The Pollution Control Department (PCD) has debunked claims by academics that some of the PM2.5 pollutants that have shrouded Bangkok and its vicinity over the past few weeks have come from Cambodia, saying winds blow fine dust particles from the neighbouring country towards the Gulf of Thailand.

The claim was made last week by Siwat Phongpiachan, head of the Centre for Research and Development of Disaster Prevention and Management at the National Institute of Development Administration (Nida) and Professor Serm Janchai, a physics lecturer at Silpakorn University's Faculty of Science.

The academics also urged more research into and cooperation on trans-boundary pollution within Asean.

In response to the claim, PCD director-general Pralong Damronthai said his agency is actively tracking "hot spots" where slash-and-burn farming is practiced -- both inside and outside of Thailand -- which are known contributors of ultra-fine, PM2.5 pollutants.

Mr Pralong said that based on a mathematical model for weather analysis developed by the United States' Environmental Protection Agency, winds blow fine dust particles from Cambodia towards Trat and the Gulf of Thailand.

Based on a Jan 20 model, there were 895 hot spots in Cambodia and it was found the fine dust particles moved from there to Trat and the Gulf. "The haze did not 'move' to the capital," he said. "The particles that caused the haze in Bangkok come from local sources."

The PCD's finding is consistent with data from the Asean Specialised Metrological Centre, which found the same thing, Mr Pralong said.

Citing Asian Institute of Technology research, Mr Pralong said PM2.5 pollutants mainly come from diesel-powered vehicles (52%), and outdoor burning (35%), while the rest are produced by heavy industries and power plants.

As concern over the PM2.5 hazard increases, the governors of the five provinces that border the capital have placed indefinite bans on outdoor trash burning. "We have also instructed authorities in Nakhon Sawan, Phichit, Kamphaeng Phet, Uthai Thani, Chai Nat and Sa Kaeo to seek cooperation from the public," said Mr Pralong.

The PCD reported that the volume of PM 2.5 on Monday continued to exceed the safety limit of 50 microgrammes per cubic metre (µg/m³) in 10 areas across the capital, as well as Samut Prakan's Phra Pradaeng district and Nakhon Pathom's Muang district.

Deputy Bangkok governor Thaweesak Lertprapan said the haze may cause an increase of influenza among young children and the elderly across affected areas.