A huge amount of hair dye product contain chemicals linked to cancer. If you get ‘all natural’ and ‘organic’ hair dye (read: expensive) it shouldn’t be a problem.
I’ve used hydrogen peroxide to bleach my hair, it’s great. Highlights look more natural if done this way instead of using a dyeing kit, and it doesn’t have all that toxic stuff.
And about men going grey young, what about losing their hair altogether? Is it muttar for a man to get treatment to regrow his hair, or is that considered a woman-thing?