Carbon monoxide detector

This article is about monitors of ambient air. For biometry devices, see Diagnosis of carbon monoxide poisoning.
Carbon Monoxide detector connected to a North American power outlet
Carbon monoxide alarm

A carbon monoxide detector or CO detector is a device that detects the presence of the carbon monoxide (CO) gas in order to prevent carbon monoxide poisoning. In the late 1990s Underwriters Laboratories (UL) changed their definition of a single station CO detector with a sound device in it to a carbon monoxide (CO) alarm. This applies to all CO safety alarms that meet UL 2034; however for passive indicators and system devices that meet UL 2075, UL refers to these as carbon monoxide detectors. CO is a colorless, tasteless and odorless compound produced by incomplete combustion of carbon containing materials. It is often referred to as the "silent killer" because it is virtually undetectable without using detection technology and, in a study by Underwriters Laboratories, "Sixty percent of Americans could not identify any potential signs of a CO leak in the home".[1] Elevated levels of CO can be dangerous to humans depending on the amount present and length of exposure. Smaller concentrations can be harmful over longer periods of time while increasing concentrations require diminishing exposure times to be harmful.[2]

CO detectors are designed to measure CO levels over time and sound an alarm before dangerous levels of CO accumulate in an environment, giving people adequate warning to safely ventilate the area or evacuate. Some system-connected detectors also alert a monitoring service that can dispatch emergency services if necessary.

While CO detectors do not serve as smoke detectors and vice versa, dual smoke/CO detectors are also sold. Smoke detectors detect the smoke generated by flaming or smoldering fires, whereas CO detectors detect and warn people about dangerous CO buildup caused, for example, by a malfunctioning fuel-burning device. In the home, some common sources of CO include open flames, space heaters, water heaters, blocked chimneys or running a car inside a garage.[3]


The devices, which retail for $15–$60 USD and are widely available, can either be battery-operated or AC powered (with or without a battery backup). Battery lifetimes have been increasing as the technology has developed and certain battery-powered devices now advertise a battery lifetime of over 6 years. Some smoke detectors are equipped with an internal rechargeable battery backup that recharges when the detector is receiving AC power. All CO detectors have "test" buttons like smoke detectors.

CO detectors can be placed near the ceiling or near the floor because CO is very close to the same density as air.[4][5]

Since CO is colorless, tasteless and odorless (unlike smoke from a fire), detection in a home environment is impossible without such a warning device. It is a highly toxic inhalant and attaches to the hemoglobin (in the blood stream) with an affinity 200x stronger than oxygen,[6] producing inadequate amounts of oxygen traveling through the body.

In North America, detectors are required in new construction in the U.S. states of Alaska, California, Colorado, Connecticut, Florida, Georgia, Illinois, Maryland, Massachusetts, Minnesota, New Jersey, New York, Oregon, Rhode Island, Texas, Vermont, Virginia, Washington (also in existing homes with some exemptions),[7][8] Wisconsin and West Virginia, as well as New York City and the Canadian provinces of Ontario[9] and Alberta (homes with a secondary suite or new homes with a fuel-burning appliance).[10]

When carbon monoxide detectors were introduced into the market, they had a limited lifespan of 2 years. However technology developments have increased this and many now advertise up to 10 years.[11] Newer models are designed to signal a need to be replaced after that time-span although there are many instances of detectors operating far beyond this point.

According to the 2005 edition of the carbon monoxide guidelines, NFPA 720,[12] published by the National Fire Protection Association, sections and, all CO detectors “shall be centrally located outside of each separate sleeping area in the immediate vicinity of the bedrooms,” and each detector “shall be located on the wall, ceiling or other location as specified in the installation instructions that accompany the unit.”

