This article was originally published in the November 1996 issue of Consumer Reports.

"Sleeping safely: Carbon-monoxide detectors can spot this poisonous gas before it’s too late."

      As the heating season starts, it’s time to consider buying a carbon-monoxide detector. Each year, about 250 fatal poisoning accidents in the U.S. are attributed to carbon monoxide. CO is a byproduct of the combustion--especially the incomplete combustion--of many common fuels. It can be generated by gasoline-powered engines, wood-burning stoves and fireplaces, charcoal grills, and kerosene heaters. CO is particularly deadly because it’s colorless, tasteless, and odorless.

      The threat of CO poisoning is most insidious when the gas collects unnoticed from normally safe sources gone bad--inefficient or malfunctioning fuel-burning heaters or furnaces--in poorly ventilated areas. Today’s tightly sealed houses have made CO a bigger problem.

      Thanks to greater public awareness of the risks of CO, sales of home CO detectors soared rapidly after they first went on the market just three years ago. More than 5 million are sold each year.

     We bought and tested 15 CO detectors made by the major manufacturers, three of them battery-powered and the rest plug-in models. Prices range from $30 to $95.

      Carbon monoxide mimics the behavior of oxygen while withholding oxygen’s benefits. Like oxygen, CO combines with blood hemoglobin in the lungs, and from there it’s transported throughout the body, not to feed but to starve the tissues. At relatively low levels it can cause headache, dizziness, nausea, blurred vision, chest pain, and fainting. These symptoms are often mistaken for common ailments like the flu. Continued exposure to high CO levels can bring on unconsciousness, brain damage, and death.

      The detectors we tested employ various methods to sense carbon monoxide. The AC-powered models have a heated metallic sensor that reacts with CO in a way that can be recognized electronically. Two of the battery models we tested use a chemically treated gel disk that darkens enough on exposure to CO to be detected by an infrared sensor within the unit. The third battery model we tested, sold by Safety 1st, employs a fuel-cell sensor that’s new to home CO detectors.

How we tested

      Our primary test was based on a standard developed by Underwriters Laboratories. In a healthy adult, the first mild symptoms of CO poisoning occur when 15 percent of the body’s hemoglobin is tied up with the gas; other people may be susceptible. UL has settled on a 10 percent blood concentration as the maximum a person should have to experience before a detector sounds off.

      But the rate at which the blood absorbs CO depends on how much of the gas is in the air, so we tested the monitors against both high- and low-level CO concentrations. At our higher concentration, 250 parts per million (ppm), we calculate that detectors shouldn’t take more than 27 minutes to sound the alarm. Most models took about half that long, although one of six samples of the First Alert FCD2DD took over an hour. The Safety 1st was the fastest to sound an audible preliminary warning, starting within a minute or so.

      In our lower-concentration test--90 ppm of CO in the air--we calculate that detectors should sound off within 107 minutes. Most of ours did, handily. But four samples of the battery-powered First Alert NICO and two of the FCD2DDs failed to meet that deadline, so we retested them at a slightly higher CO level, 110 ppm. At that level, we calculate detectors should sound an alarm within 77 minutes. Two of the First Alert NICO samples just barely did and the other two just barely missed, while the two FCD2DD samples emitted warning flashes and sounds instead of a full alarm. Close enough to pass, we decided, but hardly reassuring.

Recovery, reset, readings

      Other factors can affect performance. A reliable unit should recover fast enough from its initial state of alarm to let you know that the danger persists. So we pushed each model’s "hush" button, which is supposed to silence the horn yet leave the CO sensor active. How long it took to sound again can depend on the CO concentration, but most models re-alarmed promptly.

      Once you’ve ventilated the area, a detector should reset itself to tell you when the danger is past. In that situation, most models shut off their horns and lights within minutes. But the First Alert PICO and FCD2DD took up to an hour and a half to stop completely, even if the exposure lasted only minutes. The gel-sensor-equipped COSTAR 90D-i and First Alert NICO performed even worse, if they had been exposed for a long time.

      Several AC-powered detectors have a numerical display that tracks the CO concentration. We tested all the models with displays at concentrations from 0 to 250 ppm, the most useful range for heading off a problem. The three top-rated detectors have a digital display that continually shows the CO level and can recall the previous high level. The Nighthawk was the most accurate, though the Lifesaver and S-Tech were almost as good at levels below 100 ppm. The First Alert FCD2DD only showed values above 50 ppm, and then usually only when the alarm was sounding.

Recommendations

      There have been concerns that CO detectors are more trouble than they’re worth because of frequent false alarms. In fact, we know of one large utility that urges its natural-gas customers not to buy them.

      We disagree. False alarms have been a problem, but CO detectors are fast improving. They’re valuable if you choose one carefully and use it wisely.

      If you have a fuel-burning furnace, appliance, or fireplace, you should have at least one CO detector, ideally in a hallway or sleeping area. Our advice:

      First, look for a model with a full-range digital readout that constantly reports the CO level. An occasional glance will give you an idea of the CO levels in your home--and if the alarm sounds you can tell at once how serious the threat is.

      But don’t buy just any digital readout. You’ll probably see First Alert FCD2DD models for sale, but we recommend you bypass them since you can’t use them for routine low-level CO monitoring. Instead, look for the more useful Nighthawk, Lifesaver, or S-Tech models we tested.

      Second, most people don’t need to worry much about plug-in detectors leaving them unprotected during a power outage. If the power goes off, so will their gas- or oil-fired furnaces, removing the major sources of CO. If you heat with wood, on the other hand, it’s a good idea to have a detector with a battery backup (see Ratings) or a battery-powered detector that will keep going through a blackout. The best choice is the Safety 1st, whose new fuel-cell detector appeared much more accurate and responsive than the competition.

      Finally, if the alarm does go off, don’t automatically call 911. Heed the manufacturer’s advice: If no one in the house is feeling the symptoms of CO poisoning, it’s safe to simply open the windows and call a furnace technician. If people are feeling woozy, get out, summon the fire department, and tell them what your digital detector was reading when you left.