With particle counter to confirm the bad air cause sick

Some children easy catch a cold when the weather is changing, what is the reason?
The reason is the bad air around us. and the bad air is include the fine particulate matters.
It is easy go inside lung when we breath. we can not seem by human eye.
So we need particle counter to check them and then confirm the air is good quality or not.
Also the laser particle counter can tell you if it is really work to use the air purifier at home.