As a musician or a producer of music, your ears are your most valuable asset. If you're hoping to make a sustainable living from music and you damage your hearing, then you've lost your livelihood. The ear is such a precise, complicated and delicate assembly line of organic machinery that it is impossible to repair damage caused by exposure to loud sound. It's really strange to me that I'll meet people who do nothing to protect their ears and think that they are somehow immune to damage. I wish there was some way I could put this gently, but there isn't, so here goes: That is dumb. If you think that you can expose yourself to loud sounds and not get hearing damage, you are dumb. In addition to being dumb, you will be near-deaf by age 40.
Hearing loss is not necessarily a naturally occurring problem. Healthy people in societies that have less modern technology and less noise overall can experience perfect hearing all the way through old age.
If you've spent an hour at a noisy restaurant or bar, where you had to raise your voice to talk, then you've permanently damaged your ears. If you've spent an hour at a large concert or a dance club, then you've permanently damaged your ears.
Let's get a little more detailed. According to Stanley R. Allen, author of Audio in Media, any sound at 80 decibels or louder can be damaging to your ears, depending on how much time you spend with it. If you play an acoustic guitar at a normal volume, you are being exposed to 80 decibel level sound. Any sound at 150 decibels is instantly damaging. For perspective, a rock band generally plays at 130 decibels. If you're listening to a live rock band for more than a few minutes, you will permanently damage your ears.
You might remember that back in 2001, radio personality Rush Limbaugh, who had become deaf in one ear and near-deaf in the other, underwent cochlear implant surgery. This surgery is only for the super-rich and because of its primitive ability to pick up sound, can really only help you hear speech, not music. Anyway, my point is that Limbaugh claimed that his hearing loss was from an autoimmune disease. But really, this guy talked (loudly) on the radio every day, with headphones on to hear both his own voice and the scratchy sound of opinionated callers. Hearing loud vocals on headphones is roughly at a level of 100 decibels. We know, without a doubt that being exposed to that much noise causes permanent hearing damage. Autoimmune disease? You can be the judge. Most aging rock stars won't admit it, but they are probably deaf or near-deaf. My Audio Engineering teacher was the only one in his band to wear earplugs at their performances, and he is now the only one of the group who isn't deaf or near-deaf.
Now, I'm not telling you that you can't play your guitar just because it is at 80 decibels. In modern society, it is almost impossible to completely prevent hearing loss. Instead, simply be smart about your exposure. Always wear earplugs to concerts and at a loud workplace. You have no reason not to. Don't whine about how they're uncomfortable to wear. If you get well-made ones, their actually quite nice. I always use the AO Safety brand of disposable foam earplugs, because they are so comfortable I don't really notice them when they're in. Plus they cover the frequency spectrum pretty evenly, so you can still hear a band clearly, just slightly quieter than normal.
The mindset you should have is this: if you are going to damage your ears, make sure it's worth it. Protect your ears when you don't need to hear all that sound, so that you have more time to damage them when you actually care about what you are hearing. As one who wants to make and appreciate music for the rest of his life, I absolutely can't afford to damage my ears. If you love music and want to enjoy it for the rest of your life, you can't either.
Here are some other sound levels that you might find interesting:
Average conversation: 60 decibels
Busy street: 70 decibels
Crying baby: 90 decibels
Electric guitar amp at level 10 roughly six feet away: 110 decibels
Standing next to a Jet Engine: 160 decibels
Source: Audio in Media, Stanley Allen