What is important of handheld particle counter

Handheld particle counter is very hot selling in recent years because the dirty environment.
PM2.5 can be suspended in the air for a long time, the higher its concentration in the air content, means, the more serious air pollution.
So we need to use PM2.5 detector to test the air quality around us if it is good for health or not.
Then can use air purifier to clean the air. to test again with PM2.5 detector to know the different before clean and after clean.