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Lockdowns had lower impact on air pollution than hoped

While urban air pollution levels dropped during the first Covid-19 lockdowns in March last year, a new study has found that the changes were smaller than expected.

A team from the University of Birmingham evaluated changes in ambient nitrogen oxide (NO2), ozone (O3) and fine particle (PM2.5) concentrations arising from lockdown emission changes in 11 global cities: Beijing, Wuhan, Milan, Rome, Madrid, London, Paris, Berlin, New York, Los Angeles and Delhi.

These three air pollutants, which are mostly created by vehicles as they drive around the cities, have been shown to be harmful to human health and cause an estimated seven million deaths globally every year. The World Bank estimates that air pollution costs the global economy $3tr.

The team found that, after removing the effects of weather, the beneficial reductions in NO2 due to the lockdowns were smaller than expected. They also saw that the lockdowns caused an increase in the concentrations of ozone.

NO2 is a key air pollutant from traffic emissions, associated with respiratory problems, while ozone is also harmful to health and damages crops.

Concentrations of PM2.5, which can worsen medical conditions such as asthma and heart disease, decreased in all cities studied except London and Paris.

Lead-author of the study, professor Zongbo Shi, said: “Rapid, unprecedented reduction in economic activity provided a unique opportunity to study the impact of interventions on air quality.

“Emission changes associated with the early lockdown restrictions led to abrupt changes in air pollutant levels, but their impacts on air quality were more complex than we thought and smaller than we expected.

“Weather changes can mask changes in emissions on air quality. Importantly, our study has provided a new framework for assessing air pollution interventions, by separating the effects of weather and season from the effects of emission changes.”

The team used machine learning to strip out weather impacts and seasonal trends before analysing the data: site-specific hourly concentrations of key pollutants from December 2015 to May 2020.

Roy Harrison, Queen Elizabeth II Birmingham Centenary Professor of Environmental Health, a co-author of the study, commented: “The reduction in NO2 will be beneficial for public health - restrictions on activities, particularly traffic, brought an immediate decline in NO2 in all cities.

“Had similar levels of restrictions remained in place, annual average NO2 concentrations would have in most locations complied with WHO air quality guidelines.”

Following the relaxation of the Chinese lockdown in May, air pollution in Shanghai and Beijing was found to have soared above the levels at which it had previously been. 

Urban air pollution has also been found to make people more susceptible to suffering negative health effects from Covid-19.

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