Imagine you’re reading a 1950s who-done-it pulp novel, The Senseless Killer, about the mysterious deaths of the beloved family next door.

The investigators are stumped. There appears to be no evidence of forced entry, no apparent malice, yet the two parents and two children are found dead in the comfort of their own home.

Finally, a sharp, young investigator discovers the truth — accidental carbon monoxide poisoning — and the book’s title then holds a deeper, double meaning.

Carbon monoxide (CO) is downright deadly.

A byproduct of combustion, CO is produced by common household appliances such as gas or oil furnaces, clothes dryers, water heaters, ovens, and ranges. CO can also be produced by a charcoal grille operating in an enclosed area, a fire burning in a fireplace, or a car running in an attached garage.

It is odorless and tasteless. It’s absorbed by our lungs through normal breathing and can asphyxiate the unsuspecting.

This poisonous gas is considered enough of a threat that the Consumer Product Safety Commission and the National Association of Fire Chiefs have officially recommended that each residential dwelling have at least one carbon monoxide alarm.

Carbon monoxide alarms use various types of sensors to determine the amount of CO in a home.

Alarms must respond quickly

Carbon monoxide alarms are typically about the same size as a smoke detector. They are designed to monitor the air in a home and sound an alarm when a predetermined level of carbon monoxide is present.

Time of exposure and concentration of the gas in the air are the key factors governing the alarms. With CO exposure, a person’s hemoglobin (the oxygen carrier in the blood stream) is altered to the deadly carboxyhemoglobin.

For us humans, time is definitely not on our side, since our systems readily accept carbon monoxide over the life-giving oxygen atoms (as much as 200 times quicker than oxygen).

Therefore, the detector’s response time is based on a medical equation which combines the number of minutes of exposure and the concentration of carbon monoxide in the air, which is measured in parts per million (ppm).

Underwriters Laboratories (UL) has published the most widely followed standard (UL 2034) for predetermined levels at which a carbon monoxide detector must respond in alarm. (See Table 1.)

How they work

Carbon monoxide alarms use sensors to detect and measure the amount of CO in the air. The types of sensors used vary in their method of operation, power source needs, sensitivity, and useful life.
  • Biomimetic sensing technology: When exposed to CO, the sensing elements in biomimetic sensors darken. The darker the sensor, the greater the CO exposure.

    This technology is based on a rate-of-change principle. When exposed to CO, the sensing elements absorb photons at a rate dependent on the concentration of the CO in the environment. By monitoring the rate of change in the amount of light transmitted through the sensing elements, the concentration of CO in the surrounding environment can be accurately determined. The sensing elements also get lighter when CO decreases.

    This sensor also has a built-in “humidity buffer” that maintains a constant humidity level around the sensor to accommodate for shifts in humidity. A change in the humidity level may cause a sensor to operate improperly.

  • Electrochemical cell technology: Three platinum wire electrodes are placed in contact with an electrolyte solution to form an electrochemical sensor. The semi-permeable cell membrane allows gas to enter while preventing the liquid electrolyte from leaking.

    The gas mixes with the electrolyte solution, causing the electrode to generate a current that causes the circuit to create a voltage in proportion to the amount of CO sensed, thereby triggering the alarm.

  • Metal oxide semiconductor: Metal oxide semiconductor sensors consist of tin oxide that is heated, causing the carbon monoxide (CO) to change into carbon dioxide (CO2). This change frees up electrons, which decrease the resistance of the electrical current, thus setting off the alarm. The decrease in resistance is directly proportional to the CO concentration in the air.

Multiple-station CO alarms.

Power sources

Typically, the first decision your customers face in selecting a CO alarm is the type of power source that will best suit their needs and preferences.

1. The 120-V hardwire alarm is permanently wired to a household’s main power. It is typically used in new construction or remodeling applications, where connecting to an electrical box is easier.

Hardwire CO alarms can be single or multiple station. Multiple-station alarms are interconnected (usually up to 12 alarms) so that all will sound even if only one alarm detected CO.

2. The 120-V hardwire with 9-V battery backup functions just like the above, with the added security of a battery backup in cases where there is risk of a power outage.

3. Relying solely on a building’s main power, 120-V direct-outlet plug-ins are simple to install; just plug them into a wall outlet.

4. The 120-V direct-outlet plug-in models with 9-V battery backup are similar to the above, but have the added security of a battery backup in cases where there is a risk of power outage.

5. Finally, battery-operated CO alarms provide the greatest freedom from the electrical system and the greatest flexibility for location of the units.

Table 1. Underwriters Laboratories' standard for predetermined levels at which a carbon monoxide detector must respond in alarm (UL 2034).


Carbon monoxide poisoning is not new. Awareness of this danger, however, is much more acute and the need for accurate monitoring and measuring of levels in both homes and the workplace is becoming a necessity — even a law, in many U.S. cities.

To the consumer, having a properly placed alarm offers valuable protection from CO poisoning, the “senseless killer.”

For the contractor, CO detectors are another product that can help bind you to your customers, through interest for their health and well-being.

Sidebar: What to look for in a good CO alarm

All CO detectors have the following standard features. Some available optional features are also listed.

Standard features

  • Test alarm reset/silence button — Once the reset button is depressed, the alarm will sound off again within 6 min if the concentration of carbon monoxide is at 70 ppm or greater.
  • Visual alarm indicator — Each detector must have an LED light on the front of the unit.
  • Adhesive instruction labels — Two labels instructing building occupants of what to do when the alarm sounds off must be included with each unit. One label is to be placed near the alarm and the other near fresh air, where the household members would congregate in an emergency.

Optional features

  • Digital readout — A CO ppm digital readout is available on some units.
  • Peak level indicator — This digital readout recalls the highest CO level in ppm since the reset button was last pushed.
  • Protocol card — This is a removable card located on the unit with verbal and graphic icons that indicate what to do if the alarm sounds.