d Nutrition Examination Survey (KNHANES) from 2008 to 2011 were used to
investigate the association between blood cadmium levels and COPD. The
KNHANES survey is a repeated cross-sectional study which used a clustered
stratified random sampling method and was representative of the Korean population.
The survey method and detailed information about KNHANES have been
described previously (Yoo et al., 2011; Oh et al., 2013). KNHANES included a health
survey, anthropometric measurements, spirometry data, and blood sample for
biochemical and laboratory examination