Nearly 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.
Consumer Reports tested 10 to see how quickly they'll alert you to low-level and high-level leaks.
Testers found some took nearly 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, but carbon-monoxide alarms should be replaced after five." said Consumer Reports' Dan DiClerico.
That means you need to replace a combination alarm every five years.
That's five years from the date 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." said DiClerico.
Among interconnected detectors, the First Alert OneLink earned top ratings. It's batter operated and costs $70.
"A stand-alone alarm is probably fine if you live in a small, one-story home or an apartment." said DiClerico.
A good choice is the $40 First Alert. It plugs into the wall and also has a battery backup.
Consumer Reports says be aware 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.