A byproduct of combustion, carbon monoxide (CO) is an odorless, colorless gas that claims roughly 500 American lives each year. Often, the culprit is a furnace, water heater or other fuel-burning appliance. Carbon monoxide has become such a menace that a growing number of cities and states now require CO alarms in homes.

CO alarms beep when levels of the gas reach 70 parts per million (ppm) for an hour — sooner at higher levels — based on a voluntary industry standard. You may not notice ill effects at that level, but levels of 150 ppm over that time often cause headaches and other flulike effects. A CO level of 400 ppm can cause loss of consciousness, followed by brain damage or death. Heart patients and pregnant women are at the greatest risk.

The best carbon-monoxide alarms cost $75 or less and include interconnecting models that trigger each other so that all sound when one senses a threat. A single CO alarm may be fine for small, one-level homes. For multilevel homes, consider an interconnecting alarm at each level to warn you of problems in remote parts of the house.

Before you start shopping for a CO alarm, check whether your municipal building department or code-enforcement office requires a specific type. Then keep these points in mind at the store:

• See that it's up to snuff. Alarms should meet Underwriters Laboratories Standard 2034. Check the package for that UL certification. The standard becomes tougher in 2007, but makers say the alarms CR tested should comply.

• Pick your power. Battery alarms are easier to place than plug-in or wired models, some of which have battery backup. (Choose a central location outside or within sleeping areas and other living space. Don't put alarms inside garages, where initially high CO from car exhausts can trigger false alarms, or near doors and windows, where fresh air can cause a misleadingly low reading.) Batteries must be changed yearly.

• Look for a digital display. These show relatively precise CO levels in parts per million, rather than simply beeping. Some also show the peak level since they were reset, warning you of any spikes that occurred while you were away.

• Check the date. CO alarms lose sensitivity after five years. The manufacture date is on the back of the unit: Fresher is better. If you buy a battery-powered Senco Model One CO Alarm, make sure it was made in 2005. The 2005 model ($45) did very well in tests involving response to lower and higher levels of CO. If you already own a 2004 model, Consumer Reports suggests replacing it: Last year's unit performed erratically in this year's tests.

All the alarms CR tested sounded within 10 minutes for high levels (400 ppm) of carbon monoxide and 30 minutes for lower levels (150 ppm). The better models responded fastest, providing an extra margin of safety at higher levels.

Among stand-alone alarms, CR recommends the plug-in American Sensors AC Digital & Battery Backup CO920 ($65), and two battery-powered Kidde Nighthawk models, the KN-COPP-B (at just $40 it's a CR Best Buy), and the Smoke & CO Combo KN-COSM-B ($45), which combines fine CO sensing with smoke detection. The top-rated American Sensors provides high performance and peak memory recall, while it and the Kidde KN-COPP-B feature digital displays.

The two interconnecting alarms CR recommends both include smoke detection. The battery-powered First Alert OneLink Smoke & CO Alarm SCO500CN ($75) can trigger other OneLink alarms wirelessly. The hardwired Kidde Nighthawk Smoke & CO Combo KN-COSM-1B ($50) has battery backup.