If there were a ground level release of toxic gas in central London, how big an area would be contaminated? In the event of an emergency, this question would need to be answered rapidly. But with the complexity of turbulent mixing of the plume of gas as it travels along streets and around buildings, can this question be answered with physical basis? The DAPPLE project (Dispersion of Air Pollutants and their Penetration into the Local Environment, www.dapple.org.uk) is being undertaken by a consortium of six universities and involves a multidisciplinary approach to address these questions. Here we show how the results of DAPPLE can be integrated to give picture of three distinct regions of dispersion. Very near the source, within 4-5 building heights of the release, the plume remains narrow as it meanders along the street, meaning high fluctuations in concentration. Further downwind, on a neighbourhood scale, the plume becomes well mixed across the streets and the geometry of the street network and topology of the intersections controls lateral spread of the plume. Yet further downwind the majority of the plume mass lies above roof level. Mixing down from above roof to street level then controls ground level concentrations. Surprisingly simple rules emerge that will help inform emergency response.