Before 1974 many bath fans were just ducted into the attic, In the northern climate that has become a major Black mold issue. When I worked for our cities 2nd largest slum lord we had Section Housing Authority require us to remove the mold in the attic it two buildings. The first was cleaned with a approved Bio-cyde (high priced bleach water) and then the entire surface area sealed with Bullseyes paint. After that the other build was due for a new roof, we got a one year extension and installed new decking (plywood) and new shingles. That ended up being the cheaper method.

Now days bat vents are ducted all the way out doors to the soffit or to the hat/ridge vents.

Ivan