They are if you listen at a loud volume, simple as lol. I myself don't actually use them that much just 'cause I don't usually need to use my .mp3 player as opposed my laptop. I use them more to play electric guitar through my amp at night without waking the neighbourhood. But now I'm just rambling lol.
in all honesty, since I was about 13 or 14 I have been listening to music every day very loud with earphones (the sort Sergio has posted above), so loud I crashed countless pairs of them. And here I am, some 20 years later, and my ears are still as good as new. Strange but true. But I think in general, it can't be too healthy.
My eyesight is crap so i generally pride myself on the fact i have hearing like a dog, but theres time when i have my headphones on to loud and can really notice when my ears are struggling. I do what Sergio said and use a decent pair of headphones, though sometimes i still get abit carried away/
Use them a 2 hours per week i think. 
