We can thus say that from 1980 to 1990, the incidence of smoking decreased by 10 "percentage points" even though smoking has not decreased by "10 percent"; since a percentage indicates a ratio, but not a difference.
We can thus say that from 1980 to 1990, the incidence of smoking decreased by 10 "percentage points". It would be misleading (and wrong) to say that smoking had decreased by "10 percent", since a percentage always indicates a ratio, not a difference.