Imposing strict controls on car exhausts will only partially improve the quality of air people breathe in the UK, scientists have said. New measures to counter emissions of nitrogen oxides and other air pollutants will also be needed for buildings, heating plants and many other domestic and industrial sources in future. The warning follows the controversy that has surrounded London’s ultra low emission zone (Ulez) in which drivers are charged for their vehicles’ polluting impact. This month the mayor of London, Sadiq Khan, will expand the zone from inner London so it covers all boroughs in the city. The decision has provoked opposition from some drivers and was blamed by various Labour party figures for the Conservatives surprise byelection win in Uxbridge and South Ruislip last month. But scientists at the Wolfson Atmospheric Chemistry Laboratories at the University of York last week stressed the need to maintain air quality, particularly in cities. “Air pollution is one of the highest causes of preventable death in the UK,” Prof Alastair Lewis told the Observer last week. “The National Health Service is run at full stretch all the time, so every extra medical problem has a disproportionate effect. The crucial point is that it may be hard to make people eat healthy foods or not drink too much alcohol, but it is easy to make them breathe in clear air – as long as you provide it.” In London, there is very little difference between air quality in the inner city and in its outer boroughs – at present. Both have pollution levels that are far higher than those set as safe standards by the World Health Organization. However, the York team said they expected air quality to improve fairly rapidly in London’s outer areas as the extension of the city’s Ulez takes effect. But in the central area it will remain relatively high. Part of the problem lies with the greater congestion of the inner city. Vehicles emit more pollutants when braking and accelerating than they do when travelling at a steady speed. But there is another reason, one that has crucial implications for all urban areas. “About half of all the nitrogen dioxide that pollutes central London actually comes from buildings and not from vehicles, and while we are taking measures to improve pollution from cars, buses and lorries, we are not yet tackling those produced by houses, offices and factories,” said Lewis. The particular problem affecting buildings focuses on the way they are heated. Typical gas boilers and combined heat and power sources produce significant amounts of nitrogen oxides. “This latter issue – of space heating, as it is known – is going to become increasingly important in coming years,” said Lewis This point was backed by his colleague Prof James Lee. “There will come a time when we will find that we cannot get air pollution levels to go down because we have done as much as we can by making improvements to pollution produced by road vehicles, but still have not tackled space heating and its impact on air quality. Then we will be at impasse – unless we act now.” A measure of the problem is provided by examining how much nitrogen dioxide pollution is produced by a typical domestic gas boiler in a day. “The average output is the equivalent of driving a new diesel car for 70km,” added Lewis. “It is a non-trivial amount.” The problem is made worse because many people think that pollution from boilers and other building heating systems will disappear should gas and oil be replaced by hydrogen as a key fuel for keeping our homes, offices and factories warm. “People assume that burning hydrogen is a clean process that produces only water vapour, but that is not the case,” said Lewis. “The heat of the flame you produce by burning hydrogen causes nitrogen molecules – the main constituent of air – to break apart and form nitrogen oxides, which are pollutants. It is an issue that planners and officials often miss, but it is important.” The crucial point is that if hydrogen is going to be used as a substitute for fossil fuels, as is planned, then plans need to be put in place immediately to ensure that machines that burn it are designed to limit the pollution they will produce. “We need to think about these issues today if we want to keep cutting air pollution in future,” said Lee. “We need to reach net zero in the near future, but in a way that also keeps air pollution to a minimum.”