Summary One hundred and one histologically confirmed gastric cancer patients in Gwynedd, North Wales, were matched by sex, age and social class to two hospital inpatients without cancer. Seventy-seven of the gastric cancer cases were also matched, using the same criteria, to a patient with a confirmed cancer of a different site (excluding oesophagus). A (Stocks, 1936(Stocks, , 1937(Stocks, , 1939 and although the disease is declining in the United Kingdom as a whole, the incidence in some administrative districts of Gwynedd (North-west Wales) is still substantially higher than the national average. This fact is strikingly displayed in mapped form by Howe (1970) and by Gardner et al. (1983).The high incidence in North Wales has never been satisfactorily explained although the possibility that environmental and/or dietary factors are involved has been investigated by a number of different authors (Stocks, 1957;Davies & Wynne Griffith, 1954;Howe, 1979). Furthermore, population migration studies among the Welsh (Armstrong et al., 1983) and other races (Haenszel, 1961) would seem to exclude genetic influences as a major factor in aetiology.Many parts of Gwynedd have in the past formed 'island communities' with static populations who could thus be exposed throughout life to local environmental carcinogens. Bracken (Pteridium aquilinum) is widely distributed and one of the most successful weeds worldwide (Fenwick, 1988). It is estimated that almost 7% of Wales is occupied by bracken, and in Gwynedd the coverage is substantially greater, exceeding 20% land cover in parts of the county (Taylor, 1985).Many investigators have demonstrated the carcinogenic potential of bracken in a variety of animal species. Chronic bovine enzootic haematuria, which may be followed by bladder cancer, has been reported among cattle from many parts of the world (Pamukcu et al., 1967;Jarrett et al., 1978). In all cases there is good evidence that the animals have been affected as a consequence of grazing upon growing bracken or of eating cut bracken used as bedding; and bracken feeding experiments have confirmed the association (Pamukcu et al., 1967). The earliest experimental work was performed on rats who were given bracken fronds in their diet for 2 months; all the animals succumbed to ileal adenocarcinoma (Evans & Mason, 1965). The same workers obtained bladder tumours in guinea pigs, but in mice the most frequent malignancies produced were leukaemias and gastric carcinoma (Evans, 1984). The same malignancies can also be produced by feeding bracken spores to these animals (Evans, 1986). Among the features of bracken carcinogenicity of special interest observed in these mouse experiments are the vulnerability of the young weanling animal and the relatively long latent period before gastric tumours develop (Evans, 1987

Changes in ventilatory capacity during the work shift were studied in workers exposed to tea dust in two tea-packing plants, wood dust in two furniture factories, and virtually no dust in an inoperational power station. The FEV, and FVC in workers exposed to dust were found to decline during the work shift by a small but significant volume. The MMFR, Vmax 50% and Vmax 75%were too variable to display any trend. No dose-response relationship could be discerned between the fall in workers' ventilatory capacity and the concentration of airborne dust or microbes to which they were exposed. Bronchodilators could reverse the fall in FEV1.Many non-allergenic dusts of vegetable or animal origin have been shown to affect acutely the ventilatory capacity of exposed workers. McKerrow et alt found that dust in cotton mills suppressed the "indirect maximum breathing capacity" (IMBC) and increased airway resistance during the working day. These findings were later confirmed2 3 and others observed that the FEVo.75, FEV1, or forced vital capacity (FVC) were similarly affected in workers exposed to flax dusts,45 cotton dust,6 7 hemp dust,8 9 and jute dust.10 11 These changes were at one time considered to be byssinotic symptoms even though they occurred also in non-byssinotic subjects. In the past decade, however, workers' FEV1 and FVC in particular were found to decline on exposure to dusts in industries where byssinosis does not occur, such as in coal-mines,12 and in wool13 and tobacco processing factories.14 Similar changes tended to occur in other, occasionally more sensitive but at the same time less reproducible, indices of ventilatory function, such as the maximum midexpiratory flow rate (MMFR), peak-flow (PF), and flow volume curves (Vmax 25 %, Vmax 500%, and Vmax 75 %). Recently, Anderson et al15 showed that the FEV1 fell by a small but significant amount in people who had been rebreathing very high concentrations (2-25 mg/m3) of inert carbon-coated plastic particles for several hours in an exposure chamber.Evidence suggests that the decline in ventilatory

The prevalence at death of carcinoma of the lung in miners and ex-miners has been compared in those with and without pneumoconiosis at necropsy. The prevalence of 11 .4u/% in the group as a whole is no greater than that in the male population in North-west England. Carcinoma of the lung was present in 62 (13-1%) of those without pneumoconiosis and in 52 (9 8%) of those with pneumoconiosis.The mean age at death of those with pneumoconiosis was 713 years so that they cannot be said to have died before the age at which they would have developed carcinoma. Those with progressive massive fibrosis whose mean age at death was 72 years had the lowest prevalence of carcinoma of the lung at all ages-8-4%.For reasons stated in the text this is inevitably a biased sample. The number of those without pneumoconiosis is probably lower than the true figure because the deaths of miners and ex-miners in whom there is no suspicion of lung disease may not have been reported to the coroner or to the pneumoconiosis medical panel.There appears to be no positive link between carcinoma of the lung and pneumoconiosis. There is a surprisingly high number of smokers and ex-smokers among these miners, and this appears to have more relevance to the prevalence of carcinoma of the lung than does pneumoconiosis.The purpose of this study was to investigate the prevalence at death of carcinoma of the lung in coalminers who have worked in the Lancashire coalfields and to relate this to pneumoconiosis by direct observation of the lungs at necropsy without reference to the cause of death as stated on the death certificate.Most investigations into the prevalence of carcinoma of the lung in coalminers have been based on information obtained from death certificates and occupational statistics. Heasman (1962) has shown that the cause of death as stated on death certificates may underestimate the prevalence of carcinoma of the lung by 15 to 20%. Liddell (1973) has shown the unreliability of occupational statistics in giving the correct occupational status of a miner, especially when he has done other work after leaving the mines. Kennaway and Kennaway (1953) and Goldman (1965)