I have never seen a foreigner wear a mask during my visits to China since 1997, nor while living here now.  In a way, if you look closely, the face masks most wear here just gives you piece of mind, it will not do anything else.  Originally, they were made of a thin cloth with some cotton filling, now it is a paper cover with a filling.  The pore size will not stop most particles in the air, nor will retain contaminants from entering your body.  But please, if you do come, do not tell anyone about this, no one will believe you !. 

The quality of air will be bad in larger cities like Beijing, Shanghai, Guangzhou, Shanzhen, etc.  Also some provinces are well known for the concentration of manufacturing plants, while others do not have that many, and air quality is better.

I do live in Guangxi Province, and most of the year (depending on wind blowing direction) the quality of air is reasonable.  Besides, I do test water from the tab on a weekly basis, and I have never found a result of less than 97% purity, which is better than the levels back home.  But I still boil it and filter it afterwards prior to drinking it.

If I walk for more than a mile or two by a busy road, I find it useful to have a mask. I can strangely tell the difference

my wife makes me wear a mask till she found out about my life insurance policy and every time i put one on now she rips it off lmao