BANGKOK - Air pollution in the Thai capital of Bangkok was at unhealthy levels for most of the past week, the Pollution Control Department said on Thursday (Jan 25), the Bangkok Post reported.

The department's air and noise quality director Thaloengsak Phetsuwan said the level of fine particles up to 2.5 micrometres in diameter (PM2.5) had breached the safety limit of 50 micro-grams per cubic metre on five of the past eight days.

It said the unhealthy air quality had been caused by still and stagnant air conditions, which allows pollution from vehicles to build up in the air.

Smog has covered Bangkok throughout the week, greatly lowering visibility, The Nation reported.

Data from air-quality monitoring website aqicn.org, which uses the Air Quality Index gathered by the Pollution Control Department, on Wednesday showed that the level of PM2.5 in Bangkok had been well beyond the safe average since Saturday. The website also forecast that air quality would remain low throughout the week.

Under Thai standards, the safe level of PM2.5 is 50 micro-grams. The World Health Organisation's recommended daily average level of PM2.5 is 25 micro-grams.

PM2.5 has been proven to cause many diseases, such as heart disease, respiratory diseases, allergies, eye and skin irritation.

When the amount of fine particles in the air crosses into unhealthy levels, people could experience throat irritation and breathing difficulties, Thaloengsak said. He urged the public to wear face masks and limit outdoor activities .

According to a Greenpeace report, Bangkok ranked second on the list of Thai cities that suffered the most from air pollution last year. Heavy traffic in the city is regarded as the major reason for bad air quality.