Hundreds of Thailand's children strain to sing the national anthem, reedy voices and fragile lungs competing against eight lanes of belching traffic next to their school's open atrium in central Bangkok.

Students at Suan Lumphini School assemble each morning under a flag indicating the day's air quality – red for the worst, yellow for the second worst and blue for the best.

Thailand registers dire air pollution levels annually – its cities topped the world's most polluted for days last year – and children face the greatest risk of long-term damage to their respiratory systems.

Lalipthat Prakham, who works as a maid, said it has already made her 8-year-old daughter sick.

"I always try to make her wear a mask. I tell her to keep the mask on at all times and to avoid going outside," she told Agence France-Presse (AFP) while waiting for her child outside Suan Lumphini.

Prime Minister Srettha Thavisin has promised to tackle the issue. His cabinet approved a bill in January to reduce dangerous levels of PM2.5, particles so tiny they can enter the bloodstream.

Nanthnan Hajiub said his 11-year-old son has developed a sporadic cough.

"I'm worried about the pollution and the effect it is having on his respiratory system," the 45-year-old business owner said.

"I want the people in charge to take better care of the situation."

Not all children suffer equally

Bangkok's elite private international schools enjoy air purifiers and filtration systems in classrooms, but public establishments like Suan Lumphini must do without them.

The school is now part of a pollution study by Chulalongkorn University, which has fitted out a single classroom with a purifying air conditioning unit, known as a "clean room."

School head Suphatpong Anuchitsopapan worries about pollution affecting students' health and future.

The World Health Organization warns that as well as damaging children's respiratory systems, air pollution also puts them at higher risk of cancer and can stunt the development of their brains.

Between January and February of this year, some 910,000 people were registered as unwell thanks to pollution, said the council, which advises the government.

Teacher Jiraporn Sukpraserd loves her special "clean room" at school. She's been teaching for nearly 10 years, and now she teaches the youngest kids in kindergarten. Their classroom has a special air conditioning system that cleans the air.

Last week, the air quality was bad, which made Jiraporn worried about her students' health. Most of them are under four years old. The classroom gets really hot if the windows are kept closed, so they have to open them to let in some fresh air, even though it's polluted.

However, when they turn on the air purifier, the pollution levels decrease after 30 minutes. Jiraporn wants every classroom to have this because it helps keep the kids from getting sick.

On the other hand, Bounleua Boriharn, who owns a business, is worried because his eight-year-old daughter's classroom doesn't have a "clean room." He's concerned, especially when there's fog in Bangkok.

"The children don't really know what's happening," he said. "They just wonder why their throats hurt and why they're coughing."

He always makes his daughter wear a mask to protect her, saying, "That's all I can do."