. In 1935, two leading scientists, J.B. S. Haldane and in J. G. Priestley, posed the following puzzle. Carbon?

. In 1935, two leading scientists, J.B. S. Haldane and in J. G. Priestley, posed the following puzzle. Carbon monoxide binds to the same heme sites on hemoglobin as oxygen with an affinity 200 times as great as that for oxygen. Exposure for 1 hour to a carbon monoxide concentration of 0.1% of inspired air leads to an occupancy by CO of about 50% of the heme sites in the hemoglobin, a proportion that is frequently fatal. Yet, a person whose hemoglobin concentration is reduced to 50% due to anemia may go about work as usual. What is the key to this seeming paradox using our present day knowledge of hemoglobin?