The relaxation of novel coronavirus lockdowns throughout Europe has triggered a sudden rise in harmful emissions, as people returning to work have fired up automobiles that had sat idle for months.

Brussels, Milan, and Paris have all had significant jumps in nitrogen dioxide levels in comparison to those from the height of the lockdown, according to new analysis of European Environment Agency data by the Centre for Research on Energy and Clean Air and the Financial Times.

The billowing pollution has been exacerbated by workers resuming their commutes in automobiles instead of on public transport, out of concern that the COVID-19 disease caused by the virus spreads more easily on crowded public transportation.

That reluctance to use public transport led London Underground to report it is carrying 12 percent of its normal passenger-load.

"It is definitely a huge challenge, that we are seeing pollution volumes at or near pre-COVID-19 levels, even though in many cities traffic volumes are below pre-crisis levels," Lauri Myllyvirta, an analyst at the Centre for Research on Energy and Clean Air, which is also known as CRECA, told the Financial Times. "We risk getting pre-crisis levels of congestion and pollution already, before full economic reopening."

He said nitrogen dioxide levels in Paris had risen from 13.6 micrograms per cubic meter at the height of the lockdown to 29.7. In Brussels, they rose from 16 micrograms per cubic meter to 30.2, and Milan saw levels rise from 19 to 32.9.

London even saw its air pollution levels temporarily eclipse pre-lockdown levels on a day in late May when the relaxation of the United Kingdom's lockdown corresponded with a sunny weekend that included a public holiday, inspiring many people to use their cars to get out of town.

But CRECA said the pollution levels in most European cities remain lower than they were, and urged authorities to seek ways to preserve that.

The newsletter added that another study, by air quality researcher Rohit Chakraborty at the University of Sheffield, found UK cities, including Cambridge, Leeds, London, Manchester, Oxford, and Sheffield, had all seen double-digit growth in nitrogen dioxide.

"As the lockdown started easing there has been more traffic," he said. "With increasing traffic, we have seen NO2 levels rising gradually everywhere in the UK."

Chakraborty said an increase in the use of fertilizer on farms as people have demanded more local produce, and a rise in popularity of bonfires and barbecues had added to pollution levels.

Anne Hidalgo, the mayor of Paris, does not want things to creep back to the way they were and has called for a greener, less polluted city in future, with initiatives to encourage cycling and walking.

She told Le Monde newspaper: "Fighting against all types of pollution is a public health issue."