I mentioned this thread to a non-whistling friend and he told me that the little ear plugs you stick in your ears can cause wax build up. I guess they push the wax into the ear canal. He works in the construction business and runs heavy equipment so he uses them all the time. He and his co-wrokers have to go to the Dr. every so often to have the wax removed. So if you slowly start losing your hearing after using the plugs it might be a good idea to go get it checked out.
Two words in response to Kathy’s post - ear syringe. (I’ve used one ever since I had an ear infection as a kid.) Of course, there may be some hazard in this that I’m not aware of. Oops, that’s more than two words. This is, indeed, a subject that none of us should ignore.
Having read the link Micah suggested, it looks like I can reasonably play 2 hours a day without risk of harming myself, depending on the exact definition of ‘mezzo-forte’ and how loud piccollos are… I can probably play my Hoover for -much- more than that, or could if I hadn’t chipped the mouthpiece… 
Of course, I don’t have any loud ‘session’ whistles, so, hey.
I might put a decibel meter in front of me at some point, but I think for now I’m not going to worry too much about it… though, I’m going to get earplugs anyway for that d!*# fife, which even before this thread, I had already determined was too loud to play without them… it -hurts- to try to even do a scale and leaves my ears ringing. Yeesh!
I do, however, recall some study or studies that suggested that the continuous noise of urban life contributes far more to hearing loss than whistles are likely to, that is to say, because of the constant sounds of traffic, air conditioning, plumbing, fans, whatever, that our hearing (in urban environments) doesn’t -ever- get rested, even at night. This has certainly been true for me, to varying extents, for a long time now (though I do rest my hearing on the fairly frequent weekend in Vermont… after dark, there’s no sounds not made by those of us in camp, or the occassional scurry of wildlife, and it remains that way until birdsong starts at dawn…)
Certainly my workdays are spent surrounded by the omnipresent hum of computers, which while vastly below ‘damage levels’ in any normal sense is, well, omnipresent white noise that prevents my hearing from being rested at all during the day.
On the other hand, I’m not going to wear earplugs all day… that’s effectively the same as making myself mechanically deaf now to prevent biological deafness later! On the other hand, at least in my home situation, I can try to get low-noise appliances as much as possible, and I already prefer an open window to an air conditioner when the temperature is anything less than sweltering…
I’m running on again, I better go before I put you all to sleep with my babbling. 
–Chris