[steamrolled]

Back in the olden days, pressure-treated wood contained compounds of arsenic and chromium. This made it pretty terrible to cut, sand, burn, etc.

The warnings persist in part because older wood still has that problem, so "reclaimed wood" projects can be risky. That said, since mid-2000s, wood in the US and the EU is treated primarily with much safer copper compounds. Copper isn't hugely toxic to humans at the levels you're likely to be exposed to from wood.

To be fair, the treatment often also includes an organic fungicide (the "azole" part in "copper azole"), which is probably not understood as well as copper.

[Cthulhu_]

That puts me in mind of old railway sleepers being used for garden projects like raised beds and low walls. They were treated with ??? something, in summer the black tar would leech out of them.

[dugmartin]

The are treated with creosote: https://en.wikipedia.org/wiki/Creosote