The older aircraft were flying at lower altitudes where there is more moisture thus more moisture in the cabin. (707 cruised at 31 000' vs the 777 at 39 000').
As for long term health problems for pax, I doubt. Spend the same amount of time in the center of any major city or work in a heavy industry plant will have more effects. Actually, it is proven that airline pilots live longer and healthier than the norm.
But I reckon that it is tough for the eyes and throat and that in such a closed space you are more at risk to pick the latest flu from the Far East, India or Africa... Guess it comes with globalisation