The global Covid-19 pandemic has forced the public indoors and pushed general life to a halt around major cities of the world for several weeks now.

A slew of stringent measures enforced by countries to contain the spread of coronavirus has seen improved air quality worldwide, including in Kathmandu, according to a latest study that looked into changes in air quality in the wake of the Covid-19 pandemic in the major cities.

“The Covid-19 induced lockdown has brought down air pollution levels in all major cities,” said Uttam Babu Shrestha, one of the authors of the study. “However, the effect on air quality varied as cities have imposed different models of measures,” said Shrestha, who is also a director at the Global Institute of Interdisciplinary Studies, a think tank based in Kathmandu. “The impact of the ongoing lockdown was visible in the air quality in Nepal, too, as the country is under lockdown for over a month now.”

The study found that following the lockdown, air quality improved in the world's most polluted cities such as Bangalore, Beijing, Bangkok, Delhi, and Nanjing, as well as the major trade centers such as New York, London, Paris, Seoul, Sydney, and Tokyo.

The study conducted on 40 major cities of the world from all the six continents, shows that air pollution levels have significantly declined in cities under lockdown.

As part of study, researchers measured six major pollutants, including PM2.5 and PM10, to record how air quality improved under lockdown and travel restrictions. Researchers compared pollution levels of February and March in 2019 with February and March 2020.

The study found that PM 2.5 concentration saw a significant decline in 17 cities in February 2020 compared to February 2019, and 14 cities witnessed declines in March 2020 compared to the same month last year.

Shrestha said the cities studied were selected based on their historical level of air pollution, reported Covid-19 cases in that city, geographical setting and developmental stage, city lockdown history, and availability of air pollution data.

Nepal has been under a lockdown since March 24 with restricted public and transport movement across the country.

Due to the ongoing lockdown, the average concentration of PM2.5 (pollutants that have a diameter of less than 2.5 micrometer) decreased by 34.9 micrograms per cubic meter (μg/m3) in Kathmandu, the researchers found out after studying air quality data from January 1 to April 24.

“As the movement of people and vehicles have been stopped, and industries are not allowed to operate, it's evident that air quality has improved in Kathmandu,” said Shrestha. “But we also thought of studying other factors like wind and humidity and found that air quality still improved in Kathmandu.”

According to Shrestha, the average concentration of PM2.5 recorded a further decline of by 22.6 μg/m3 after accounting for the humidity and prevailing wind during the lockdown period. PM2.5 and PM10 had decreased from 135.4 μg/m3 to 108.3 μg/m3 and 54.6 μg/m3 to 42.2 μg/m3 respectively in Kathmandu before and after the lockdown was enforced. Air quality has also improved in Biratnagar and Pokhara after enforcement of the lockdown.

But even after the improvements, air quality in Kathmandu is still not suitable for certain groups such as senior citizens and people with respiratory ailments. The World Health Organization’s air quality guideline stipulates that 24-hour mean PM2. 5 should not exceed 25 μg/m³ and that for PM10 should not exceed 50 μg/m³.

“Although air quality has improved under the lockdown, the quality is still unhealthy for certain age groups,” said Shrestha. “Concerned authorities who imposed stringent measures for containing Covid-19 should treat air pollution with similar urgency as air pollution is also a leading global health issue.”

According to Bhushan Tuladhar, an environmentalist, the improvement in air quality due to the Covid-19 measures should be tapped into to keep the air clean.

“Stopping of vehicle exhaust due to the lockdown and frequent rainfall has improved air quality in Kathmandu. Existing air pollutants could be due to forest fires and westerly wind,” said Tuladhar. “Even after the lockdown is over, people may not go outside freely for some time, nor use public transport. We have to cash in on the public’s desire to breathe clean air.”