Listening at a high volume for a long time is what hurts. Engineers can give you all sorts of numbers in db and such which is meaningless to you unless you a way to measure the level out of your earphones and/or calibrate them. So it's really up to you to use your own judgement about loudness. I think anything less than 50% should be OK but from using my own computer to play internet stuff, the level they supply varies a lot. Some videos I have to turn the volume all the way up to even hear it and the next one would blow my head off at the same level.
One rule I have heard is if someone else can hear what you are listening to, it's too loud.
Sorry I couldn't reply to your private message but when I hit reply there was no place to type a message.