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