As a general rule, (according to the mags/books I've read..) is to record and mix at the lowest volumes possible. Sure do a final mix at a higher volume, but your long sessions should always be at low volumes. And what also helps is to stop, listen to some natural sounds (wind, noise, tv, etc..) this helps your ear drum relax. Also, make sure to clean your ears out often. Ear wax build up can cause permanent damage as well. (yuck!)
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Yah I know, stupid ain't I?