A metric for the assessment of health effects of diesel exhaust particles is proposed and a methodology to determine it from experimental data is presented. The metric treats separately the emitted volatile and non-volatile mass fractions. The appropriate metric for health related effects of the non-volatile mass fraction (primarily soot), termed the physical effect, is taken to be the particle (active) surface area distribution. The corresponding metric of the volatile mass fraction (unburned hydrocarbons, sulfates, nitrates), termed the chemical effect. is taken to be the volatile mass concentration distribution of the nucleation mode and the mass concentration distribution of the condensed volatiles on the accumulation mode. The latter is proportional to distribution of the (active) surface area concentration of the accumulation mode. A methodology to derive the distributions (metric) from filter-based mass and number distributions measurements is suggested. Nine experimental particle number distribution, emitted by light duty Euro 1, 3 and 4 vehicles, with and without diesel particulate filters or catalysts, at different speeds and using different fuels, are analyzed to determine characteristic volatile mass and non-volatile surface area concentration distributions. These distributions become the required input distributions for lung deposition calculations, thereby defining the appropriate metric to estimate health effects of diesel exhaust particles. (C) 2009 Elsevier Ltd. All rights reserved.