I had experienced frequent cough. Every time I start coughing, it'll last for over a month'.
Last last I found out that the dust on top of my cupboard causing it. The cupboard is right in front of my Aircon. When turn on Aircon, the dust blow off.
How I frequently clean the cupboard, no more coughing. When I feel like starting to cough, I clean the cupboard, then all is fine.