The prevalence of coalworkers' pneumoconiosis: its measurement and significance.

Abstract

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 unneces sarily 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 theo retical 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. Preva lence 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 forward looking studies designed to measure attack rates and progression rates and the factors influencing them. Both these reasons have led the epidemio logical 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, Cochrane, Gilson, and Higgins, 1956); 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 tuber culosis (Cochrane et al, 1956); and the relationship between simple pneumoconiosis and bronchitis (Higgins, Oldham, Cochrane, and Gilson, in the press).