It has long been recognized that emissions from traffic have a negative impact on human health. In recent years there has been emerging consensus that the main influence is due to particulate matter. From an economic point of view these negative effects are external costs caused by traffic that, if not accounted for in decision making regarding transport, will result in a non-optimal allocation of resources leading to welfare losses. To be able to implement road pricing measures, but also for the evaluation of other control measures through benefit-cost analysis, information on the external cost of traffic emissions is needed. In the Impact pathway approach (IPA), that has been developed in the ExternE projects, the external cost is calculated as the product of exposure, effect and value. In this study the effect we focus on is health impacts (mortality). Regarding particulate matter (PM) there is recognition among the research community that there are different types of PM and that it is likely that their impact on human health differs. Still the current practice is to treat fine PM (which are considered to be most detrimental to health) as equally harmful irrespective of origin. In the TESS project the purpose has been to investigate how important the external health cost of road traffic generated PM is in relation to the cost of other sources of PM. To do this we have both investigated how the exposure varies between sources but also assessed if it is reasonable to assume that the impact differs between PM from different sources. Whether or not to assume that PM of different origin is equally harmful is of particular interest in Sweden where non-exhaust PM makes a large contribution to the concentrations of PM in urban areas. In the project we have used Stockholm as a case study and we have focused on mortality since this is the health impact that has been found to have the largest impact on health cost in other studies.