The rebound was likely due to industrial activity, the researchers said, adding there were concerns that after months of unusually low pollution levels, a drive to kickstart economic activity was causing emissions to spike.

“There are early warning signs that China’s recovery from the COVID-19 crisis is reversing air quality gains,” the Helsinki-based Centre for Research on Energy and Clean Air (CREA), which produced the study, said.

Average levels of some air pollutants in China dropped in February to significantly below levels for the same period in 2019, as lockdown measures shuttered factories, curbed electricity demand and slashed transport use as swathes of the population stayed home.

But average levels of some pollutants have since rebounded, and were higher in the 30 days ended 8 May compared with the same period in 2019, CREA said in its analysis of data from 1,500 air quality monitoring stations in China.

This was true of nitrogen dioxide, sulfur dioxide, and fine particulate matter, suggesting a rebound in industrial activity drove the trend, CREA said.

Regions with factory clusters reported bigger increases in nitrogen dioxide emissions. Densely populated urban areas – where emissions of the gas are mostly from vehicles, rather than factories or power plants – showed smaller increases.

After months of lockdowns, China is reopening its economy as the outbreak comes under control, although some cities – such as Shulan, in the north east – have reimposed lockdown measures after reporting clusters of new coronavirus infections.

Overall passenger transport use in China remains lower year-on-year, but CREA said concerns about catching the coronavirus had led people to choose private cars over public transport as lockdowns eased, contributing to the rise in air pollution.

In Europe, cities including London, Milan and Brussels have expanded cycle lanes to encourage people to choose bikes over cars as containment measures start to lift.

But many emissions-intensive sectors are desperate to return to work. Public health campaigners said the China study showed governments would need to take tougher measures to clean up industries, to avoid a sustained surge in health-damaging air pollution.

“There is no reason to think that going back to normal would not have the same consequences – namely, pre-crisis pollution levels,” Zoltan Massay-Kosubek, policy manager for clean air at the non-profit European Public Health Alliance, said of easing lockdowns in Europe.

“The question is what will be the new normal.” (Reporting by Kate Abnett Editing by Frances Kerry)