A range of samples of textile filters varying in mean fibre size, packing fraction and area weight have been produced and subjected to filtration efficiency tests with monodisperse aerosols of sizes between 1.5 and 9.0 mum, and with standard synthetic dust. Although there is considerable scatter, sensible correlations exist between the results of the standard tests and those of the monodisperse aerosol tests for all sizes of particle. The performance data points of filters that owe their efficiency to fine fibre size, tighter packing or increased thickness all tend to fall on the same plot. A simple curve-fitting procedure, which would be strictly valid for filters that varied in thickness only, fits all of the data equally well. The penetration of monodisperse aerosol depends on a single parameter and that of the synthetic dust on two parameters, which are related to mean and standard deviation of the penetration of the various size components of the aerosol. The filters were also subjected to loading tests with the synthetic dust. If clogging is defined as the quotient of the pressure drop across a loaded filter and that across the same filter in the clean condition, correlation exists between the clogging rate and the mean fibre size, but convincing correlations with other parameters could not be found. Crown Copyright