All TTFC categories showed an increased risk of disease (COPD, chronic bronchitis, and emphysema separately) compared to those with TTFC>60 minutes (P <0.05). The point estimate of COPD risk conferred by shortened TTFC was of greater magnitude for chronic bronchitis compared to emphysema, particularly among those with TTFC ≤5 minutes (ORs 2.29 and 2.99 for emphysema and chronic bronchitis, respectively).
