During 1990 all present and retired china clay workers in the United Kingdom were invited to take part in a chest health survey. A total of 4401 The relation between total exposure to china clay dust and x ray film category is such that a typical non-smoker worker employed in the most dusty of current occupations may expect to reach the lower limit of category 1 after about 42 years continuous employment in that job at current exposures. Both forced vital capacity (FVC) and forced expiratory volume in one second (FEV,) were found (as in other studies) to decline with age, more rapidly for smokers than non-smokers at the rates for FVC of 0 035 Vy and 0 033 Vy, whereas for FEV, the rates are 0*039 Vy for smokers and 0*034 IIy for non-smokers. Changes in x ray film category are also related to lung function, a change of one major category being equivalent to about six years of ageing in its effect on FEV,.
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 ofpulmonary health and occupational exposure. The information available, particularly on occupational history, was more detailed than in previous studies of 1961, 1977, and 1981 In 1985 a chest x ray survey was carried out by the Nottingham based mobile unit of the National Coal Board using 40 x 40 cm films. It was offered to all people employed in the china clay industry in the south west of England. The subsequent analyses were based on a group of workers from the largest single employer, which represented 94% of the china clay industry for which full occupational details were available. For this group an overall response rate of 65% was achieved. For china clay employees in potentially dusty occupations (where measured respirable dust concentrations may exceed 0 5 mg/m3), however, the response rate was higher, at approximately 85%.
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