NHANES has been conducted continuously since 1999, providing
an ongoing assessment of health, health-related behaviors,
nutrition, and environmental chemical exposures in the U.S.
population. Using a stratified, multistage, probability cluster
design, NHANES obtains a representative sample of the noninstitutionalized
civilian U.S. population (additional detail is
available at http://www.cdc.gov/nchs/nhanes.htm). Each year
approximately 5000 persons participate in the Survey, which
includes a household interview, health behavior interview, standardized
physical and medical examinations, and collection of
blood and urine specimens. Adult smoking information was
obtained in the household interview. Smoking status was defined
using two self-reported variables as current cigarette smokers,
former cigarette smokers, and non-smokers (defined as smoking
fewer than 100 cigarettes in a lifetime). Serum cotinine was used
to assess the validity of self-reported smoking status.