An important but little understood factor in the determination of mainstream smoke yield is the impact of air flows during the smoking process. Although the ambient air velocities surrounding cigarettes in an analytical smoking machine during the smoking process are defined in ISO3308, and in turn referenced by the Health Canada Intense method, the specification lacks detail concerning vectors and stability of air flow. These are considered to contribute to both the absolute yields obtained during smoking but also to the repeatability of measurements. A series of experiments were undertaken to understand the origins of the air consumed during smoking, where the smoke generated goes and how seemingly simple changes within the smoke hood can change yields. In particular user exposure to smoke , as evidenced by CO exposure, was examined and it was found that the user is not exposed (less than 1 ppm CO measured compared with OSHA PEL based on an 8 hr TWA 50 ppm or the ACGIH TLV of 29 ppm) provided overall extraction was maintained. It was observed that adding a barrier for ETS has the unexpected consequence of increasing yield variability. This can be explained by examining the detailed path whereby air impinges on the smoked cigarette. Some methods of controlling air flow were investigated including the use of various “air straightening” systems which were used to reduce smoke hood edge effects, reduce turbulence in the smoke path and how these efforts changed yields and variability of yields.