Created a super-lethal fort some time ago. Might've posted about its beginning. So far, gone underground. It was built at the border of mountains, rocky wasteland, and I think taiga. Two "Terrifying" and one "Untamed Wilds" biomes. Beautiful, just beautiful. Native gold all over the dang place on the mountain, hematite beneath it. No coal, but I have trees. And I plan on slaughtering the elves anyways, so trees won't be an issue. I need to aggravate goblins, though .... So I did some Dwarven Science. Ordered a dwarf out into the "heinous dust" cloud. Waited. And waited. And then the yak, who I'd seen get coated in it earlier, starts flashing. So I say, what the heck? Do I have an extra animal there? While on his page, I accidentally hit Z. He's completely rotten and swollen with his own blood.
So that's what it does. Four dwarves down for a test, plus a yak cow and bull. But I butchered the yaks before the dwarves started dropping. And the doctor .... spends two seconds diagnosing one of them, begins surgery "remove rotten flesh". His patient died while still being operated on. As did the other three. Oh well, it was a test anyways: and now we know, it's lethal. Gruesomely so. Reload save to continue on sans suicide test.
I just needed to know whether or not I had to lock them in. Also discovered a trick of burrows that allowed the dwarves to hop between them in an emergency burrow. (I was migrating the fort to the soil rather than the rock as expansion was taking too long.) That caused the extra dwarf casualties.