The importance of the time factor in the measurement of dust exposure.

Measurements of dust concentrations for the estimation of the danger of pneumoconiosis have been carried out in various industries for many years. In some, such as the South African gold fields, they have been made regularly for nearly 40 years, while in others, such as the British coal-mines, regular dust sampling on a national scale has only been carried on for about five years. In other industries again, such as foundries, normally only occasional samples are taken in an attempt to make some estimate of the likely danger of pneumoconiosis. In all this work the main emphasis has been placed on the factor of concentration and little is heard of the possible importance of the duration of exposure. It is the purpose of this paper to attempt to arrive at an estimate of the relative importance of these two factors, since such an estimate is essential for the proper planning of dust sampling and dust suppression procedures. It must be emphasized that the discussion is concerned only with dusts which are of such low solubility that, if they are dissolved at all, their rate of solution is very low compared with their rate of inhalation and deposition in the lung. Examples of such dusts are quartz and coal, which between them produce most of the serious cases of pneumoconiosis in this country (Ministry of National Insurance, 1952). For the purpose of this discussion we shall assume, as a first approximation, that the severity of a pneumoconiosis due to a particular dust will be dependent upon the amount of that dust in the lungs. This is obviously not entirely true, but it is necessary to make some assumptions in order to be able to define the problem. Such evidence as exists (Badham and Taylor, 1938, 1940; King and Nagelschmidt, 1945) suggests that even when there are marked changes in pathology, e.g. from simple pneumoconiosis to massive fibrosis in coal-workers' lungs, there is a rough correlation between the amount of dust in the lungs and the severity of the disease. Our problem therefore is, What are the relative effects of concentration and time on the rate of inhalation and the retention of dust in the lungs ? In the absence of any other information, the natural assumption would seem to be that concentration and time are equally important, i.e., that the amount of dust retained in the lungs is proportional to the product of the two, so that an exposure to a concentration C for a time T will have the same effect as an exposure to a concentration