I've been DJing for a 10 1/2 years now and I'm definitely cautious now about my hearing. I've been using in-ear monitors BUT I notice I always like to DJ with them loud. Someone told me that after a while the ears get fatigued and so you keep them loud... and four hours a night doing it 5 nights a week can probably take a toll on hearing loss. I know in-ear monitors for DJing can actually be worse if used this way. Several professionals have told me that the best way to do it is to use ear plugs with over-the-ear headphones on.