I would not rely on the OSHA recommendations for CO exposure for aviation operations for several reasons, the most important of which are that these limits are based on sea level partial pressures of oxygen (about 120 torr). At 10,000 feet, the partial pressure of oxygen is down to a little over 80 torr. At altitude, the effects of CO poisoning will be amplified due to several effects, including (1) lower hemoglobin-O2 saturation, which will allow higher hemoglobin-CO occupancy at altitude than at the same CO concentration at sea level, (2) lowered blood CO2 levels, which will decrease the ability of hemoglobin to release O2 to tissues or relinquish CO (indirect Bohr Effect), and (3) additional degradation of the ability of hemoglobin to release O2 to tissues due to the increased occupancy of hemoglobin by CO. (This latter effect it the primary reason that CO is a physiological poison. 25% occupancy of hemoblogin by CO is enough to effectively prevent oxygen delivery to tissues. Oxygen remains stubbornly bound to hemoglobin.)
It is estimated that sustained exposure to 50 ppm CO increases the effective physiological altitude by about 5000-6000 feet. That level of CO could push a healthy pilot flying at 9,000 or 10,000 feet beyond effectively safe oxygen altitudes. Those in less than superb cardiovascular condition could encounter hypoxia symptoms at lower altitude in the presence of CO. Remember, your amperometric CO detector measures absolute concentrations (pressures) of CO. As you increase in altitude, the ratio of that (constant) CO concentration to oxygen concentration increases for a particular "ppm" reading. It is the CO/O2 ratio that determines how effectively CO can compete with oxygen for hemoglobin binding.
I would not regard any persistent level of CO in the cockpit as safe during flight. My CO meter reads zero in flight. During taxi, I occasionally see up to 10-12 ppm wafting through the open canopy, presumably from exhaust pipe discharge being swirled around by the propeller.
