"We are planning to install the device on 25 buses within July in the first phase," Khurelbaatar Bulgaa, deputy head of the Ulan Bator city's transportation department, told Xinhua.

According to Khurelbaatar, the installation of diesel particulate filter (DPF) on public buses has just started with non-refundable aid by the Japanese International Cooperation Agency.

He added that the filter can remove 80 percent of diesel particulate matter.

"The filter should be cleaned after every 250 kilometers. A public bus runs in the capital city 200-300 kilometers per day. So, we decided to install the three DPFs on a bus in order to not disrupt the operation of public buses," he said.

Currently, more than 930 public buses are operating in the capital Ulan Bator, he said, and the cost of the installations of three DPFs on a bus is some 40 million Mongolian Tugriks (16,200 U.S. dollars).

Air pollution is the most pressing issue in Ulan Bator where more than 800,000 residents, over half of the city's population, live in slums. They have to rely on burning raw coal and other flammable materials such as plastics and old tires to stay warm and cook meals during the six-month-long winter season.

The Mongolian government announced in March that it would impose a ban on the burning of low-grade coal for domestic use in the capital beginning on May 15, 2019.

Since the early 2000s, the government of Mongolia in cooperation with international organizations have carried out a number of measures directed at reducing air pollution in Ulan Bator. However, the city has not seen a significant reduction in air pollution levels.

The city's air pollution on Jan. 30 exceeded 133 times above the safety level set by the World Health Organization.