CO Detectors that meet the UL standard measure both high concentrations of CO over short periods of time and low concentrations of CO over long periods of time.

Detectors sound an alarm before the level of CO in a person's blood becomes crippling.

The price range for detectors that meet the UL 2034 standard is currently between $35 and $150. Beware of inexpensive CO detectors that don't meet UL standards.
Some cheap CO detectors made of plastic or cardboard change color instead of sounding an alarm. You must check the visual indicator regularly in order to determine whether or not CO is present. Because of this, the detector must be placed in a high traffic area if it is to be useful. The major drawback of this kind of system is that if CO concentrations build up rapidly at night while family members are asleep there will be no warning of the danger. These detectors also have a limited life span.

Installation
Since CO gases spread evenly and rapidly throughout the house, CO detectors should be installed on the wall or ceiling outside bedrooms to alert occupants who are sleeping.

Safety Devices on Appliances
Sine the late 1980's, vent safety shutoff systems have been required on furnaces and vented heaters. These safety shutoff systems protect against blocked or disconnected chimneys and vents. Oxygen depletion sensors (ODS) have also been installed on unvented gas space heaters since the 1980's. These ODS protect against the production of CO caused by insufficient oxygen for proper combustion.

Although these devices (ODS and vent safety shutoff systems) serve as a line of defense against CO poisoning, they are no substitute for regular servicing by a professional. In addition, many older appliances may not have such safety devices. In any case, a CO detector is important in any home as a line of defense against this deadly gas.