Self-selected blocking of vents on low-yield cigarettes.

Blockers of vents in ultra-low-yield cigarettes had higher levels of carbon monoxide (CO) and salivary cotinine than did nonblockers. None of the blockers reported that they blocked vents. Exposure differences seemed not to be due simply to vent blocking, but also to be the result of syndromes of heavier (blocking, more cigarettes per day, and starting earlier in the morning) or lighter smoking (not blocking, fewer cigarettes per day, and starting later in the morning). The results are interpreted in light of the boundary model. Cigarette smoking and brand selection should be studied as they occur naturally, as well as in experimentally contrived studies.