Smog Question (Shanghai)

Hello all,

I am considering an English teaching position in Shanghai, as well as a couple other cities. Shanghai would be a no-brained for me, however, I am concerned about the smog. What can I expect from the smog? Inability to go outside? Litmited outdoor activities? Should I wear a mask? How frequently is the smog bad? What are your thoughts, and how significant should this play into me picking a city?

Thanks,

-Mark