Air pollution in the UK has recently re-emerged as a major policy problem. Insights from the agenda-setting literature are used here to explain the extent to which air quality has become a national political issue, and a local issue in London. The article explores the development of a problem stream in which scientific experts and non-governmental actors have since the late 2000s, provided evidence related to the economic and particularly health costs of air pollution, and increasingly framed the problem as urgent. Key focusing events have also contributed to increasing media coverage and public concern. Explanations are offered for the limited policy response at the national level compared to London. The research is based on primary data which includes 16 elite interviews with key actors, manifestos from London Mayoral and UK general elections, and local and national government policy documents. In London there is broad cross-party consensus and relatively high public concern, and a mayor acting as a policy entrepreneur, able to build upon pre-existing congestion charging policy. In contrast, at the national level there is a shared understanding that air pollution constitutes a problem, but less consensus on its scale amongst the public and across political parties, fragmented policy making and responsibility for the issue and little agreement on the appropriate policy response.