If 70% of chimney smoke re-entered nearby buildings, then those times when most buildings were burning, the air inside them would be nearly as smoky as what come out of the chimney. It's also very hard to believe outside of a tenement arrangement. In a village where there's lots of space between houses, and assuming all particulate matter fell back within the bounds of the village (itself a big stretch), it's hard to see how two-thirds of smoke would find its way back into buildings.