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... UL listed carbon monoxide alarms will not alert you to low levels of carbon monoxide. Carbon monoxide alarms keep people from dying; they don't eliminate all health risks. Underwriters Laboratory standard UL2034 requires carbon monoxide alarms to not sound off when exposed to carbon monoxide levels under 30 parts per million (ppm). This means that if you have a furnace with a CO reading of 25 ppm in the flue gas, you could place the carbon monoxide alarm right inside the furnace's vent and it would never go off.
Even at higher CO levels, UL listed CO alarms might take a long time to sound off - long enough for you to start experiencing symptoms of carbon monoxide poisoning. Seattle home inspector Charles Buell just wrote a post on this topic, where he tells the story of how a kitchen stove was producing CO levels at over 600 parts per million, but the UL listed CO alarm never made a peep.
Does this mean that low levels of carbon monoxide are safe? No. There are numerous studies saying that low levels of carbon monoxide exposure cause health problems; there are links to many such studies at CO-Experts Home Page. Even though this web site looks like it was designed in the early 90's without being touched since, it has some good information.