General Information

In 1997, Underwriters Laboratory (UL) revised its standards for Carbon Monoxide (CO) detectors. Now, all UL-listed units must pass a stability test and come with detailed instructions. Alarms are required to sound at 85 decibels within 189 minutes (3 hours, 9 minutes) if 70 parts per million (ppm) of CO is present; within 50 minutes if 150 ppm is present; and within 15 minutes if 400 ppm is present. Previous requirements said the alarm had to sound within 90 minutes at 100 ppm, within 35 minutes at 200 ppm, and within 15 minutes at 400 ppm. The standards are now geared toward detecting lower percentages of CO than before, as well as reducing the possibility of false alarms.

Other revisions to the UL standard include mandating that units ignore low-level concentrations of CO for at least 30 days without sounding an alarm signal, that they ignore concentration levels of 70 ppm or less for at least one hour before the alarm goes off, that they feature a manually operated reset button and contain no additional warning signals besides the alarm. An engineer at UL confirmed that Standard 2034 was revised significantly, and that the changes went into effect on October 1, 1998.

Many reviewers and consumers are most familiar with the First Alert series of CO detectors. However, most experts say these units have a slow response time and an even slower ability to recover and reset after CO levels are safely reduced.

There are a number of different types and brands of carbon monoxide detectors on the market today: They can be most easily characterized by whether they operate on household current or batteries. Underlying this, in most cases, is the type of sensor employed in the detectors operation. Detectors using household current typically employ some type of solid-state sensor which purges itself and resamples for CO on a periodic basis. This cycling of the sensor is the source of its increased power demands. Detectors powered by batteries typically use a passive sensor technology which reacts to the prolonged exposure to carbon monoxide gas.

 Household Current
Battery Operated 
Ease of Installation
More difficult- requires outlet near detector or 'hard wiring'.
Less difficult. Can be placed anywhere needed.
No maintenance required during life of product (5 to 10 years). Detector sensitive with age.
Requires periodic replacement of battery/sensor module every 2 to 3 years at a cost of ~$20.
Reaction Time/ Exposure Level Display
Gives continuous display of CO levels updated every few minutes.
Reaction time depends on concentration level and duration of exposure. Display information is limited.
Reset Time
Will reset immediately once CO problem is corrected.
Reset time depends on exposure concentration and duration. May require removal of sensor pack. A silence button however, is now provided/ required.

Carbon Monoxide Physiological Effects

Carbon monoxide inhibits the blood's ability to carry oxygen to body tissues including vital organs such as the heart and brain. When CO is inhaled, it combines with the oxygen carrying hemoglobin of the blood to form carboxyhemoglobin. Once combined with the hemoglobin, that hemoglobin is no longer available for transporting oxygen. How quickly the carboxyhemoglobin builds up is a factor of the concentration of the gas being inhaled (measured in parts per million or PPM) and the duration of the exposure. Compounding the effects of the exposure is the long half-life of carboxyhemoglobin in the blood. Half-life is a measure of how quickly levels return to normal. The half-life of carboxyhemoglobin is approximately 5 hours. This means that for a given exposure level, it will take about 5 hours for the level of carboxyhemoglobin in the blood to drop to half its current level after the exposure is terminated.

Since one can't easily measure COHb levels outside of a medical environment, CO toxicity levels are usually expressed in airborne concentration levels (PPM) and duration of exposure. Expressed in this way, symptoms of exposure can be stated as follows:

Long-term suggested limit for residential areas.
35 PPM
8 hours
Maximum exposure allowed by OSHA in the workplace over an eight hour period. (8 hour TWA)
200 PPM
2 to 3 hours
Mild headache, fatigue, nausea and dizziness. OSHA Ceiling.
400 PPM
1 to 2 hours
Serious headache - other symptoms intensify.
Life threatening after 3 hours.
800 PPM
45 minutes
Dizziness, nausea and convulsions. Unconscious within 2 hours. Death within 2 to 3 hours.
1200 PPM

1600 PPM
20 minutes
Headache, dizziness and nausea. Death within 1 hour.
2500 PPM

Unconsciousness within 30 minutes.
3200 PPM
5 to 10 minutes
Headache, dizziness and nausea. Death within 1 hour.
6400 PPM
1 to 2 minutes
Headache, dizziness and nausea. Death within 25 to 30 minutes.
12,800 PPM
1 to 3 minutes

NOTES: CO is slightly lighter than air. It is easily moved by drafts and air currents within buildings. It easily permeates walls and ceilings.