Bangkok, Sep 30 (IANS): Thai authorities on Monday issued an alert over an increase in air pollution in Bangkok and recommended the people to exercise precaution, especially minors, the elderly and the sick.

The pollution control department, in a statement said that PM2.5 - particles 2.5 microns or less in diameter - levels were between 40 and 78 micrograms per cubic metre (µg/m3) in the capital and its surrounding cities.

The country's authorities consider anything exceeding 50 µg/m3 to be unhealthy, whereas the World Health Organization recommends that PM2.5 levels should not exceed 25 µg/m3, according to Efe news.

The department, in a statement, urged people in higher risk categories, such as children, pregnant women, the elderly and the sick to stay indoors.

Prime Minister Prayut Chan-ocha tweeted encouraging people to use masks and called for construction firms and factories to cooperate to reduce air pollution.

According to the authorities, the increase in pollution is due to the scarcity of rainfall in recent days.

Chengdu (China), Hanoi, Bangkok, Hong Kong and Seoul on Monday topped the list of the major cities with the highest levels of air pollution, according to Air Visual, a platform that measures air pollution around the world.