PARIS — It was touted last spring as the silver lining of the coronavirus crisis: the lockdowns and travel bans might wind up being a boon for the environment. Air pollution numbers were down, urban birds could be heard chirping and photos circulated of blue skies above some of the typically smoggiest cities in the world.

No place more emblematic than New Delhi, by some measures the world's most polluted city, that appeared cleaner than it had in recent memory. Air pollution levels in April were a fraction of their usual size across much of South Asia — with India's Indo-Gangetic Plain recording one-fourth the pollutants it had in April 2019.

But now, even as the second wave of infections is prompting new shutdowns and continuing to limit international travel, experts see signs that the would-be silver lining was mostly just an illusion. Le Monde"s India correspondent reports that the past few weeks Delhi residents have woken up each day surrounded by a misty haze of pure pollution. Emission levels have boomed again in November, recording 1,021 micrograms of fine particles per cubic meter of air. That's 40 times the upper limit recommended by the World Health Organisation.

Rising particles worldwide: In the last few weeks, air quality has collapsed across the world, too:

• In Lahore, Pakistan, the air pollution has also reached hazardous levels — about 12 times the recommended limit.

• In Milan, Italy, air pollution rose above the recommended limits even while the city was under lockdown as hospitals battled the second wave of coronavirus infections.

• The rise was also observed throughout the Balkans and in Eastern Europe.

Nothing new? Spikes in air pollution occur seasonally as hundreds of millions of people turn on the heating and resort to more polluting transport in the winter. It adds to the emissions injected into the atmosphere by industrial and agricultural activities.

A missed opportunity: But some argue that this resurgence of air pollution is a defeat because the world had managed to curb emissions during the first wave of months-long lockdowns.

• In April 2020, big European cities like Paris, Madrid, Milan and Rome saw a 50% drop in air pollution compared to typical levels, according to the Royal Netherlands Meteorological Institute.

• A study by researchers in British universities found that CO2, nitrogen oxides and other emissions fell up to 30% globally in April.

Dust and smog in Kathmandu, Nepal— Photo: Prabin Ranabhat/SOPA Images/ZUMA

The long view: Environmentalists say that even if the drop in emissions occurred during lockdowns had been sustained, they would not be enough to prevent the climate from rising 1.5 °C above pre-industrial temperatures.

Economy v. ecology? The new rise in emissions happens as several governments scrap environmental restrictions in the hope they would boost economic recovery.

• In the northern Italian region of Lombardy — still one of the European regions worst-affected by COVID-19 — local authorities have lifted pre-existing bans on polluting vehicles.

• Indian Prime Minister Modi has also eased the country's environmental constraints, seemingly in an attempt to revive the economy. He has auctioned coal mines, the most polluting fossil fuel and eliminated the duty to run environmental impact assessments and public consultations before starting new industrial projects.

• The Indian federal government has repeatedly ignored the dangers posed by high levels of air pollution, advising residents to eat carrots or start the day with music as a remedy in 2019, according to Le Monde.

• Last week, a dangerously high quantity of ozone in the air prompted Mexico City authorities to reintroduce a day-time ban on polluting vehicles, according to Animal Politico.

Why it matters: Other than being the main drivers of man-made climate change, emissions are also one of the world's leading causes of death.

• Some seven million deaths a year are attributable to air pollution, according to WHO estimates. Diseases linked to air pollution include diabetes, headaches, chronic bronchitis, irreversible airflow limitations, cancer, type-2 diabetes and neonatal mortality.

• Air pollution also weakens the human respiratory system, making it more vulnerable to the onset of other diseases.

Bringing it full circle: Several studies have also linked air pollution to a greater risk of dying from COVID-19. A study by Harvard University scientists found that each extra microgram of tiny particulate matter (PM2.5) per cubic metre of air over the long term increases COVID-19 mortality rate by 11%.