One example of a mechanism for adding pollution is called a cooker. Aggressively frying foods can cause particle pollution. In more backward parts of the world, burning wood for the cooking and heating may contribute.
Of course, I expect most members here have more restrained cooking methods.
However, I'm still surprised that no one has come forward and declared possession of or an interest in getting an air filter. The cost seems quite modest when compared with the health risk.
I am presuming your aircon filters are not effective for pm2.5 here, but am open to being corrected on that (by anyone who has read the specifications)
The filter in my hotel aircon unit looks like a mosquito net.