At 7 a.m. the air quality index (AQI) hovered between 101 and 150 in the downtown area, the city Department of Natural Resources and Environment said.

At the park near Thanh Cong Lake in Dong Da District and around the Hang Ma Ward police station it rose above 150.

An AQI reading of above 100 is considered unhealthy for humans.

The U.S. embassy recorded 155, a level that requires old people and those with heart and respiratory ailments to stay indoors, and the French embassy on Tran Hung Dao Street reported 145.

IQAir, a Swiss air quality technology and monitoring company that operates the index, said Hanoi's AQI level of 145 on Tuesday morning made it the 15th most polluted city in the world.

It warned that from Wednesday the pollution would rise to dangerous levels with the index climbing to an alarming 194, classified as "very unhealthy," on Saturday.

Independent air quality analysis system PamAir recorded 214 at Van Con Commune in Hoai Duc District. Monitoring stations in Thach That and Quoc Oai districts, far from the city’s downtown, recorded 199.

Hoang Duong Tung, president of the Vietnam Clean Air Partnership (VCAP), said Hanoi and northern provinces are entering a period of severe pollution.

"In addition to the main reasons such as heavy congestion, large-scale construction and straw burning, weather patterns during winter also make pollution levels worse than in other seasons."

He warned that people should wear masks to protect themselves while waiting for authorities to take measures.

Air pollution is not new in Hanoi, but remains as pressing an issue as ever.

City authorities have blamed the low air quality on large-scale construction, large number of private vehicles, intensive industrial activity like steel and cement production, and coal-fired power plants.

Hanoi plans to stop burning straw and other waste from next year to reduce pollution

Vietnam suffers an estimated $10.8- $13.2 billion worth of economic losses due to ambient air pollution each year, or equivalent to 5 percent of GDP.