TTFC was strongly associated with COPD, even when participants were stratified by other smoking covariates. The association persisted across stratum of smoking duration (Fig 1). For example, risk of COPD increased as TTFC shortened among both those who smoked for 40 years or less and those who smoked for 40 y or more, for whom the ORs (95% CI) were 1.17 (0.43–3.18), 2.08 (0.92–4.69) and 3.19 (1.19–8.53) for TTFC 31–60, 6–30, and <5 minutes compared to TTFC >60 minutes, respectively (Fig 1 and Table B in S1 File). Similar patterns were observed across stratum of other examined smoking phenotypes including cigarettes per day, pack-years, years smoked, and age at smoking initiation.
