The famous but simple Haldane Equation relates the partial pressure of carbon monoxide in the air, the partial pressure of oxygen in the air, and the concentrations of carboxyhemoglobin and oxyhemoglobin in the blood. "M" is known as the Haldane Constant and is equal to approximately 200. The equation was first published by John Scott Haldane at the turn of the century.

COHb will be increased by increasing the partial pressure of CO (ie., increased ppm CO), and/or by decreasing the partial pressure of ambient oxygen (ie., depleting oxygen).