According to the 2009 edition of the IRC,[13] published by the International Code Council, section R315.1, "For new construction, an approved carbon monoxide alarm shall be installed outside of each separate sleeping area in the immediate vicinity of the bedrooms in dwelling units within which fuel-fired appliances are installed and in dwelling units that have attached garages", and section 315.2, "Where work requiring a permit occurs in existing dwellings that have attached garages or in existing dwellings within which fuel-fired appliances exist, carbon monoxide alarms shall be provided in accordance with Section R315.1."

Installation locations vary by manufacturer. Manufacturers’ recommendations differ to a certain degree based on research conducted with each one’s specific detector. Therefore, make sure to read the provided installation manual for each detector before installing.

CO detectors are available as stand-alone models or system-connected, monitored devices.[14] System-connected detectors, which can be wired to either a security or fire panel, are monitored by a central station. In case the residence is empty, the residents are sleeping or occupants are already suffering from the effects of CO, the central station can be alerted to the high concentrations of CO gas and can send the proper authorities to investigate.

The gas sensors in CO alarms have a limited and indeterminable life span, typically two to five years. The test button on a CO alarm only tests the battery and circuitry, not the sensor. CO alarms should be tested with an external source of calibrated test gas, as recommended by the latest version of NFPA 720. Alarms over five years old should be replaced but they should be checked on installation and at least annually during the manufacturers warranty period.


Early designs were basically a white pad which would fade to a brownish or blackish color if carbon monoxide was present. Such chemical detectors were cheap and were widely available, but only give a visual warning of a problem. As carbon monoxide related deaths increased during the 1990s, audible alarms became standard.

The alarm points on carbon monoxide detectors are not a simple alarm level (as in smoke detectors) but are a concentration-time function. At lower concentrations (e.g. 100 parts per million) the detector will not sound an alarm for many tens of minutes. At 400 parts per million (PPM), the alarm will sound within a few minutes. This concentration-time function is intended to mimic the uptake of carbon monoxide in the body while also preventing false alarms due to relatively common sources of carbon monoxide such as cigarette smoke.

There are four types of sensors available and they vary in cost, accuracy and speed of response,[15] listed below. The latter three types include sensor elements that typically last up to 10 years. At least one CO detector is available which includes a battery and sensor in a replaceable module. Most CO detectors do not have replaceable sensors.


The detector consists of a pad of a coloured chemical which changes colour upon reaction with carbon monoxide. They only provide a qualitative warning of the gas however. The main advantage of these detectors is that they are the lowest cost, but the downside is that they also offer the lowest level of protection.


A biomimetic sensor works in a fashion similar to hemoglobin which darkens in the presence of CO proportional to the amount of carbon monoxide in the surrounding environment. It uses cyclodextrins, a chromophore, and a number of metal salts. This can either be seen directly or connected to an infrared source of photons such as an IR LED and then monitored using a photodiode. Battery lifespan usually lasts 2–3 years with conventional alkaline, but a lithium battery will last the life of the product. The biotechnology based sensors have a useful operational life of 6 years. These products were the first to enter the mass market, but because they cost more than other sensors they are mostly used in higher-end areas and RVs. The technology has been improved and is the most reliable technology, according to a report from Lawrence Berkeley National Laboratory.[16] The technology is the only one tested false alarm free and is preferred by those with larger facilities like hospitals, hotels and apartments that use air fresheners, alcohols and other disinfectants where the cost of one false alarm is very high. This technology was invented in the United States and is manufactured in California.


This is a type of fuel cell that instead of being designed to produce power, is designed to produce a signal current that is precisely related to the amount of the target gas (in this case carbon monoxide) in the atmosphere. Measurement of the current gives a measure of the concentration of carbon monoxide in the atmosphere. Essentially the electrochemical cell consists of a container, two electrodes, connection wires and an electrolyte - typically sulfuric acid. Carbon monoxide is oxidized at one electrode to carbon dioxide while oxygen is consumed at the other electrode. For carbon monoxide detection, the electrochemical cell has advantages over other technologies in that it has a highly accurate and linear output to carbon monoxide concentration, requires minimal power as it is operated at room temperature, and has a long lifetime (typically commercial available cells now have lifetimes of five years or greater). Until recently, the cost of these cells and concerns about their long term reliability had limited uptake of this technology in the marketplace, although these concerns are now largely overcome. This technology is now the dominant technology in USA and Europe.


