I had a pretty lazy search around, but I can't find any solid studies, only articles making reference.
A couple of them have mentioned "110-120 dB for over an hour a day will do damage!", but if you're listening to headphones at ~110dB, you may as well be at a rock concert or in a night club (both places where I wear ear plugs).
The study they've been talking about (Northwestern University? No idea) also mentions a completely fucking useless "60/60 rule", where they say you should listen on 60% volume for an hour a day max.
Even for a rule of thumb, that's way too vague and unhelpful.
I suspect the "ZOMG HEADPHONES ARE KILLING OUR CHILDREN!" cries are more because people just plain turn shit up "too loud".
Once you've
done damage to your ears, you'll be more likely to crank it
further, which only makes things worse.
For me, the whole point of head/ear/canal phones is that they're pleasantly listenable at very low dB levels (again, YMMV - audio volume is freakin' hard to measure, visualise and discuss at the best of times).
Edit:
Oh snap, just found something more useful:
http://www.headwize.com/articles/hearing_art.htm
Not hard science, but bits are based on existing reports and recommendations, and others are reasonable assumptions, by my measure at least.
In loudspeaker reproduction, sounds must travel several feet before reaching the listener's ears. By the time they arrive, a portion of the high frequencies have been absorbed by the air. Low frequencies are not absorbed as much, but they are more felt through bone conduction than actually heard. With headphones, the ears hear all frequencies without any attenuation, because the transducers are literally pressed against them. Thus, when listening to headphones at the same effective volume level as loudspeakers, headphones may still transmit louder high frequencies that are more likely to cause hearing damage.
^Makes sense. We tend to lose upper frequency hearing as we go, and this doesn't help matters. Strike one for the phones.
Also of note:
Another hearing phenomenon that seems to be more noticeable with headphones is a decreasing sensitivity to sound levels over time, as the ears adapt to loud sounds.
Keyword is "adapt". This is misused.
Any time your ears are "getting used" to a volume which once felt a little too high, or at the threshold of what you could handle, you have done damage to your ears.
Again, I wish I could cite something more solid, but meh.
Indoors in a quiet listening environment: According to the Airo study, listeners in a quiet room set headphone volumes at an average of 69 dB, a little less than the average sound level in a restaurant. With open-air headphones, the ability to hear normal conversation through the headphones is a good indicator that the volume level is safe. Because closed-ear headphones acoustically isolate the listener, normal conversation may not be audible when wearing these types of headphones. Instead, a safe volume level may be set by moving one earcup off and comparing the level in other earcup with that of normal conversation.
I'd like to think this is me, but without any even remotely accurate method of measurement, I can't honestly say.
However, if someone wanders up behind my desk at work, I've noticed they can't tell if I'm listening to music, or just have the phones in silently. That's something, at least.
So yeh, if you're the kind of person who thinks 90+ dB is "moderate" volume for your headphones, you're doing damage by listening more than a few hours a day.
But by my yardstick, 90+ dB qualifies for Very Loud.
/shrug