Although Vietnamese authorities were able to anticipate annual weather patterns that typically cause serious declines in air quality in Hanoi and Ho Chi Minh City, it took three weeks for the first health warnings to be issued, leaving millions exposed to dangerous levels of air pollution.

The levels of hazardous particles in air, known as PM2.5, in Vietnam’s two largest cities have been consistently on the rise since early September and currently sit well above recognized safety limits, according to statistics released by the Vietnam Environment Administration (VEA).

PM2.5 refers to atmospheric particulate matter (PM) that has a diameter of less than 2.5 micrometers, approximately three percent the diameter of a human hair.

"Local people, especially children, the elderly, pregnant women and people with respiratory issues should refrain from sitting in traffic or participating in outdoor activities," the government said in a statement released on Tuesday.

"People should wear masks and goggles while outdoors," it added.

The Hanoi administration has named 12 primary sources for pollution in the city and neighboring provinces, including low rain, the burning of coal for power generation, and open agriculture burning.

VEA general director Nguyen Van Tai explained that the haze blanketing both Hanoi and Ho Chi Minh City in the past few weeks is known as ‘photochemical smog,' a phenomenon typical of the transition period between seasons in late September and early October.

Weather conditions prevent pollutants in the air from being dispersed quickly, he said.

Questions have been raised as to why no official health warnings were issued by Vietnamese authorities until Tuesday, three weeks since residents first noticed the smog, despite the hazardous air quality having been identified as a predictable issue.

By the time of the first government-issued warning, air quality in Hanoi and Ho Chi Minh City had already been deemed “unhealthy” by air quality monitoring systems.

“Authorities took no measures to respond to three-weeks of air pollution and were even slow in giving out warnings,” said Nguy Thi Khanh, director of non-profit organization GreenID.

If data shows that air pollution increases during September and October, warnings should have been issued “well before the pollution began,” according to Prof. Dr. Phan Van Tan, a hydro-meteorological expert.

The PM2.5 levels recorded in Hanoi last month were the highest in five years, VEA said.

Over the past few weeks, air quality in Hanoi was so bad that it topped the list of the world’s most polluted cities on multiple occasions, beating out both Bangkok and Beijing, according to independent online air quality index monitor Air Visual.

In Ho Chi Minh City, PM2.5 levels have fluctuated between 27.9 and 67.4 micrograms per cubic meter since September 20.

The World Health Organization recommends an air quality standard for PM2.5 of no more than 10 micrograms per cubic meter.

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