Thin wires of the semiconductor tin dioxide on an insulating ceramic base provide a sensor monitored by an integrated circuit. This sensing element needs to be heated to approximately 400 °C in order to operate. Oxygen increases resistance of the tin dioxide while carbon monoxide reduces resistance. The integrated circuit monitors the resistance of the sensing element. Lifespans are approximately five to 10 years.

The large power demand of this sensor means that it is usually powered from the mains. A battery-powered, pulsed sensor is available with a lifetime in months.

This technology has traditionally found high utility in Japan and the far east with some market penetration in USA. However the superior performance of electrochemical cell technology is beginning to displace this technology.


Although all home detectors use an audible alarm signal as the primary indicator, some versions also offer a digital readout of the CO concentration, in parts per million. Typically, they can display both the current reading and a peak reading from memory of the highest level measured over a period of time. These advanced models cost somewhat more but are otherwise similar to the basic models.

The digital models offer the advantage of being able to observe levels that are below the alarm threshold, learn about levels that may have occurred during an absence, and assess the degree of hazard if the alarm sounds. They may also aid emergency responders in evaluating the level of past or ongoing exposure or danger.


Portable CO detectors are also available; these are typically used for professional applications or in some cases by consumers such as property managers for maintenance and diagnosis issues (i.e. sourcing a CO leak). Most offer real time measurements of CO down to a few ppm (usually shown on a digital display), and are more expensive than home safety CO detectors (e.g. ~$250 vs $25). There are two types of portable detectors, one that is designed for aircraft, cars and trucks. They will warn the driver and passenger if there is a CO hazard. Another type is used by industrial hygienists and first responders. Digital, fast responding portable type CO detectors are usually a better choice for real time "on the go" applications as they respond to low levels of CO in seconds rather than minutes or hours (which is the case for UL2034 listed residential alarm).


Wireless home safety solutions are available that link carbon monoxide detectors to vibrating pillow pads, strobes or a remote warning handset. This allows those with impediments such as hard of hearing, partially sighted, heavy sleepers or the infirm the precious minutes to wake up and get out in the event of carbon monoxide in their property.

Legislation in the United States

The installation of CO detectors is imperative to ensure protection from inhalation of toxic gas. More and more states are legislating for their installation as a mandatory feature.[17]

House builders in Colorado are required to install carbon monoxide detectors in new homes in a bill signed into law in March 2009 by the state legislature. House Bill 1091 requires installation of the detectors in new and resold homes near bedrooms as well as rented apartments and homes. It took effect on July 1, 2009. The legislation was introduced after the death of Denver investment banker Parker Lofgren and his family. Lofgren, along with his wife and children were found dead in their home near Aspen, Colorado on Nov. 27, 2008, victims of carbon-monoxide poisoning.

In New York State, "Amanda's Law" (A6093A/C.367) requires one- and two-family residences which have fuel burning appliances to have at least one carbon monoxide alarm installed on the lowest story having a sleeping area, effective since February 22, 2010. Although homes built before Jan. 1, 2008 are allowed to have battery-powered alarms, homes built after that date need to have hard-wired alarms. In addition, New York State contractors have to install a carbon monoxide detector when replacing a fuel burning water heater or furnace if the home is without an alarm. The law is named for Amanda Hansen, a teenager who died from carbon monoxide poisoning from a defective boiler while at a sleepover at a friend's house.[18]

Alaska House Bill 351 requires a carbon monoxide detector be installed in dwelling units that contain or are serviced by a carbon based fuel appliance or other device that produces by products of combustion.

