Lockdowns restricting travel and industry imposed to halt the spread of coronavirus have resulted in unprecedented reductions in deadly air pollution around the world, new analysis shows.

Major cities that suffer from the world's worst air pollution have seen reductions of deadly particulate matter by up to 60% from the previous year, during a three-week lockdowns period.

Researchers from IQAir -- a global air quality information and tech company -- studied 10 major cities around the world which have relatively high numbers of coronavirus cases and Covid-19 lockdown measures.

The study compared levels of harmful microscopic particulate matter known as PM 2.5. The pollutant, which is smaller than 2.5 micrometers in diameter, is considered particularly dangerous as it can lodge deep into the lungs and pass into other organs and the bloodstream, causing serious health risks.

Seven out of the 10 cities studied, including New Delhi, Seoul, Wuhan and Mumbai, saw significant improvements in air quality. Those with historically higher levels of PM2.5 pollution witnessed the most substantial drops in pollution.

Analysts chose the three-week timeframe to reflect either the period with the strictest lockdowns, or -- during longer lockdown periods such as in Wuhan -- to coincide with the peak number of daily reported coronavirus cases.

The report was released to coincide with the 50th anniversary of Earth Day, which this year is focused on climate action.

Air pollution is already a global public health crisis, as it kills seven million people each year, according to the World Health Organization. Reducing global heat-trapping emissions is the best way to clean up our skies and prevent avoidable pollution-related deaths, scientists say.

The report's authors say that while they do expect air pollution to rise again when economies restart after coronavirus, "out of these extraordinary circumstances, we can see how changes in our society's activities can have a momentous impact on our environment and the air we breathe," said IQAir's marketing specialist Kelsey Duska.

Cleaner air around the world

The Indian capital New Delhi -- which frequently tops the world's most polluted city lists -- saw a 60% reduction in PM2.5 levels from March 23 to April 13 from the same period in 2019.

Both New Delhi and the country's commercial center Mumbai experienced their best March air quality on record in 2020.

During the initial three-week lockdown period, the number of hours rated as "unhealthy" in New Delhi dropped from 68% in 2019 to 17% in 2020.

On March 25, India placed its entire 1.3 billion population into lockdown, closing factories, markets, shops, places of worship and suspending most public transport services. The world's largest lockdown was then extended to May 3.

India is one of the world's most-polluted countries and an average resident is exposed to air pollution that exceeds the World Health Organization's target for annual PM2.5 exposure by more than 500%.

Meanwhile, the South Korean capital Seoul saw a 54% drop in PM2.5 levels from February 26 to March 18 from the previous year.

South Korea's air quality ranks among the worst of Organization for Economic Cooperation and Development (OECD) countries, with some of the highest levels of particulate matter pollution. Last year in March, the government declared air pollution a "social disaster."

In February, South Korea had one of the largest coronavirus outbreaks outside mainland China but aggressive testing and contact tracing methods have brought cases there under control.

And the Chinese city of Wuhan, where the deadly virus was first identified, saw a 44% reduction in air pollution levels from February 26 to March 18 from the same period last year.

The city of 11 million people in central China's Hubei province was the first city to impose a complete shutdown after Chinese authorities struggled to contain the spread of the coronavirus -- an unprecedented move at the time.

After 75 days those restrictions began to be lifted on April 8 -- a milestone in China's fight against Covid-19 as the country reported nearly zero new local infections.

Over the course of Wuhan's 10-week lockdown, the city experienced its cleanest air quality on record for the months of February and March. The average concentration of PM2.5 plunged from 63.2 and 43.9 micrograms per cubic meter in February and March 2019 respectively, to 36.8 and 32.9 in the same months this year. The World Health Organization considers anything above 25 to be unsafe.

Elsewhere, other major cities experienced cleaner air. Los Angeles saw its longest stretch of clean air on record, over 18 days from March 7 to 28. PM2.5 concentration levels were down by 31% from the same time last year, and down 51% from the average of the previous four years.

And in Europe, London, Madrid and Rome all experienced reductions in their PM2.5 compared to 2019 during their lockdown periods.

Calls to pursue green deals

While suddenly closing all factories and banning cars from roads is not a sustainable solution tackle climate change, the IQAir researchers said there are ways to preserve healthier air conditions.

These include supporting green deals in government stimulus packages, shifting towards sustainable sources of energy for power generation, limiting individual's purchases to primarily essential goods, opting for cleaner modes of transportation -- including walking and cycling -- and encouraging a shared economy of goods, according to Duska.

"In our recovery from the pandemic, it's important that we strive to preserve the cleaner environment, which protects our health from another invisible killer, air pollution," she said.

"We hope that the urgency in tackling this global pandemic may be matched in addressing air pollution."