What is the most harmfull in the around air

We know that the most harmful to the person in the air pollutants is fine particulate matter less than 2.5 microns in diameter.
It's called PM2.5, which is easy breath into lung, and bad to human health.
PM2.5 and other small particles can easily through the filter from normal air purifier, activated carbon,
such as material, back into the air in the harm to human health.
With the air purifier which with HEAP filter can stop the PM2.5 give you the best air.