Thailand has blamed Cambodian farmers for thick clouds of smog that have led to the closure of hundreds of schools in the capital, Bangkok.
But increasing levels of smog in Chiang Mai and Bangkok have also resulted from construction, diesel use and crop fires in surrounding Thai regions.
Harmful levels of microscopic PM2.5 particles in the atmosphere have sparked health warnings.
Pollution is lying over eastern Thailand due to low pressure, lack of winds and dry conditions.
Hillside wildfires in Cambodia have been burning for more than a week. Plantation burn-offs from sugar, corn and rice fields in the central, northern and northeastern Thailand and similar burn-off on Cambodian farms have been blamed. Wildfires are also burning in Laos.
The Bangkok Metropolitan Administration closed public schools, saying most of the smoke was from Cambodia, carried by light easterly winds.
PM2.5 levels have exceeded 100 microns per cubic metre this week.
Readings have been taken of up to 188, almost four times the upper safe limit imposed by the Pollution Control Department and approaching eight times the World Health Organisation limit.
Bangkok and Chiang Mai have both been listed with the world’s top five worst air pollution this week.
The Bangladeshi capital Dhaka registered the worst pollution today, followed by the Indian capital Delhi, Lahore in Pakistan, Mumbai and the Chinese city of Chengdu.
Harmful microscopic dust can clog the lungs and lead to illness and breathing problems.
Bangkok municipal spokesman Pongsakorn Kwanmuang said public schools would be closed until tomorrow (Friday).
Last year water was sprayed to increase humidity, which helps to absorb particles of pollution.
”Unfortunately, this mitigation does not appear to be effective, since the volume of water is minuscule compared to actual rain,” said scientist Worasom Kundhikanjana.
Central Thailand’s Kampaeng Phet measured the country’s worst reading today at 218.
Beijing, once infamous for its toxic air, has reduced smog levels and dropped down a list of the world’s most polluted cities, according to the 2019 World Air Quality Report.
India dominates the list, accounting for 14 of the top 20.
The worst-ranked city, Ghaziabad, is a Delhi suburb.
According to the index, Chiang Mai, Hanoi, Jakarta and Seoul saw sharp increases in PM2.5 levels last year. Since 2017, Jakarta’s pollution increased by 66 per cent, making it the worst polluted Asean city.
Concrete Bangkok. Picture credit: Asean Economist