Abstract: A compact portable unit incorporating a noxious gas concentration alarm system and method adapted to setting off an alarm to warn of the presence of a dangerously high level concentration of carbon monoxide in an environment and when lower levels of concentration are present for time periods which are predetermined to be hazardous to an individual in the environment. A specific embodiment is disclosed in which two or more timed levels of carbon monoxide concentration are arranged to set off an alarm and/or automatic shutdown of the CO producing mechanism when the measured time of presence of each concentration level is hazardous to an individual in the environment.