Almost 500 Americans die from carbon-monoxide poisoning each year. And more than 15,000 end up in emergency rooms. Faulty heating appliances and blocked chimneys and vents are usually to blame.

A carbon-monoxide alarm can help make sure you stay safe. Consumer Reports tested 10 to see how quickly they'll alert you to low-level and high-level leaks. Some took almost twice as long as others to sound. Most of the alarms tested are just for carbon monoxide, but a couple are combined with a smoke alarm.

Keep in mind that while smoke alarms should be replaced after 10 years, carbon-monoxide alarms should be replaced after five. That means you need to replace a combination alarm every five years. That's five years from the date you'll find on the back of the alarm.

It's also important to have a carbon-monoxide detector on each level of your house—the basement, ground floor, and the upper level.

It's a good idea to get interconnecting alarms that signal each other. That way you'll be warned about a leak in the basement even if you're asleep upstairs. Among interconnected detectors, the First Alert OneLink SCO501CN earned top ratings. It's battery operated and costs $70.

Consumer Reports says be aware that interconnected carbon-monoxide alarms usually only communicate with alarms made by the same company. So if you're installing several CO alarms, check to be sure they are all compatible with each other, as well as with your fire alarms.

A stand-alone alarm is fine if you live in a small, one-story home or an apartment. A good choice is the $40 First Alert CO615. It plugs into the wall and also has battery backup.

Consumer Reports has no commercial relationship with any advertiser or sponsor appearing on this Web site.

Copyright © 2005-2009 Consumers Union of U.S., Inc.

Complete Ratings and recommendations on all kinds of products, including appliances, cars & trucks, and electronic gear, are available on Consumer Reports' Web site. Subscribe to ConsumerReports.org.