A lot of coughing and sneezing in public transport in Bangkok now.
The trend seems to be that air pollution in Bangkok is now Dec Jan Feb and up North Feb to end of April, so that makes 5 months of the year and all of tourism high/peak season.
Apart from burning in and around Thailand (fields/agricultural waste/household waste/sugarcane fields-to increase sugar contents-/charcoal burning in households) there is pollution and/or dust from traffic but also a lot from construction and airplanes.
The relatively cooler air keeps pollution to the ground.