chockablock investigated that here on HN, at
https://news.ycombinator.com/item?id=6864088 :
> So according to these rough calculations the great smog of 1952 probably had a PM2.5 of about 700 ug/m^3, quite similar to the current Shanghai smog, and lower than levels regularly reported in other Chinese cities.
The current PM2.5 for Beijing, at http://aqicn.org/city/beijing/ , is 162 - "unhealthy".