The worst I ever saw personally was around Beijing on Christmas Eve 2008. I guess this link should work. Beijing is under the grey murk bounded by that brown mountainous ridge, on the right of the image. I've also checked recently to corroborate the reports of the last week or so.
The thing I always wonder about is the scope of the public health issue that this must surely engender. I guess they just endure it, which is their tradition in the first place, but thoroughly reinforced by their totalitarian government.
I worked in China from fall ‘76 to spring ‘77. Went through Beijing, Harbin and Chengdu on the way to various job sites. There was only very mild smog in Beijing and almost perfectly clear air in Chengdu and Harbin.
They should have been smart enough to install western pollution equipment as they were growing.