, 217-226. Role of dust in the working environment in development of chronic bronchitis in British coal miners. In the course of a long-term prospective study of chronic respiratory disase in British coal miners the effects on pulmonary ventilatory function of exposure to airborne dust, of simple pneumoconiosis, and of chronic bronchitis have been examined in a group of 3581 coalface workers.The men were employed in 20 collieries throughout the British coalfields. Their cumulative exposures to coal mine dust in the respirable range (1-5 ,um) were calculated from detailed dust sampling results at their work places during a 10-year period and from estimates of earlier exposures based on records of their industrial histories.A progressive reduction in FEV1.0 with increasing cumulative exposure to airborne dust has been demonstrated. This effect was evident also in a subgroup of the men studied who reported no signs of mild bronchitic symptoms (cough and phlegm for at least three months in a year).Among men with pneumoconiosis there was no evidence of a reduction of FEV1.0 in excess of that attributable to their dust exposures, smoking habits, age, and physique.

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The British medical public have in the past been so well served by the Registrar-General's mortality data that there has been little interest in other medical indices. They may, too, have been unnecessarily discouraged by imagined difficulties associated with the measurement of such other indices as prevalence and attack rate. As, however, interest in the acute killing diseases subsides and increases in the chronic disabling diseases, it is becoming slowly apparent that mortality rates are insufficient. The Registrar-General, for instance, has to disregard rheumatoid arthritis, and information about excess mortality due to coalworkers' pneumoconiosis can be gleaned only by complicated deductions from death rates from other better-known respiratory diseases.The prevalence rate is not in itself, from a theoretical point of view, a very satisfactory index in that it is a secondary and not a primary epidemiological measurement. It varies directly with the attack rate and indirectly with the case fatality rate. It is thus possible for similar prevalence rates to represent very different attack and case fatality rates. Prevalence is, however, important for two reasons. In the first place, it measures better than any other index the actual load on the community socio-medically as well as from an administrative point of view. It measures the size of a problem. Secondly, a prevalence study is a necessary first step for "forwardlooking" studies designed to measure attack rates and progression rates and the factors influencing them. Both these reasons have led the epidemiological section of the Pneumoconiosis Research Unit (P.R.U.) to carry out measurements of the prevalence of pneumoconiosis in many different populations. The results are summarized in this article. The results of investigations into other aspects of pneumoconiosis have been, or are being, summarized in other papers, e.g., the relationship between radiological category and disability (Gilson and Hugh-Jones, 1955; Carpenter, ; the correlation between simple pneumoconiosis and exposure to dust (Roach, 1953); the relationship between radiological category and expectation of life (Carpenter and Cochrane, 1956); the factors influencing the attack rate of progressive massive fibrosis (P.M.F.) (Cochrane and Miall, 1956); the factors influencing the radiological progression of P.M.F. (Cochrane and Carpenter, 1956); the tuberculous infectivity of P.M.F. (Carpenter et al., 1956); the effect of exposure to coal dust on the attack rate of tuberculosis ; and the relationship between simple pneumoconiosis and bronchitis (Higgins, Oldham, Cochrane, and Gilson, in the press). MethodologyTerminology.-It has not been necessary to invent any new epidemiological indices. "Death rate ", " prevalence rate ", and " attack rate " are used in the customary and well defined senses. Only two points deserve comment. The word " incidence" has not been used at all, and " attack rate " has been preferred to " inception rate ". " Incidence" is, we think, an excellent word, but its orig...

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