The CO71A monitors, records, and alerts you to the presence of dangerous carbon monoxide in ambient air. The visual display constantly indicates precise quantities while the audible and visual alarms respond to various threshold levels. Carbon Monoxide (CO) - is a lethal poison produced when certain fuels are burned. It can rapidly accumulate even in areas that might appear well ventilated. Because CO is colorless, tasteless, odorless and non-irritating, it can overcome an exposed person without warning. The CO71A Features CO sensor self test, preset alarms 3-color warning light and audible alert, MAX capture, low battery indicator, CO detection to 999 ppm, backlit display and one button auto zero. Holster with clip included.