Although recent studies have shown a positive association of exposure to ultrafine particulate matter (PM) with adverse effects on human health, it is not yet clear which PM components or properties of these particles may cause these responses. In the context of human exposure, depending on ventilation and air exchange ratios and in the absence of major indoor sources, an appreciable fraction of the indoor ultrafine aerosol is of outdoor origin. This study examined volatility of penetrating ultrafine outdoor particles, predominantly from freeway emissions, into indoor environments where other particle sources were minimized and no cooking activities took place. A tandem differential mobility analyzer (TDMA) system was used to study particle volatility at two apartments, 15 and 40m downwind of the I-405 Freeway in Los Angeles, CA. The first differential mobility analyzer (DMA) selected particles of a certain diameter and subsequent heating of this monodisperse aerosol allowed for detection of changes in particle diameters by measuring the resulting size distribution with a second DMA. Aerosol volatility was examined by measuring changes in particle diameters as well as volume and number concentrations. Results suggest that outdoor particles are more volatile than indoor aerosols. Increasing temperature from ambient to 130 degrees C decreased and broadened indoor and outdoor aerosol mode diameters, however greater mode decreases were observed for outdoor particles. Furthermore, outdoor particles lost more of their volume upon heating than indoor aerosols. No significant particle losses due to volatilization were observed at 60 degrees C for either indoor or outdoor aerosols. A greater number of outdoor than indoor particles was lost at 110 degrees C. Heated outdoor particles with diameters greater than 45 nm showed bi-modal distributions, indicating that some of the aerosol is composed of primarily non-volatile particles, whereas the remaining particles are composed of mainly volatile material and consequently shrink. Evaluation of outdoor particle volatility as a function of distance to the freeway revealed that aerosol volatility decreases with increasing distance from the source. (c) 2004 Elsevier Ltd. All rights reserved.