Product review (5 stars): Very sensitive
I bought this because I installed an unvented gas heater in my basement to heat my office area.

CO2 meter arrived a few days before plumber arrived to hook up heater, so I plugged it in to get an idea of baseline CO2 levels without this new heater. Of course, humans exhale carbon dioxide. Meter is about 5 ft from my body, at 9 o'clock when I am facing 12 o'clock at my desk. When I sat down, meter read 451 parts per million. After an hour or so it read 568 ppm, just from me breathing. When two plumbers were here to hook up heater to gas line, 3 of us were within 10 feet of the meter. Reading at start was 465. Reading with 3 of us exhaling for an hour was 760.

Don't know how accurate it is, but it sure is sensitive. Easy to operate. Alarm can be set at different CO2 levels. A little pricy compared to carbon monoxide meters, but well worth the money with an unvented gas heater 6 ft behind me. BTW, carbon monoxide (CO) meter is at 3 o'clock when I am facing 12 o'clock. CO2 to my left and CO to the right. Can't be too careful with unvented combustion heaters, IMO.