Carbon monoxide detectors are as important to home safety as smoke detectors. In fact, the U.S. Consumer Product Safety Commission recommends every home be equipped with a carbon monoxide detector to prevent life-threatening buildups.

However, every make and model of CO detector has different features. "All CO detectors are not created equal," says Chris Rovenstine, marketing vice president for Nighthawk Industries. Before you make your purchase, consider the following questions, as your safety may depend on which features you choose.- Is it UL-approved? Currently, the only existing standard for CO detectors is Underwriters' Laboratories UL 2034. Manufacturers whose products comply with this voluntary safety standard are entitled to mark those products as "UL Listed." Check the box for the UL seal to ensure a minimum level of performance.

- Is it electrically or battery-powered? AC-powered detectors that plug into standard electrical outlets will leave you unprotected during power outages, but they are not prone to false alarms because they rely on metal-oxide sensors that clear themselves quickly after CO concentrations subside to safe levels. Battery-powered units use a chemically treated disk that darkens on exposure to CO and that sometimes triggers nuisance alarms because it takes longer to clear. In addition, battery models require you to remove the sensor module for 24 to 48 hours after an alarm, which probably will leave you unprotected longer than if you had lost your electricity.

- Do you have to replace the sensor after a certain period of time? This, again, will depend on whether you choose an electric or battery model. Sensors in battery-powered units have to be replaced every two years or if they have absorbed too much CO. Replacements cost about $20. AC-powered models require no replacement sensors and have life expectancies of 7 to 10 years, depending on the brand.

- At what CO level is the alarm activated? Under UL 2034, detectors must alarm within 90 minutes at minimum CO levels of 100 parts per million (ppm). Some detectors are set to sound off at levels as low as 15 ppm. You will probably want to choose a product somewhere in between as the UL standard protects against acute symptoms of CO poisoning in healthy people, but it doesn't address two issues: chronic health problems that may occur after prolonged low-level exposure, and high-risk groups such as the elderly, infants and small children, pregnant women, smokers and those with heart or respiratory conditions who are especially sensitive to CO.

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- Is there a mechanism to distinguish between high and low CO levels? Some detectors alarm at 100 ppm, period. Others sound intermittent alarms or have warning lights at lower levels, enabling you to take action before a life-threatening emergency develops. And some have digital readouts that give a continuous visual display of the CO levels in the air. This not only alerts you to a brewing problem, but it helps you determine how to respond. (Should you call 911 at midnight or just open a window?) And it helps emergency personnel assess the seriousness of the problem in an alarm situation. This can be important because the typical response to an alarm is to open doors and windows to let the CO dissipate. By the time the fire department or heating contractor arrives, CO levels may have dropped, and that make it impossible for them to gauge the extent of the danger. If the homeowner knows the original CO reading, however, there is a better chance investigators will be able to diagnose the problem.

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