Carbon monoxide (CO) detectors for home use have been around for 25 years. Joseph Priestly, the English chemist, first differentiated between CO and CO₂ in the late 18^th century, but he did not know that the gas was toxic. Once science identified CO as a potential killer, the need for a convenient, accurate detector became plain. Laboratory equipment, like the spectroscope, worked, but was not a practical solution. The vast improvement in sensor development in the 20^th century now makes these essential detectors relatively inexpensive and very reliable.

History

Aristotle observed the potentially fatal effects of carbon monoxide poisoning; he knew that burning coals produced toxic fumes, but he did not know the fumes’ chemical composition. In 1800 Scottish chemist William Cruikshank discovered that the toxic fumes contained carbon and oxygen. Increased industrialization"Beware of carbon monoxide". Image credit: National Archives and Records Administration, 1941-1945. in the 19^th and 20^th centuries, with concomitant increases in burning fuels like coal, resulted in many more opportunities for CO poisoning.

As a result, demand for CO detectors came initially from industry. The Gas Record for April 13, 1921, reports the invention of a “very sensitive carbon monoxide detector” by G.H. Burrell. The detector combined a syringe for sampling air and a container of a proprietary chemical that changed color in the presence of CO. In 1925, Chester Gordon and James Lowe of AT&T patented a detector for use in confined spaces, like manholes, where gases could collect. Their detector combined a reagent in a glass vial covered with absorbent cotton. To test for CO, a user crushed the glass, allowed the reagent to saturate the cotton, and watched to see if the covering grew dark.

These early sensors indicated the presence of CO but not its concentration. Industry demand for more complex sensors picked up in the 1970s and 1980s, resulting in biomimetic, electrochemical and semiconductor sensors, which are described below. Battery-powered home CO detectors followed the popularity of battery-powered smoke alarms; the first such devices shipped in 1993. A combined CO and smoke detector hit the market in 1996, followed by a remote-control-operated version in 2000.

Sensor Technology

CO sensor technology comes in four flavors, each with different applications. Sensors other than opto-chemical measure both CO presence and concentration. Determining the point at which a concentration is too high is not straightforward; a low concentration over a longer time or a high concentration for a short time can be equally deadly. Sensors that calculate concentrations before sounding an alarm include a method for handling these calculations.

CO pad detector. Image credit: Victory of the People, CC BY 2.0Opto-chemical detectors are the direct descendants of the device Gordon and Lowe patented in 1925. A chemically-infused pad changes color when it reacts with CO. Such a detector is inexpensive and useful when incorporated in a badge for a worker to wear in a factory or other site where CO could be present.

Biomimetic or biotechnology-based detectors, introduced in the early 1990s, reveal both the presence and concentration of CO. The “bio” in the technology’s name comes from hemoglobin, which darkens proportionately in the presence of CO; the darker color indicates a higher concentration. Current detectors use a combination of cyclodextrins, a chromophore and metal salts. Biomimetic detectors require electricity, usually from a battery; lifespan depends on battery type and averages six years. According to testing done by Lawrence Berkeley Laboratory, this is the only type of sensor that is false-alarm free. These detectors are more expensive, but the expense is justified for applications like hospitals, nursing homes and other institutions where false alarms can be expensive.

Electrochemical sensors, the dominant technology, are a type of fuel cell that produces a signal current, the size of which is directly related to CO concentration. The cell consists of a container, two electrodes, connection wires and an electrolyte. The sensor oxidizes CO and produces CO₂ at one end and consumes oxygen at the other. The amount of CO measured is directly related to CO2 output. This technology has minimal power requirements and operates at room temperature. Electrochemical sensors last about five years.

Semiconductor sensors consist of two tin dioxide wires on a ceramic base in an integrated circuit. The sensing element has to be heated to 400 degrees C to operate. A change in resistivity indicates the presence of CO. This technology uses so much power it is usually powered by a building’s electrical system. Hardwired versions can last up to 10 years; battery-powered versions, a few months.

Designs and Features

Typical domestic CO detectors are contained in smallish plastic cases, along with the batteries that power them and the alarm that goes off at a predetermined CO level. Some detectors plug into a standard electrical Home detector: Image credit: Santeri Viinamäki/CC-SA 4.0 outlet.

Newer models are getting “smarter.” Sending an alarm message to a smartphone is a standard feature, along with a sound or voice alert in the building where the detector is installed. Some models connect to Z-Wave-compatible hubs to allow syncing with other smart devices; others are compatible with Amazon’s Alexa and Samsung’s SmartThings. Several also serve as nightlights.

As early as the 1980s, an inventor built a factory-grade CO detector that also opened a louver when it sensed an unacceptable CO level. At the 2008 CES Skylink introduced a particularly comprehensive home-safety system based around a Wi-Fi enabled garage door opener. A CO detector can communicate via Wi-Fi with the garage door and tell it to open when CO is too high.

Not all CO detectors are mounted in a building. Portable devices allow inspectors to test multiple parts of a building to find localized pockets of CO. G.H. Burrell first used his detector in a steel mill office, where the staff was complaining about headaches. By carrying the detector around the building, he discovered that an underground flue was not venting properly. Levels of CO in the bloodstream can be tested with a “blower” similar to that used to detect alcohol levels.

Standards and Laws

In the U.S., CO alarms must conform to Underwriters Laboratories 2034 standard. The standard is based on a threshold carboxyhemoglobin (percent COHb) in the blood as a biomarker for time to alarm. UL has a separate standard (2075) for what they define as detectors – passive indicators that show the presence, absence and level of CO.

States have responsibility for providing laws and regulations concerning CO detectors. As of January 2017, 32 states required CO detectors in homes, 32 by statute and 11 have established regulatory standards. A few states require the detectors in hotels (15) and schools (five).