Maybe his voice loss was due to the smoke but I can’t help but think singing in a rock band for as long as Bono has must take its toll on the voice. I like his voice but it does sound like he often sings at his absolute highest which can cause one to strain.
Bingo! That's also my opinion.
I first became aware of this when I met a man at work who was "vocally challenged" in everyday conversation. This was about 40 years ago but I never forgot it. We became friends because he was a drummer/vocalist in a local band. One day I asked him, why he sounded like that. Reply: he developed polyps on his vocal cords from singing incorrectly for many years.
Then, when I was taking vocal lessons, long after that, one of the first things my teacher made sure he taught me was to sing from the stomach and not the throat. He stated that a trained singer can sing under any circumstances (sore throat, tiredness, etc) because they learn to bypass the throat which most rock musicians don't seem to take the time to learn. But that's how Opera singers can sing three hour operas.
I forget myself sometimes to "bypass" the throat, or keep it lubricated with a lozenge, and by the end of only one hour I'm sometimes hoarse (unless I eat a McDonald's exactly 30 minutes before performance time).