In July 2011, California required installation of carbon monoxide detectors in existing single-family homes, with multifamily homes following in 2013.[19] CA Law 2015 require all new installation of smoke and CO alarms to be 10 year non serviceable type.[20] Existing alarms may not need to be replaced for home owners, see local codes. Required alarm location also vary per local enforcing agencies.


North America

The Canadian Mortgage and Housing Association reports, "The standards organizations of Canada (CSA) and the United States (Underwriters Laboratories or UL) have coordinated the writing of CO standards and product testing. The standards as of 2010 prohibit showing CO levels of less than 30 ppm on digital displays. The most recent standards also require the alarm to sound at higher levels of CO than with previous editions of the standard. The reasoning behind these changes is to reduce calls to fire stations, utilities and emergency response teams when the levels of CO are not life threatening. This change will also reduce the number of calls to these agencies due to detector inaccuracy or the presence of other gases. Consequently, new alarms will not sound at CO concentrations up to 70 ppm. Note that these concentrations are significantly in excess of the Canadian health guidelines,"[21] (and also in excess of US Occupational Safety and Health Administration (OSHA) Permissible exposure limits, which in 50 ppm.)[22]


In the UK a domestic/Type-B alarm compliant with BS EN 50291:2001 should emit an audible alarm after about 3 minutes exposure to 300 ppm CO, or 10 to 40 minutes at 100 ppm, or 60 to 90 minutes at 50 ppm.[23]


  2. NFPA 720: Standard for the Installation of Household Carbon Monoxide (CO) Warning Equipment, 2005 Edition, Annex B Dangers of Carbon Monoxide, B.1 Carbon Monoxide, Table B.1 Symptoms of Carbon Monoxide Exposure Based on Concentration
  3. U.S. Consumer Product Safety Commission, Carbon Monoxide Detectors Can Save Lives (CPSC Document #5010), archived from the original on 2009-04-09, retrieved 2007-07-29
  4. Metropolitan Utilities District, Carbon monoxide detectors, archived from the original on 2007-08-14, retrieved 2007-07-29
  5. CO Alert, Placement of Carbon Monoxide Detectors Important, retrieved 2009-01-11
  6. Guyton A C: Medical Physiology 12ed. 2010, page 502
  7. "WAC 51-51-0315: Section R315—Carbon monoxide alarms.".
  8. "RCW 19.27.530: Carbon monoxide alarms — Requirements — Exemptions — Adoption of rules.".
  9. Retrieved 2009-01-28. Missing or empty |title= (help)
  10. (PDF) Retrieved 2012-05-28. Missing or empty |title= (help)
  11. "Kidde Sealed Lithium Battery Power Carbon Monoxide Alarm with Digital Display C3010D". Retrieved May 10, 2016.
  12. NFPA 720: Standard for the Installation of Household Carbon Monoxide (CO) Warning Equipment, 2005 Edition
  13. 2009 International Residential Code® for One- and Two-family Dwellings
  14. "Top 5 Things to Know About CO," LifeSafety magazine, Fall 2006
  15. Guide to Prevent Carbon Monoxide Poisoning, retrieved 2007-07-29
  16. Gundel, Lara A.; Michael G. Apte; Albert R. Nematollahi (1998). Carbon Monoxide Detector Technology Comparison: Response to Various Gases (PDF) (Technical report). Ernest Orlando Lawrence Berkeley National Laboratory. LBNL-40556. Retrieved 2014-01-14.
  18. "Welcome Page". New York State Office of Fire Prevention & Control. Archived from the original on 2010-05-21. Retrieved 2010-03-18.
  19. Senate Bill 183
  21. Canadian Mortgage and Housing Association Archived April 11, 2013, at the Wayback Machine.
  22. "Carbon monoxide". Centers for Disease Control and Prevention (CDC). April 11, 2016. Retrieved May 10, 2016.
  23. Ei 204EN CO Alarm User Instructions
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