Trees planted in parks and along the streets of Dhaka city are mostly covered with layers of dust, giving a hint to what the world's most polluted air can be like.

Every single day, the dust accumulation of these trees was 436 tonnes on average, found a study titled "Assessment of total dust accumulation on tree leaves in Dhaka city" which was conducted by the Center for Atmospheric Pollution Studies (CAPS) at Stamford University.

CAPS collected leaves from as many as 77 species of trees from Gulistan Park, Chandrima Udyan, Ramna Park and Botanical Garden between February 9 and February 12 to assess the presence of dust on those.

After testing those leaves in laboratory, CAPS found out that trees in Gulistan Park had the highest amount of dust whereas trees in Botanical Garden had the lowest.

As per the report, each square millimetre of the leaves accumulated 4.53 micrograms of dust and the accumulation was 13.59 micrograms in 72 hours.

In total, the trees across Dhaka retain 1,310 metric tonnes of dust in 72 hours and 436.7 tonnes of dust a day, according to the report.

Talking to The Daily Star, experts said trees usually fight back against pollution but the trees in Dhaka were in danger. Three trees in the city were at risk due to the emission from the city's vehicle and dust.

Contacted, Prof Mohammad Kamruzzaman Majumder, director of CAPS, said the dust accumulation by leaves are higher in those areas that have high frequency of vehicular movement.

During the night time, vehicles move very fast spreading dust in relatively larger areas than the ones moving slowly, he said.

"Apart from this, unplanned and uncoordinated street digging is one of the major main reasons for spreading dust," he added.

HARDEST HIT FOR DHAKA TREES

The survival chances of street-side trees in Dhaka dropped by 30 percent due to pollution, read a report of SN Applied Sciences journal that was published on October 12 last year.

The study was jointly conducted by botany and chemistry departments of Dhaka University.

During the survey in 2017 and 2018, the researchers collected samples of Debdaru, Mehagani and Jackfruit trees from the roadsides of Farmgate and Gulshan, residential areas of Dhanmondi and Uttara, and Botanical Garden.

Prof Abdus Salam of the university's chemistry department, said, the capital city had a small number of trees and if the pollution continues trees would be threatened.

"Trees are stressed in the high pollution areas. We have not found any tree yet that performs better in polluted areas," he added.

He then said that immediate steps should be taken to save the lives of humans and trees.