In 1958, 1,317 men working in the Detroit area were selected for study on the basis of two criteria: they were aged 40 to 65 years and working full-time. They were classified into four working groups: (1) no industrial exposure, (2) general industrial exposure, (3) 20 years of silica-dust exposure, and (4) 20 years of silica-dust exposure with roentgenogram evidence of silicosis. They were classified further as to whether they had chronic bronchitis, which was defined as a daily cough for six months, productive of at least a teaspoon of sputum a day. In 1964, the same group of men were reexamined using the same questionnaire as was used in 1958. A spirogram and chest roentgenogram were obtained as well. Altogether, 1,056 men were followed, which includes 123 men who died during this time. In this sample of the Detroit male population, the presence of chronic bronchitis did not have any significant adverse effect on health, working ability, or ventilation of these men over a six-year period.
Brinkman GL, Block DL. The Prognosis in Chronic Bronchitis. JAMA. 1966;197(1):1-7. doi:10.1001/jama.1966.03110010053016