Wearing headphone is not that bad but using it for more than 30 minutes at a go is bad.
When you place on headphones, you cover your ears from the pure air, which increases development of bacteria by 700% in 1hr. Your ear drum normally can not bear increase of bacteria in that amount so it starts to react by paining or hearing damage in long term.
The second thing is, when you are listening to fast music/ high beat sound which goes direct to your brain due to headphones, it affects the fine tissue/membrane of the brain sooner or later listener becomes hyper or energized which is bad in long run.
The number of headphone-wearing pedestrians seriously injured or killed near roadways and railways has tripled in six years, according to a U.S. study.
One more thing you should know:
The number of headphone-related death accidents cases increased from 16 in 2004-2005 to 47 in 2010-2011, according to the study posted today in the online journal Injury Prevention. 34 of the 116 total cases in the study particularly mention horns or sirens being sounded before the victims were struck, wrote researchers including Richard Lichenstein at the University of Maryland Hospital for Children.