Northern provinces are on course for severe air pollution next weekend due to fires and still air in the region as temperatures rise across the country over the forthcoming long break.
The Pollution Control Department on Sunday said the magnitude of pollution would depend on the number of hot spots detected later this week.
Air quality from the central up to the northern and northeastern regions has worsened over the past few days, with increasing density of particles with a diameter of less than 2.5 micrometres (PM2.5). The tiny air pollutants cause respiratory problems and kill thousands each year.
Car exhausts are a major source of the particles in Bangkok, but smoke from slash-and-burn farming and the burning of crop residues, along with forest fires, are blamed for the thick smog in locations outside the capital - with the huge increase in corn plantations thought to be particularly culpable in the North.
Bangkok and Chiang Mai were in the top five most polluted cities in the world on Sunday. The capital was in second place after Beijing, while the northern city was ranked fifth, according to the IQAir ranking at 4.30pm. IQAir monitors air pollution around the globe.
Bangkok and Chiang Mai, however, were not the most polluted areas in the country. That title went to Muang district in Ubon Ratchathani where PM2.5 hit 123 µg/m3, compared to 93 in Bangkok's Wang Thong Lang district and 63 around Chiang Mai's Doi Suthep.
In the North, the most hazardous haze was in Muang district of Uttaradit, with a PM2.5 reading of 95.
The Pollution Control Department (PCD) advised people in the so-called orange and red zones to spend less time outdoors through Monday to avoid breathing problems.
Temperature forecast in Bangkok. (Screen capture from Meteorological Department website)
Air quality would be better from Tuesday until the forthcoming weekend, the PCD added.
The country begins the five-day Songkran holiday on Wednesday.
Weather forecasters predicted higher temperatures across the country this coming week. Temperatures in some northern areas could go up to 40°C, making them the hottest in the country.
The highest mercury in Bangkok would be 36°C this week, the Meteorological Department added.