It is always so easy to bash Western nations when Thai government is always hiding something. From your own link.
https://www.iqair.com/us/thailand/bangkok
"However, despite the readings classing it at a moderate level, there are times when it crosses rapidly over this line to significantly worse pollution with levels of PM2.5 and other contaminants found lingering in the air. A large number of schools were closed in January of 2019, an unprecedented order at the time because the level of air quality has only something that has recently been made more transparent and available for the average citizen, as well as the government keeping a closer eye on it."