Air pollution has remained at lower levels in UK towns and cities despite a return to near-normal traffic levels after the easing of coronavirus restrictions, according to research. Analysis of data from more than 100 urban roadside locations shows nitrogen dioxide pollution levels were 30% below normal at the end of June, despite HGV traffic being back at 95% of normal levels, vans at 90% and cars at 75%. Scientists think the small reduction in traffic that remains is enough to cut congestion on the roads and that this has an outsized impact in reducing dirty air. People may be staggering their commutes to work, which would also cut congestion, they said. David Carslaw from the University of York, who led the analysis, said the finding could help cities improve the way they cut pollution and that this is particularly important during the pandemic, given the growing evidence that dirty air could make Covid-19 more deadly. Nitrogen dioxide is mainly emitted by diesel vehicles and has been at illegal levels in most urban areas since 2010. It is responsible for an estimated 23,500 early deaths every year. At the height of the lockdown, NO2 levels plunged by 56%, as traffic fell to levels last seen in 1955. “Things are not back to normal according to the air quality data,” said Carslaw. “We think a big part of that is the effect of congestion.” Congestion ramps up emissions because vehicles are forced to repeatedly accelerate and brake, as well as sitting in one location in between. “Everyone would appreciate improved air quality and this suggests we don’t need such savage reductions in road traffic [as seen during lockdown] to achieve that. If you can reduce traffic by 10-20% and remove a lot of the congestion, that may have a disproportionate effect on the emissions.” The impact of congestion on air pollution is difficult to study because of its chaotic nature and the fact that emissions control systems on vehicles perform differently at different speeds. “There isn’t a great deal of data on how congestion affects air pollution,” said Gary Fuller of Imperial College London, who was not involved in the analysis. But he said congestion was a plausible cause of the lowered air pollution. Carslaw’s team used statistical tools to account for the effect of variable weather, which affects pollution levels. The data will help scientists to understand the link between congestion and air pollution. “It’s timely that we understand it better,” said Carslaw. “The last thing we want during the Covid-19 pandemic is increased air pollution as well, either for people that are currently suffering or who may be susceptible. There’s a strong reason to make sure air pollution is low during this period.” Different types of vehicle emit different levels of pollution. Stricter regulation of HGVs means they can emit less pollution than diesel cars, which could indicate that reduced car traffic is the most important factor. However, Carslaw said congestion was the most likely reason for the lower than expected pollution levels. The UK government’s advisers said last week that air pollution was likely to be increasing the number and severity of Covid-19 infections. This followed calls from MPs for action and a warning from lawyers that the government had a legal obligation to urgently review its air quality strategy. An air pollution inquiry by a select committee of MPs is also examining delays in the rollout of clean air zones in cities as a result of the pandemic.