Figure 1.

Details of sampling and recruitment for baseline through Wave 3. Wave 2 attempted to recruit all subjects who indicated a diagnosis of asthma or chronic obstructive pulmonary disease (COPD) at baseline. Wave 3 attempted to recruit all subjects followed in Wave 2 plus those who indicated a diagnosis of allergic rhinitis or obstructive sleep apnea at baseline. The participants in the current study included subjects with COPD who completed Wave 3 follow-up, were current nonsmokers, and had urine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol measured.