Early researchers would have been puzzled to see ground-level ozone breaching World Health Organisation guidelines across the UK during late March and early April this year. At this time some places still had laying snow and daytime temperatures were only just above freezing. This ozone was not caused by pollution from a single city. It was due to the build up of pollutants over the whole northern hemisphere during the winter, with contributions from Europe, North America and Asia forming a reservoir of pollutants ready to react in the spring sunshine. Three-way catalysts on petrol cars have gone a long way to control summertime smog in Europe and North America. However, despite many millions of people being exposed to springtime ozone above human health thresholds there is no explicit international mechanism to tackle the methane and carbon monoxide emissions that are causing ozone to increase globally.