Being exposed to very loud noise just once probably won't damage your ears much but having to live with it on a daily basis for years can and will cause irreversible harm to your ears if you don't do anything about it.
So what can we do to protect ourselves?
I've seen people walking out of stage in rehearsals and refusing to play until the brass or percussion were moved away from them! You can try that depending on how the politics in your band work. In some places, it would work; in others it would be at you own risk...
I've personally tried negotiating with the percussionists to see if they could move a bit. They sometimes agree to move a few inches back but in general, they tend to be quite territorial about their space and rarely move even when it's the conductor asking! So not very helpful here either...
Plexiglass screens will reduce the noise in your ears. The problem is that there isn't always space for them. When you use a screen, make sure it's as close to your head as possible behind you. Otherwise the sound waves will "go around" it and it won't be as efficient.
Ear plugs work but they can be uncomfortable. You can find special ear plugs designed for musicians though. They might be a good investment if you're sitting in front of the trumpets or next to the piccolo. I personally have some silicone ear plugs that I can shape the way I want. I place them in my ears without pushing them all the way in so I can reduce the noise level and still hear quite well. For me, this is the last resort solution when diplomacy and screens have failed. I don't like it very much but at least my ears will live another day!
Young musicians tend to neglect that issue. I know because I didn't pay much attention to it myself until I got a good wake up call once. I got hurt so bad in a rehearsal that I had to file for work injury to the orchestra management. The doctor said it was a concussion of the nerves. I took some tests to determine how my ears were affected and it was obvious that my left ear was significantly hurt. A bit like when you walk out of a dance club; you feel like everything is "muted". For me, I had headaches and extremely sensitive ears for a few weeks. Fortunately, it got better after a while.
I've seen many brass players losing a rather high percentage of their hearing ability after 20-30 years playing in an orchestra. Some of them are almost deaf and need hearing aids to do their jobs. Don't let this happen to you. Do your best to protect your ears even if it's a bit uncomfortable. At the end your health is more important. Ear damage is irreversible and you only have one set of ears so don't wait until you go deaf or develop tinnitus to take good care of them.