I've transported a couple of people for grinder related injuries as well as treated a few on the oil rigs. Never think even a 4" won't rip you open in a heartbeat and the larger 7" grinders are extremely dangerous! We had an LTI earlier this summer when an experienced welder pulled the cord instead of the plug on a 4", didn't insure it was de-energized and then went to change the cutting disc. He somehow bumped the paddle and it spun up enough to bite him in the thumb.
Years ago while cutting an exhaust bolt lose under one of my FJ40s I got a splinter of metal in my eye and had to have it removed. It somehow slipped in around my safety glasses that I had on. Ever since then I've always worn safety glasses and a face shield along with leather gloves. Make sure there is no loose clothing or anything else that they thing can grab too.
You need to double check the rpm ratings against your tools on everything that rotates but I also refuse to use any of the Chinese made cutting or grinding disks or their wire wheels/cups as I've seen them come apart more than once. The dollar saved just isn't worth the risk. Other common cause of incidents are using the wrong tool for the job, i.e. grinding with a cutting wheel or taking the handle off.