As part of the « Sustainable mobility in the Brussels-Capital Region » project, the Centre for Economic and Social Studies on the Environment is currently developing a methodology for the assessment of the physical effects and external costs caused by air pollution generated by road traffic in an urban area. The approach considers a sequence that follows the five following main steps: human activities, emissions, immissions, physical impact and external costs. This paper describes specifically the relationships between the first three steps of the sequence. As the first step, we present a module that allows the assessment of the contribution to air pollutant emissions of traffic, domestic and office heating as the main urban sources. Road traffic emissions are assessed using both the COPERT methodology and traffic data for the Brussels-Capital Region. Emissions from other activities are taken into account on the basis of statistical data and suitable emission factors in relation with economic and meteorological data. Three pollutants nitrogen oxides, particulate matter and sulphur dioxide are considered. As the second step, we describe a model that establishes the relationship between the emissions previously calculated and the pollutant concentrations in the ambient air. This relationship uses an empirical model based on a hybrid statistical-deterministic distribution. The model is designed as a statistical relation between economic, meteorological and environmental variables given in the form of daily data series for the period 1994-1996. The type of relations used in order to represent the dependence between these variables is also discussed. The results make it possible to determine the responsibility of each sector of activity in the overall immissions measured in the Brussels area. A comparison is presented between the values measured by the monitoring network and those evaluated by the model. The impact of road traffic in the air pollution in Brussels is discussed for the period considered.