The filtration efficiencies of various types and makes of filters used in the cabs of agricultural vehicles have been measured in the laboratory using monodisperse aerosols. Both new and used filters were tested, though in the latter case areas of minimal dust loading were selected as samples for testing in order to obtain results close to those expected from unladen filters. Monodisperse aerosols of urea, in the respirable and near-respirable size range, were used. The results showed that pleated paper and needlefelt filter materials were poor at capturing small particles, though they have the advantage of robustness and low pressure drop. Pleated glass fibre materials performed much better but with the disadvantages of high pressure drops and fragility. The electrostatic filters tested had a low pressure drop and high filtration efficiency, but the form in which they were used was not that which would result in the optimal efficiency, resistance and lifetime.