Great smog of 1952 had a measured peak daily average of 1620 ug/m^3 of total suspended matter (TSM) [1].

Reported levels in Shanghai of up to 602.5 ug/m^3 PM2.5 (suspended particles < 2.5 microns).

We can convert from TSM to PM2.5 using ballpark conversion given by: PM10 = 0.6TSM [1; notes to table 3] PM2.5 = 0.75PM10 [2].

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.

Open access sources: [1] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1240556/pdf/ehp1... (Table 3 reports total particulate during the great fog of 1620ug/m^3, and estimates PM10 as 60% of that number = 972)

[2] http://www.epd.gov.hk/epd/english/environmentinhk/air/guide_...