The attack rate of progressive massive fibrosis.

A radiological follow-up of the miners and ex-miners in two Welsh mining valleys whose chest radiographs showed simple pneumoconiosis in 1950/51 has been carried out to study the factors associated with the attack rate of progressive massive fibrosis. The radiographs were read in pairs, and also with the pairs separated, the radiographs being randomized and identifying features concealed. The relative value of these two types of reading is discussed. An attempt to investigate the importance of exogenous tuberculous infection by comparing the attack rate in the two mining valleys, in one of which great efforts had been made to eradicate tuberculosis, failed. The reasons for the failure are discussed. No evidence was found of any association between the attack rate of progressive massive fibrosis and age, energy expenditure at work, smoking habits, body type, exogenous tuberculous infection, or endogenous infection as measured by the presence of primary complexes in the first radiographs. The only factor related to the attack rate was the average category of simple pneumoconiosis. The attack rate is zero at category ½ and rises to 30 or more per 100 in eight years for category 3. It is argued from this that the logical way to control the appearance of progressive massive fibrosis is to concentrate on preventing miners reaching category 2 of simple pneumoconiosis. Evidence is also presented that considerable progression of simple pneumoconiosis has been occurring during the past eight years amongst coal-face workers at the collieries in this area.