It is customary to measure the percentage of carbon monoxide hemoglobin (CO-Hb) in studying CO poisoning. There are a number of techniques for measuring CO-Hb, however, of which the procedures are usually very cumbersome, time-consuming and necessitating the preparation of blood samples. In the population survey, those methods are, therefore, thought to be inadequate. Ringold et al. developed a method to estimate the percentage of CO-Hb by measuring the concentration of CO in the expired air. In the present study the authors collected samples of expired air in polyvinyl bags (18 cm × 24 cm 0.1∼0.15 mm thick) which had shown no significant CO losses for 2 days, according to Jone's 20 second breath-holding method, and analyzed samples using a commercially available nondispersive infrared analyzer, Horiba Model LIA-2. Interferences by carbon dioxide and water vapor were avoided by means of soda lime and silica gel respectively. Blood CO-Hb percentage which was measured by Conway microdiffusion method well correlated to the expired air CO concentration in the range of lower concentrations. Medical students (119) were examined in relation to their cigarette smoking habits and the result showed that number of cigarettes smoked in a day had significant correlation with the CO concentration of the expired air. Average expired air CO concentrations of 276 school children in Tokyo on very fine days in autumn were 1.5-2.0 ppm in the ambient air with CO at about 1.0 ppm. On the other hand, school children (1383) in rural Japan in winter showed a little higher average expired air CO (3.0-5.5 ppm) regardless of the presence of heaters in the class room (ambient air CO at 0.5-3.0 ppm). This expired air CO is thought to be accumulated CO emitted from traditional rural Japanese heating. But a small amount of expired CO concentration which was higher than that of the ambient CO might be due to endogenous CO.

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