@article {1433580,
	title = {Smoking duration alone provides stronger risk estimates of chronic obstructive pulmonary disease than pack-years},
	journal = {Thorax},
	volume = {73},
	number = {5},
	year = {2018},
	month = {2018 05},
	pages = {414-421},
	abstract = {BACKGROUND: Cigarette smoking is the strongest risk factor for COPD. Smoking burden is frequently measured in pack-years, but the relative contribution of cigarettes smoked per day versus duration towards the development of structural lung disease, airflow obstruction and functional outcomes is not known.

METHODS: We analysed cross-sectional data from a large multicentre cohort (COPDGene) of current and former smokers. Primary outcome was airflow obstruction (FEV/FVC); secondary outcomes included five additional measures of disease: FEV, CT emphysema, CT gas trapping, functional capacity (6 min walk distance, 6MWD) and respiratory morbidity (St George{\textquoteright}s Respiratory Questionnaire, SGRQ). Generalised linear models were estimated to compare the relative contribution of each smoking variable with the outcomes, after adjustment for age, race, sex, body mass index, CT scanner, centre, age of smoking onset and current smoking status. We also estimated adjusted means of each outcome by categories of pack-years and combined groups of categorised smoking duration and cigarettes/day, and estimated linear trends of adjusted means for each outcome by categorised cigarettes/day, smoking duration and pack-years.

RESULTS: 10 187 subjects were included. For FEV/FVC, standardised beta coefficient for smoking duration was greater than for cigarettes/day and pack-years (P,
	keywords = {Aged, Cigarette Smoking, Cohort Studies, Cross-Sectional Studies, Female, Forced Expiratory Volume, Humans, Male, Middle Aged, Pulmonary Disease, Chronic Obstructive, Pulmonary Emphysema, Risk Assessment, Surveys and Questionnaires, Time Factors, Tomography, X-Ray Computed, Vital Capacity, Walk Test},
	issn = {1468-3296},
	doi = {10.1136/thoraxjnl-2017-210722},
	author = {Bhatt, Surya P and Kim, Young-Il and Harrington, Kathy F and Hokanson, John E and Lutz, Sharon M and Cho, Michael H and DeMeo, Dawn L and Wells, James M and Make, Barry J and Rennard, Stephen I and Washko, George R and Foreman, Marilyn G and Tashkin, Donald P and Wise, Robert A and Dransfield, Mark T and Bailey, William C}
}