Physical Danger
It's a dangerous world out there, but it's even more dangerous if you work with heavy equipment and machinery. Expect your classic, playground-grade injuries—cuts, bruises, and strains—but also expect them on a much more serious level. That's just par for the course when you work with your hands. You'll be wearing protective equipment, but even then you're at a higher risk for injury than most people (source).
Being a millwright is like working at an all-you-can-survive buffet of injuries. A basic list of hazards includes back injuries, overexertion, falling and slipping, and getting struck by...well, anything, really. Get used to hearing about being struck; it's bound to happen in your line of work (source). We're guessing that's why the rule about hard hats is so strictly enforced.
When a job is this dangerous, the injuries stack up. A bad one could stop you from working for weeks, months, or even years. By the time you're nearing retirement age, you're probably going to be feeling pretty old. Stick by those safety rules, and maybe take up yoga.