Because carbon monoxide (CO) is freely diffusible despite being tightly bound, the proportion of total hemoglobin occupied by CO (%COHg) can be estimated by measuring the concentration of CO in expired gas. The graph relates exhaled CO in parts per million (ppm) to approximate %COHg, and may be useful in estimating overall exposure to smoke. The relationship assumes that patients are adult, are free of lung disease affecting diffusion, and have not been exposed to smoke immediately prior to measurement. Exhaled CO may be used as a patient education device, or as confirmation of abstinence. It is not accurate enough to be considered a direct measure of %COHg.