aluminum is stable in the sense that the outer layer oxidizes rapidly and prevents the rest of the metal from reacting with anything. the reason thermite is so exothermic is because of massive amount of surface area provide by the powdered metal. a chunk of aluminum simply isn’t dangerous.
I’m not well versed in this but I was under the assumption that anyone who worked 32 or more hours was eligible for some kind of insurance through their employer, which is why so many people are stuck at 31 hours a week.