What is the reason cause the PM2.5

PM2.5 always called as find particlate matter, it always in the air.
But it become more and more harmfull now since the bad pollution.
motor vehicle exhaust is the main reason one.  then smoking is the second one.
But We can not see or even fell it. so we need particle counter to test air quality.
After use the handheld particle counter test the air quality then we can clean the bad air.
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