China clay workers in the south west of England: analysis of chest radiograph readings, ventilatory capacity, and respiratory symptoms in relation to type and duration of occupation.

In 1985, employees in the china clay industry were offered chest x ray examinations and 4478 (52.6% of the total workforce) accepted. Of these, 4167 workers and pensioners of the largest single employer also completed occupational histories, respiratory symptom questionnaires, and underwent ventilatory capacity tests. The x ray readings (read to the 1980 ILO classification) of the 4167 workers and pensioners were analysed to seek relations between the indices of pulmonary health and occupational exposure. The information available, particularly on occupational history, was more detailed than in previous studies of 1961, 1977, and 1981. Analyses show that in the improved operating conditions of recent years the average worker exposed to dust only after 1971 would not expect to develop category 1 pneumoconiosis through a full working life in any of the industry's occupations. For those with exposure before 1971 the category reached will depend on the amount of early exposure, but the rate of development of pneumoconiosis since 1971 is about half that before 1971. The milling of china stone, a practice that ceased over 15 years ago in the china clay industry, had by far the largest effect on x ray category, whereas of the current occupations, employment in china clay attritor mills has the greatest effect. Ventilatory capacity is related to x ray category as well as age, and results for loss of ventilatory capacity in relation to age, x ray category, and smoking habits were similar to the results in previous studies. Respiratory symptoms are associated with smoking class and a loss in ventilatory capacity (FVC or FEV1), FEV1 being the most dominant. Allowing for this, there was no further effect for years of exposure, x ray category, or age.