As you approach Tower Bridge from the north side (the same side as Tower Hill tube station), along Tower Bridge Road, walk on the right-hand side of the road, next to the Tower of London, and look at the lamp posts and you'll notice one that isn't what it first seems as it's thicker than the other posts and there's no light on top.
This is a chimney/flue for the Royal Fusilier's room under the bridge which would have been used by the guards when on duty protecting the Tower of London. The room would have had a coal fire and what we see above on the bridge is the where the smoke would have come out.
The London Clean Air Act came into force on 1956 and smokeless fuel could only be used in urban areas. The staff I spoke to at Tower Bridge think this would have been the end of this room being used.
