Adele Mbelle travels from Kuala Lumpur to attend school in Bangkok. The 22-year-old, who has three years of high school left to go, joined the thousands of other Thai children who chose to stay home from school Tuesday.

“I don’t think I will get the best time to study, because there are too many people in class,” said Adele, who said she was not concerned about the quality of her education. “I am worried about traffic noise. I worry about the smell from the frying-pan vendors on the streets around here.”

The air-quality index in Bangkok reached a maximum reading of 358, the most lethal on Tuesday, according to the government’s Air Pollution Control Department. That means people’s health can be endangered.

Some 21 schools closed for the day, six of them in the eastern Pattaya area, that is home to one of the country’s worst air pollution episodes. Air pollution levels in this area remain particularly hazardous.

“That type of high concentration of people travelling, that’s where it happened,” said Anannyo Prasertsots, director of air pollution control department. “It’s not an accident that it happened today. We know that the weather forecast and the weather forecast that there are these heavy pollutants on the way.”

An index measuring air quality is released every Tuesday at 8 a.m. and must be updated every morning. The most recent edition, released on Tuesday, showed the air pollution index across much of Thailand at 226, up three points from Monday, and indicating health and safety risk.

If high or very high levels of pollution are detected, government officials are to immediately close schools.