The particle penetration characteristics and solid volume fractions of 1.57 and 0.99 mm thick stainless steel porous metal filters with pore sizes from 100 to 2 mu m were determined experimentally. The penetration measurements were performed with monodisperse NaCl and latex particles at face velocities of 3.0, 6.1, and 12.1 cm s(-1). The penetration data were fitted by non-linear regression with a simple equation that takes into account the diffusion and impaction mechanisms. The effective fiber diameters for the filters based on the measured pressure drops were also calculated, and they agreed with the effective single-collector diameters obtained from the fitted equations. Loading of filters was studied by sampling indoor air for one and two months in the laboratory and in an apartment. The penetration characteristics with monodisperse NaCl particles and the pressure drops of these filters were also measured. Loading of particularly the small pore size filters was shown to decrease penetration significantly with longer sampling times. The increase in pressure drop during sampling is a good indicator of mass loading. The filters have been used in a portable diffusion battery and the increased pressure drop was used as a determinant of a filter change or the end of sampling. The porous metal filters are cleanable and reusable, and electrostatic effects, that may cause errors when sampling with membrane filters, are avoided. (C) 2000 Elsevier Science Ltd. All rights reserved.