This is somewhat the pink elephant in the room when it comes to ANC. Last year lots of people complained about a simultaneaous reduction of ANC across several brands and models brought with firmware updates. One suspicion was that Bose, Sony, Apple have some info about possible damage from ANC and did this to avoid liability. I'm wearing ANC headphones up to 12 hours a day now for 5 years. I didn't experience any damage as far as I can tell but I am worried about it. There are several possible ways how wearing ANC headphones that long might cause damage. The build up of relative humidity might for example promote infections. But the big question is if the ANC itself might cause damage. As far as I know there are no conclusive studies on that.