This is a good try, but I am looking for something much, much worse. Like 30 billion people living in complete destitution with massive famines sweeping the world ever seven years killing off 40% of the population each time and leaving the rest in pain and agony. Or only one in twenty children survive past the age of seven, the rest dying terribly. A world so bad it makes you uncomfortable to even think about it. I'm also thinking about this more on geological time scales - we can probably come up with a world where such suffering has been going on at least long enough for evolutionary pressures to be significantly compensating for some of the dreadful environmental effects.

I do like the idea of lead poisoning. The only thing difficult with that from the story perspective would be the relative difficulty in rectifying that problem but that's probably fixable with some creativity. Easily accessible WMPs wasn't the direction I was thinking of but might work and is definitely your most horrifying suggestion from my view.