SUMMARY: Small samples of hay were shaken in a perforated drum in a wind of 4·2 m./sec.; the liberated dust cloud was sampled with the cascade impactor for microscopical examination, and with the Andersen sampler for identification of organisms in culture. The results of testing batches of hay showed large differences in microbial content. Twenty-eight batches classed as ‘good hay’ gave up to 3 million spores/g. dry wt. hay, mainly with spp. and . Dust blown from seventeen batches of ‘mouldy hay’ differed greatly in composition and had from 5 to 250 million spores/g.: abundant forms included spp., spp., , bacteria, many actinomycetes. Fourteen batches of mouldy hay associated with cases of ‘farmer's lung’ disease were generally similar in mould content to the batches of mouldy hay, but had many more spores of and , and were specially characterized by very many actinomycetes. Farmer's lung hays tended to neutrality (averaging pH 7·0, as compared with pH 5 to 6 of other hays), and were rich in thermophilic organisms, commonly with hundreds of millions of actinomycete spores/g., a large proportion of which grew readily at 60°. Hays associated with farmer's lung appear to have heated spontaneously to a higher temperature during maturation than the other hays. Dust from all hays contained particles of higher plants, but there were not obviously more in farmer's lung than in other hays. All types of spores were shown to be potentially able to penetrate to the deeper parts of the lung (especially the spores of actinomycetes, and ), but whether these organisms play any part in the aetiology of the disease is unknown.


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