Open pore polyurethane foam (PUF) can be used effectively as a substrate for conventional inertial impactors with both high particle collection efficiency and minimal vaporization of semi-volatile particle components. The collection characteristics of PUF as an impaction substrate were studied as a function of PUF density, Reynolds number, impaction substrate diameter, and nozzle-toplate distance. The conventional impaction substrate of the PM2.5 Harvard Impactor sampler was replaced with the PUF substrate. The use of PUF resulted in significant changes in the collection efficiency curve, with the 50% cut-off size. © 2001 Taylor & Francis Group, LLC.