Parisian pollution is nothing compared with that of Beijing or Mexico City. Yet Paris, with 11 million people crammed into a region just 20 kilometres across, is to take centre stage in a new research project on the impact of megacities on air pollution. The MEGAPOLI project, which starts next month, will focus on building regional air-pollution models for every city in the world with a population of more than five million. It encompasses 23 research organizations from 11 European countries, along with 24 collaborating partners outside Europe.