Unfortunately most HEPA based air purifiers only catch PM0.3 and some down to PM0.1, leaving over 90% of all particles in your indoor air.
Smog, traffic pollution, virus, combustion from coal power plants, mold mycotoxins are all finer than PM0.1.
These particles cause severe health problems such as cardio- and vascular disease, COPD, cancer, and asthma.