The Diffusion of Carbon Monoxide out of Cigarettes

In some cigarette designs, diffusion of carbon monoxide out of a cigarette as the gases are drawn from their formation region towards the mouth end of the cigarette is more important than air dilution in controlling carbon monoxide mainstream delivery. The diffusion is a three-stage process: radial diffusion through the tobacco bed, diffusion through the paper, and diffusion away from the outer surface of the paper. Measurements have been made of the diffusion coefficient of carbon monoxide through paper, and the diffusion coefficient from the cigarette. The former has a value of 4.4 × 10^-3 cm² s^-1 for paper of permeability 70 cm³ min^-1 (10 cm²)^-1 (10 cm water)^-1, whereas the latter has the much lower values of 1.5 to 1.9 × 10^-3 cm² s^-1 (dependent on external air movement) for an unlit cigarette wrapped in the same paper. In a lit cigarette, the combined diffusion coefficient is increased by about 50 %, due to thermal effects. The diffusion of carbon monoxide away from the outer surface of the paper is a significant component of the overall diffusion coefficient determined for cigarettes. This diffusion rate is affected by the external air movement round the cigarette. Thus the draught conditions under which cigarettes are smoked could have an important influence on the carbon monoxide delivery determined. For inherently porous cigarette papers, the diffusion coefficient of carbon monoxide through the paper is approximately proportional to the square root of permeability. For a perforated paper, the diffusion coefficient is dependent on the permeability of the base (unperforated) paper, and virtually independent of the degree of perforation. These observations are explicable using a simple model of the paper structure.