The city of nine million ranked second only to India's New Delhi on IQAir's ranking of most polluted cities on Thursday morning, improving slightly in the afternoon.

According to the Swiss monitoring company, levels of PM2.5 pollutants -- cancer-causing microparticles small enough to enter the bloodstream through the lungs -- were vastly higher than the World Health Organization's recommended daily exposure limit.

"I have experienced difficulty in breathing out on the streets these days," resident Dang Thuy told AFP on Thursday, adding she had bought two new air purifiers for her apartment.

Hanoi authorities, in an administrative order made public Thursday, urged people to limit time outdoors and said schools can close if the situation deteriorates.

The order instructed officials to crack down on illegal waste burning and take measures to control the dispersion of dust at construction sites, including covering trucks and spraying water to keep tiny particles from becoming airborne.

However, AFP journalists observed construction sites operating normally, with trucks arriving and departing without the required coverings.

"Authorities have been quite active on paper only. Nothing has worked yet and the terribly toxic air remains in our city," said Thuy.

AFP journalists also saw people burning trash on Thursday along the Day river in Hanoi's western outskirts.

"In the past we led a poorer life, but I often roamed along the river with so much fresh air," said 70-year-old Hung, who gave only his first name.

"Now we all have multi-storey brick houses, motorbikes and cars, but leaving the house anytime is a pain. I have glued myself to this face mask."

According to the WHO, a number of serious health conditions, including strokes, heart disease and lung cancer, are linked to air pollution exposure.

Experts say pollution in Hanoi is a result of widespread construction, as well as emissions from the huge number of motorbikes and cars that criss-cross the capital every day.

Emissions from coal plants to the north and agricultural burning exacerbate the problem.

Authorities have announced plans to ban gas motorbikes from central Hanoi during certain hours starting in July next year.