Clouds hover over the blue sky at India Gate during the lockdown to limit the coronavirus on April 20, 2020 in New Delhi, India.

Confinement measures imposed to slow the spread of the coronavirus pandemic have resulted in an unprecedented fall of deadly air pollutants around the world, according to new research published on Earth Day.

IQAir, a Swiss-based air quality technology company, compared measurements of the world's deadliest air pollutant — "fine particulate matter" known as PM2.5 — before and during the Covid-19 outbreak in 10 major cities.

The findings, published Wednesday, revealed a "drastic drop" in air pollution for almost every city under lockdown when compared with the same period a year earlier.

New Delhi recorded a 60% fall of PM2.5 from 2019 levels, Seoul registered a 54% drop, while the fall in China's Wuhan came in at 44%.

The study also found that Wuhan experienced its cleanest air quality on record through February and March, while Los Angeles experienced its longest-ever stretch of clean air — meeting the United Nation's recommended air quality guidelines.

The data was based on a three-week time frame to reflect either the most stringent lockdown measures in each city or to coincide with the peak number of daily reported Covid-19 infections.

Major cities with historically higher levels of deadly air pollutants witnessed the most substantial drop in PM2.5, the analysis found.