I remember old timers talking about fumigating textiles mills with cyanide when they closed Saturday evenings. They would close up the mill, set up water buckets all over, then quickly drop pellets in the buckets which reacted with water to produce cyanide gas, and get the heck of Dodge.

I've used phosphine gas pellets. The pellets react with moisture in the air to make phospine gas, which diffuses well through stored grain. That stuff will kill you dead easily as well. A local fellow nearly killed his fool self with the stuff. He was on top of a grain bin dropping the pellets in and accidently dropped the container down the hatch. Like a fool, he climbed down in to retrieve the cannister and nearly killed himself. He got out, but IIRC had to go to the hospital.

I thought he should have better sense than that, and it turned out they diagnosed him Alzheimer's a couple of years later.

-Richard