Pilots tend to think of carbon monoxide gas as something produced by a defective muffler, a faulty exhaust system, or a heater leak in the aircraft cabin. When they perform their preflight check of the aircraft and find no breaks or cracks, they feel reassured.

Yet, one of the more common sources of carbon monoxide intoxication in an aircraft is tobacco smoke. Carbon monoxide makes up about 3% of cigarette smoke and from 5% to 8% of cigar smoke. A one-pack-a-day cigarette smoker is walking around with about 4% to 8% of his blood saturated with carbon monoxide. At ground level, he may be untroubled by this, but altitude flying changes the picture.

Carbon monoxide has an attraction for the red blood cells which is 200 times greater than that of oxygen. If a molecule of carbon monoxide unites with a molecule of haemoglobin, which ordinarily carries oxygen, they stick together like glue. Oxygen doesn't stand a chance in the competition for haemoglobin. Thus, the red blood cell cannot again carry oxygen into the system until the carbon monoxide is expelled. For the pilot at altitude, whether he is hypoxic because of low oxygen availability or whether he is poisoned by carbon monoxide, the effect is the same.

Tobacco does more than deprive the body of oxygen because of the carbon monoxide content in smoke. It lowers the sensitivity of the eye and cuts night vision by approximately 20%. Moreover, nicotine increases the body's heat production 10% to 15% above normal creating added oxygen demands. Ironically, the same cigarette that increases the demand for oxygen also reduces the supply.

Careful tests have shown that the carbon monoxide in tobacco smoke can lower the pilot's tolerance to altitude by as much as 5,000 to 6,000 feet. In other words, medically speaking, pilots who smoke are already "at altitude" before they ever leave the ground. If you smoke, you will need to use your oxygen systems earlier than a non-smoker would during ascent. If you classify yourself as a moderate-to-heavy smoker, use your oxygen at all altitudes during night flying. You will find day flying more comfortable and safer - if you use oxygen above 5,000 feet.