Low smoking exposure and development and prognosis of COPD over four decades: a population-based cohort study

Background A diagnosis of COPD is mainly considered in individuals with >10 pack-years of smoking. We tested the hypothesis that low smoking exposure, below the critical threshold of 10 pack-years, increases risk of COPD and leads to poor prognosis. Methods We followed non-obstructed adult smokers from the Copenhagen City Heart Study for COPD, defined as a forced expiratory volume in 1 s (FEV 1 )/forced vital capacity <0.70 and FEV 1 <80% predicted, and for related clinical outcomes. First, we followed individuals for 5 years according to baseline smoking for risk of developing COPD, and thereafter for up to four decades for severe exacerbations and death. Results In 6098 non-obstructed smokers, 1781 (29%) developed COPD after 5 years of follow-up: 23% of individuals with <10 pack-years of smoking at baseline, 26% of those with 10–19.9 pack-years, 30% of those with 20–39.9 pack-years and 39% of those with ≥40 pack-years. During four decades of follow-up, we recorded 620 exacerbations and 5573 deaths. Compared to individuals without COPD with <10 pack‑years of smoking, multivariable adjusted hazard ratios (HRs) for exacerbations were 1.94 (95% CI 1.36–2.76) in those without COPD and ≥10 pack-years, 2.83 (95% CI 1.72–4.66) in those with COPD and <10 pack-years, 4.34 (95% CI 2.93–6.43) in those with COPD and 10–19.9 pack-years, 4.39 (95% CI 2.98–6.46) in those with COPD and 20–39.9 pack-years and 4.98 (95% CI 3.11–7.97) in those with COPD and ≥40 pack-years. Corresponding HRs for all-cause mortality were 1.20 (95% CI 1.10–1.32), 1.31 (95% CI 1.13–1.53), 1.59 (95% CI 1.40–1.79), 1.81 (95% CI 1.62–2.03) and 1.81 (95% CI 1.55–2.10). Conclusion Low smoking exposure below the critical threshold of 10 pack-years increases risk of COPD in middle-aged adults within 5 years, and these individuals have increased risk of severe exacerbation and early death over four decades.