Hoang Van Thuc, deputy head of the Vietnam Environment Administration under the Ministry of Natural Resources and Environment, said the capital had "good" air quality on 15.6 percent of days last year, "average" on 62.8 percent, "poor" on 14.1 and "very poor" on 2.1 percent, Tuoi Tre newspaper reported this week.

That translates into around 60 days of "poor" or "very poor" air.

The data was collected from 12 surveillance stations across the city, including two of the environment administration and one of the U.S. embassy.

Areas surrounding certain streets like Pham Van Dong, Hang Dau and Minh Khai were more likely to have recorded "poor" or "very poor" air quality due to their high traffic density and numerous construction projects, Thuc said.

However, while the pollution in major cities like Hanoi and HCMC occasionally exceeded permitted levels, it was not a major concern to people’s health, and "people should not be too alarmed," he said.

"On humid days fog appears and limits circulation within the atmosphere, which prevents pollutants from being diffused ... and they linger in the lower atmosphere near the ground, which worsen Hanoi’s air quality," Thuc said as to make an example.

Hanoi was ranked the second most polluted city in Southeast Asia by IQAir Airvisual last month, with the capital’s average PM2.5 level last year being 40.8 micrograms per cubic meter of air as opposed to 45.8 in 2017.

The World Health Organization Air Quality Guideline recommends an annual mean exposure threshold for PM2.5 of 10 micrograms to minimize health risks.

PM, or particulate matter, refers to a mixture of solid particles and liquid droplets found in the air. PM2.5, also described as super fine particles, is a fraction of the width of a human hair, and is released by vehicles, industry and natural sources like dust.

Deputy Minister of Natural Resources and Environment Vo Tuan Nhan said IQAir Airvisual report’s findings were not precise.

"Due to the lack of complete data from 11 Southeast Asian countries, there’s no convincing evidence to conclude Hanoi is the second most polluted city in Southeast Asia."

The report studied air quality in over 3,000 cities globally, including 20 cities in four Southeast Asian countries: Indonesia, Thailand, the Philippines, and Vietnam.

Nhan also blamed Hanoi's worsening air quality on the dense traffic and emissions from construction projects, industrial facilities and waste burning.