Usually the back third or so and on long flights it was horrible. Also, the cabin pressure in those days was thinner than it usually is now and the air dryer.

The air was somewhat drier but also of higher 'quality' because less of it was recirculated; most was fresh intake from the atmosphere, heated, passed through the cabin and dumped.

The introduction of widebody twins in the early 1980s coincided with increased air recirculation, in order to reduce bleed-air load on the engines, and until the 787 and A380 it was often over 60% recirculated.

Ironically the elimination of smoking led to a big increase in recirculated air.

You mean it could have been higher quality if not for all the smoking. I remember it being pretty miserable on longer flights.