Air quality of Kathmandu Valley deteriorated yet again today, as a thick layer of smog blanketed the valley. Air pollution in the valley had come down after rainfall on Wednesday.

The hourly data of air pollution at pollution measuring units inside the valley show that pollution is at its worst between 7:00am and 10:00am.

The fine particulate matter (PM 2.5), considered the most hazardous pollutant, was 305 microgram per cubic metre at 10:00am at the US Embassy in Maharajgunj, Bhaisepati, and Jhamsikhel. At 7:00am, PM 2.5 was 260 μg/m3, 273 μg/m3, and 285 μg/m3 at the three places, respectively.

In Bhaktapur, PM 2.5 was 299 μg/m3 at 7:00am, not changing much at 10.00am; at the US Embassy, Phora Durbar, PM 2.5 was 330 μg/m3 at 7:00am. Air pollution came down slightly between 10:00am and 3:00pm. The pollution, however, increased after 3:00pm.

Environmentalists say that lower temperature is one of the major reasons for increased air pollution in the morning. Indu Bikram Joshi, deputy director general of Department of Environment, said smog blocked the sunlight, bringing the temperature down.

"Decreased temperature traps cold air, with small pollutants on the lower level."

He said after 10:00am, the temperature starts increasing and pollutants ascend to higher levels.

Pollution is unlikely to come down unless there is a strong westerly wind or heavy rainfall inside the valley, Environment Engineer Riya Shrestha told THT.

Other reasons for the hazardous haze shrouding the valley are wildfires raging around the valley for the past few weeks, vehicular and industrial emission, and burning of waste materials. To worsen the situation, air movement inside the valley has remained stagnant, one of the reasons for dismal rainfall in the last several months.

Health authorities have advised people to stay at home and avoid morning exercise and other outdoor activities.

Anup Bastola, consultant tropical medicine, Sukraraj Tropical and Infectious Disease Hospital, said, "Exercising in the morning can be harmful, as people tend to inhale more unhealthy air while exercising. " He said the number of people with respiratory ailments, allergies, ENT problems, and breathing difficulties have increased in the past few days. "Air pollution can be damaging for children, elderly, and ailing people."

A version of this article appears in the print on April 6, 2021, of The Himalayan Times.