I can remember seeing smog in LA that nearly reached this level in the 60's. Sometimes your car would cut out due to a lack of oxygen to burn.

And the smog in Dallas in the middle 60's hung over the city like a purple cloud.

But the smog in China in question appears to be from coal burning. I cannot find any articles that seem to know the source of the smog, but if anyone knows I would find the information interesting.