Edit: 10ebbor offers another method, which just reinforces my idea that "I can't think of a way to do it, therefore no one can and we'll just let employers get away with anything." is a stupid fucking idea.
Anyway, to my original post: Dangerous is lose-of-life and lose-of-mobility/limbs above a global threshold for that industry (obviously mining will be more dangerous than trucking will be more dangerous than warehouse work will be more dangerous that burger-flipping)
Too poor is some combination of debt-to-income, poverty levels, and food/shelter/electricity availability/consumption. Too low a level, or too high a debt-to-income, and obviously something is wrong.
Too much degradation is when global sea-levels rise, temperature increases across the globe, areas become de-forested or polluted, animals and plants in the region (esp. if they're endemic) become either genetically degraded (cannot breed with a wide enough population) or go extinct, etc.
Who gets to decide? No single person. Economics, biological and environmental sciences, hospital rates and public health statistics.
This is all thought off the top of my head, Lagslayer. People eminently more qualified and with a wider knowledge-base than I would be able to come up with better and more nuanced ideas than just me, sitting in my bedroom and thinking for a few minutes. You can do better than that, surely.
Assuming you aren't playing devils advocate.