Alarming pollution readings have been recorded in both of Vietnam’s largest cities Hanoi and Ho Chi Minh City during 2016, raising health concerns amongst locals.

Hanoi’s 2016 Air Quality Index (AQI) value, calculated based on the PM2.5 annual mean concentration, was twice as high as the safe limit set out by the World Health Organization (WHO), experts said at a seminar on pollution and public health in Hanoi on Tuesday.

PM, or Particulate Matter, is the term for a mixture of solid particles and liquid droplets found in the air. PM2.5 particulates are fine particles with diameters 2.5 micrometers or less.

PM2.5 readings are converted into an AQI value, which reveals the level of pollution the monitored air has on a scale from 0 to 500, with 500 being the most polluted.

PM2.5 particulates are as thin as 1/30 the width of a human hair, and therefore can easily pass through lung tissues and be absorbed into the bloodstream, causing adverse health effects, according to Nguy Thuy Khanh, CEO of Green ID, a Hanoi-based sustainability advocate organization which co-organized Tuesday’s seminar.

According to a Green ID survey conducted in 2016, the PM2.5 annual mean concentration in Hanoi was 50.5 micrograms per cubic meter of air.

Hanoi suffered 123 days of excessive PM2.5 levels in 2016 according to Vietnamese standards, and 282 days according to WHO’s air quality guidelines.

These readings were higher than those recorded in China’s Guangzhou City the same year, the group noted.

The PM2.5 annual mean concentration in Ho Chi Minh City was lower, at 28.3 micrograms per cubic meter, while its citizens went through 14 and 175 days of unsafe PM2.5 levels according to national and WHO standards, respectively.

Heavy industrial zones located to the east of Hanoi and thermal power plants outside the city contributed greatly to air pollution in the capital, based on satellite data collected by Green ID.

Traffic vehicles, construction sites, garbage incineration as well as household cooking were also named as causes of Hanoi’s worsening air pollution.

A late 2016 survey of over 1,400 Hanoi residents, more than 86 percent of which are under 40, found that over 70 percent of respondents claimed to have noticed respiratory problems in themselves and their family members.