I would have to disagree with this statement. I'm a fire lieutenant, we carry CO meters & sometimes, just for sh*ts & giggles (and to test the meter), I'll get one of the guys to blow cigarette smoke into the machine. It reads anywhere from 35 to 90 ppm. If someone is smoking a butt while filling your tank &, let's say for argument's sake, you get the low side, 35 ppm, then take your tank to 100 fsw, that's 105 ppm. Suck on that for 45 minutes, not to mention getting the high end. Repete that for a few days & we're looking at a "YIKES" situation