Martinez CH, Kim V, Chen Y, Kazerooni EA, Murray S, Criner GJ, Curtis JL, Regan EA, Wan E, Hersh CP, Silverman EK, Crapo JD, Martinez FJ, Han MK. The clinical impact of non-obstructive chronic bronchitis in current and former smokers. Respir Med. 2014 Mar; 108(3):491-9.