I spent over ten years as a captain on the older 146 aircraft and although it had many good features one of it's worse ones was the quality of the air in the cabin pressurisation system. During the period I was flying it I suffered more colds and flu-like symptoms than at any other period in my life . I returned to Boeing 737 aircraft after that and almost immediately my health began to improve. Since retiring from flying and taking up simulator instruction about four years ago I virtually never suffer from colds or similar ailments at all! I attribute the extremely poor air quality on the 146 to two factors-
1. The high leakage rate through the engine oil seals into the bleed air.
2.The requirement to recirculate a high proportion of the cabin air to compensate for the inadequate thrust/bleed air from the engines (every 146 pilot knows how underpowered this aircraft is!)
Quite often apon initially starting a day's flying on the 146 it was common to start the APU and then select the highest possible cabin temperature in an attempt to try and "burn off" the residual deposits in the ducts before the rest of the crew and passengers boarded the flight!
Although the 146 and the Boeing 757 were known to be the worst offenders in the "dirty air" club I'm sure that eventually all new large aircraft will follow Boeing's lead and revert to indirect cabin pressurisation-- nothing new really-- the good old DC8 had cabin turbo compressors roaring away in the nose. (The downside of those was that most pilots ended up with with significant hearing loss!)