There are several plausible mechanisms by which shorter TTFC confers a greater risk of COPD. For example, highly dependent smokers inhale more deeply [19], which leads to a higher dose of cigarette-associated carcinogens [20–22]. There may also be other smoking differences between smokers with and without COPD that are not captured by established smoking metrics but are associated with TTFC; these factors include how much of the cigarette is smoked before it is extinguished and how long smoke is retained in the lungs after inhalation and prior to exhalation.
