A:Answer Hi George, thank you for your questions. Underwriters Laboratories Inc. Standard UL2034 requires residential CO alarms to sound when exposed to levels of CO and exposure times as described next. They are measured in parts per million (ppm) of CO over time (in minutes). • If the alarm is exposed to 400 ppm of CO, IT MUST ALARM BETWEEN 4 and 15 MINUTES. • If the alarm is exposed to 150 ppm of CO, IT MUST ALARM BETWEEN 10 and 50 MINUTES. • If the alarm is exposed to 70 ppm if CO, IT MUST ALARM BETWEEN 60 and 240 MINUTES. * Approximately 10% COHb exposure at levels of 10% to 95% Relative Humidity (RH). The unit is designed not to alarm when exposed to a constant level of 30 ppm for 30 days. - Maria
