It is called the "Burning Season" or "Smoke Season". Air pollution often reaches hazardous levels especially in March and April. The main reason is the burning of forest undergrowth in mountain forests all over the North to make harvesting a certain type of black mushrooms easier. These mushrooms are sold at local markets and exported to China. Another reason is agricultural burning, cane, corn, rice etc.. I usually spend 2 months in Danang, VN as soon as the air starts getting really bad here.
These are the best months of the year with good air quality. In just a couple of months many people will start coughing and suffering from the air pollution again.
Amarant Suvarnabhumi Airport Hotel. Spacious balcony from day 1. After the first negative Covid test you can spend 2x1 hr/day in the garden or by the pool. Food is excellent too with a choice of Thai, Western or Japanese. I paid 53.000 THB for a nice and spacious room in December. The staff was always very nice and helpful.