In this paper we investigate the importance of exposure calculations for the health cost estimation of traffic emissions with the impact pathway approach. We focus on particles and the effect they have on chronic mortality since this have been shown to impose the largest costs to society. Up to now particles have been treated as a homogeneous type of pollutant though research in recent years indicates that in an urban area there is at least three different kinds of particles that are likely to impose impacts on the population exposed. The problem is that the harmfulness and the dispersion differ between different particles. This, together with population density, will make the exposure to particles vary between different areas and hence the cost. In this paper, we develop a model for exposure and cost calculations. With simulations we also investigate the importance on costs of various assumptions used in exposure calculations. The implication of our findings is that the current limit values based on PM10 do not target the emissions that give rise to the largest cost. The current limit values may result in reduction measures that have a small impact on the total cost of the emissions from traffic.