worksite supports most OEL alarm objectives for CO and will provide automatic continuous display for CO levels from 10-500 ppm. The detector may be obtained for about $100. 

IH use

In the early days of industrial hygiene, single-use CO detector tubes i.e. colorimetric change by hand pump was common. Today, portable professional detection for CO, such as for confined space entry or IH use, requires an electronic detector that is quick to respond, frequently calibrated against a known standard for high accuracy and is overall more robust with expanded capabilities.  

Professional CO detectors cost about $200-$400; and, some detectors with features such as data logging may cost much more. Do your homework before selecting the detector that best fits your need.


Is a residential CO detector good enough to use at your worksite? The answer depends upon organizational objectives and context such as building code requirements. If CO may exist at a worksite, its concentration should be measured.