An alternative way of constructing a concentration effect curve is to determine the percentage of a population of patients showing a defined response at various drug concentrations. For example, with phenytoin, the therapeutic response might be defined as >80% decrease in the frequency of fits and the adverse effect defined as the proportion of patients developing nystagmus on looking sideways. These are called quantal (population) concentration response curves and have the same shape and parameters as the graded concentration response curves referred to above.
Another special case of the Bohr effect occurs when carbon monoxide is present. This molecule serves as a competitive inhibitor for oxygen, and binds to haemoglobin to form carboxyhaemoglobin .  haemoglobin's affinity for CO is about 250 times stronger than its affinity for O 2 ,  meaning that it is very unlikely to dissociate, and once bound, it blocks the binding of O 2 to that subunit. At the same time, CO is structurally similar enough to O 2 to cause carboxyhaemoglobin to favor the R state, raising the oxygen affinity of the remaining unoccupied subunits. This combination significantly reduces the delivery of oxygen to the tissues of the body, which is what makes carbon monoxide so toxic . Interestingly, though, this toxicity is reduced slightly by an increase in the strength of the Bohr effect in the presence of carboxyhaemoglobin. This increase is ultimately due to differences in interactions between heme groups in carboxyhaemoglobin relative to oxygenated haemoglobin. It is most pronounced when the oxygen concentration is extremely low, as a last-ditch effort when the need for oxygen delivery becomes critical. However, the physiological implications of this phenomenon remain unclear.