The overall aim of IMPROVE (Implementing Methodologies and Practices to Reduce air pollution Of the subway enVironmEnt) is to provide a benchmark study that will lead to real improvement in subway air quality. Published research has shown that in some cases air quality in subway stations and access tunnels can be poor, although the amount of information available remains rather limited and piecemeal in character. Physical measurements frequently consider only one aerosol size fraction, sampling campaigns may be limited in time and place, and chemical analyses are usually partial and/or small in number. IMPROVE LIFE is bringing together all information published in different cities across the world over the last 20 years, identify the main pollution sources, and ascertain just where we are in our understanding of the subject. The understanding and conclusions reached from this initial state-of-the-art overview of the problem will then be applied to the underground rail system in Barcelona.