Table 1. Characteristics of the study sample with valid spirometry data (n=8,969).

Values are presented as number (%), unless otherwise indicated.

^*Subjects are greater than 40 years of age, mean±SD. ^†Chi-square test for categorical variables. ^‡By t-test for continuous variables. ^§Data from the 5th Korea National Health and Nutritional Examination Survey. llIncludes former and current smokers.