About 2.4 million people in Thailand have sought hospital treatment for medical problems linked to air pollution since the start of the year, health officials said, as toxic smog chokes parts of the country.
Bangkok and the northern city of Chiang Mai were among the most polluted cities in the world on Thursday morning, according to air quality monitoring firm IQAir.
Opas said respiratory problems, dermatitis, eye inflammation and sore throats were among the most common medical issues.
Health officials have urged people to use high-quality N95 anti-pollution masks, close windows and doors, minimise time outside and exercise indoors.
Smoke from forest fires and farmers burning crop stubble have polluted the air in parts of the country, according to experts.
Thailand is home to more than 70 million people and its poor air quality has become a hot button issue ahead of the country's May 14 election, with the incumbent government accused of not doing enough to tackle the problem.
