Illustration: Harry Campbell
Despite what your mother told you, what you don't know—or, for that matter, see or smell or taste—can hurt you. In the case of carbon monoxide, it can kill you. The invisible, odorless gas, a by-product of all fuel-burning appliances, heating devices, and engines, is responsible each year for more than 200 deaths (and possibly thousands more unreported, nonfatal poisonings) in the U.S. alone. Yet only one third of houses have CO alarms, and only a handful of states require them. Winter is the riskiest season for CO poisonings, so if you're among the unprotected, now's the time to act.

To understand how these alarms work, it helps to first understand the way carbon monoxide affects the body. When inhaled, CO combines with hemoglobin, the oxygen-carrying component in blood, to produce carboxyhemoglobin (COHb). Because COHb bonds with oxygen so tightly—200 times stronger than regular hemoglobin—cells slowly suffocate. Likewise, it's difficult for the body to expel CO, so even small doses can build to toxic levels. The brain doesn't recognize the danger because it regulates breathing rates based on carbon dioxide levels, not the body's need for oxygen. Poisoning victims may complain of headache, fatigue, or nausea, but they never receive a clear warning from their bodies to walk away from a potentially deadly situation.

That's where CO alarms come in. By constantly monitoring the air for the gas, which is almost always present in minute quantities, the detector calculates whether levels are high enough to pose a risk of CO buildup in the body. If so, the device triggers an alarm of 85 decibels or more, loud enough to jolt most anyone out of peaceful slumber. (Most deaths from CO poisoning occur while the person is sleeping.)

It's easy for an alarm to sense CO. The trick is to make one that reliably tracks the minute fluctuations in levels over time and sounds a warning only when the cumulative exposure indicates a health risk. That requires a complex interaction between sensors and microcircuitry, which made such technology prohibitively expensive until 1993, when First Alert introduced the first battery-powered CO alarm for home use.

Today, prices range from $20 up to $100, depending on the type of sensing technology and the sophistication of the features. There are basic battery-powered or plug-in models, or fancier versions that talk, connect to a wireless network, or work in combination with a smoke alarm. But they all meet the same performance standard—UL 2034—set and enforced by Underwriters Laboratories, so you're safeguarded no matter what kind you choose. Besides, installing one is no more difficult than mounting a smoke alarm. What are you waiting for?
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