Students at Le Quy Don primary school will have physical education (PE) lessons in the classroom and gymnasium instead of the schoolyard, the school's vice-principal Ly Thi Son said. All the doors and windows will be closed and students are advised not to go out during their recess.

"These are temporary solutions but they are necessary and useful for our students because we have good facilities and infrastructure," said Son. 

"We have not seen any students whose health affected by polluted air so far," she said, adding that outdoor activities at the school will be encouraged after the air in Hanoi gets better.

Vietnam-Australia School in Nam Tu Liem district also adopted several solutions including delaying outdoor matches, cancelling outdoor physical activities and closing all the windows.

On September 30, Lomonosov primary school made a notice asking teachers to move all PE lessons to the gymnasium. Classroom windows are now required to be closed at 7 a.m. everyday instead of 9 a.m. 

To reduce students' exposure to the air, the school board also advised parents to pick up their children earlier, right after they finish lessons at 3:40 p.m. or after-school activities at 5:30 p.m.

Principal Nguyen Thi Thu Mai said that the school has ordered air quality monitors from Pam Air to track the air quality and report to parents, teachers and students. The installation will be finished by October 5.

On Wednesday morning Hanoi took the top spot in the world for worst air quality in a ranking of more than 10,000 cities worldwide, with an AQI of 180, a level where there's increased risk of adverse affects to the heart and lungs, especially among children, elders and people with respiratory or heart diseases.

Officials said the low quality of air in Hanoi is caused by construction, a growing number of cars and motorcycles and heavy industry, including steel works, cement factories and coal-fired plants.

On Tuesday, the Vietnamese government warned its people to limit outdoor activities because the air quality in the country has persistently been at unhealthy levels for days.

"Local people, especially children, the elderly, pregnant women and people with respiratory issues should refrain from traffic and outdoor activities," the government said in the statement.

PM2.5 levels are forecast to stay high over the next days, particularly at night and early in the morning, it added.