They sound an alarm when levels of the toxic gas are too high for a designated amount of time. When CO detectors first hit the market, no universal specifications existed regulating when an alarm would sound. For example, one detector's alarm might trip at 50 parts per million, while others would go off at 20 ppm, said Tom Bier, Janesville Fire Department marshal. A level of about 30 ppm usually is considered dangerous enough that a person will experience dizziness, fatigue and nausea. Many detectors today plug into wall outlets and feature alarms along with lights.
Some models have digital displays that recall the higher levels of carbon monoxide recorded.
He recommended the display-type models because, if someone began feeling dizzy or nauseous and was concerned that it might be from carbon monoxide, he or she could just hit the display button rather than waiting until levels are high enough to set off an alarm. Some CO detectors that plug in also have battery backups in case the power goes out. Less expensive detectors operate solely on batteries, which should be changed regularly, similar to smoke detectors. Bier said carbon monoxide detectors should be placed in the basement or wherever the furnace and water heater are located. Other detectors should be upstairs near living areas, especially sleeping areas.