Sanya air pollution isnt bad at all.
It's right next to the ocean and so ocean breezes and the monsoon season often clears up whatever pollution there is.
Definitely better than Beijing and HK.

For me, I find that pollution in London is far worse than in Shanghai.
Shanghai's air quality is ok.
By the end of a day walking around in London, my sinuses and throat would really act up...plus all that soot from blowing the nose...
In comparison, nothing of that sort happened when in Shanghai or Sanya.

Beijing air quality can be very bad during the spring sandstorms.
The entire city can be covered in yellow dust several days a year.