Carbon Monoxide
Carbon monoxide isn’t usually a problem with well-maintained aircraft, however, it’s enough of a risk that many people use CO monitors to detect concentrations while flying. Most sound an alarm at a preset level and many read the concentration in parts per million (PPM). One of the students brought his monitor and we used it on a flight. The monitor read 13-15 ppm when the door of the Cherokee was open while taxiing and 3-4 ppm in flight. The question though was is that good or bad?
Wikipedia has an article on carbon monoxide that reports various concentrations.
Carbon monoxide commonly occurs in various natural and artificial environments. Here are some typical concentrations:
0.1 ppm - natural background atmosphere level 0.5 to 5 ppm - average background level in homes 5 to 15 ppm - levels near properly adjusted gas stoves in homes 100-200 ppm - Mexico City central area from autos etc. 5,000 ppm - chimney of a home wood fire 7,000 ppm - undiluted warm car exhaust - without catalytic converter
A different article has the effects on the body of various concentrations.
In the United States, OSHA limits long-term workplace exposure levels to 50 ppm.
From this information it looks like even the levels found when starting up aren’t particularly worrisome.
The levels while flying are good—about the same as in most homes. Both levels are below the threshold where effects are noted.
Several sources state that the effects are amplified at high altitude, but I can’t find any specific numbers. It makes sense though. CO isn’t released by the hemoglobin and as you get higher there is less oxygen in the air so the effects of hypoxia—not necessarily CO poisoning—are definitely amplified.
When pre-flighting, look for gray-white powdery residue around the gaskets for the exhaust pipes, at the seams of the muffler, and where the tailpipe connects to the muffler. Also look for any cracks in the system. The gases are under pressure so you may find residue a few inches away from the leak on the firewall, cylinders, or cowl.
Mike Busch has an interesting article on the prevalence of CO poisoning in aircraft accidents and he tests some units that were on the market in 2003.