Modern copper-based treatments--e.g. ACQ or CA--still cause skin and eye irritation. If you try to sand it so that people sitting in your new chair don't get a nasty splinter somewhere best avoided, you can compromise the effectiveness of the treatment (even when the treatment gets full penetration, it's still most effective on the outer layers you're now sanding away). Plus, while the dust you create when working with it might not include arsenic, it's still nasty to breathe in and can cause respiratory problems. Staining P/T wood can be a whole ordeal in itself, and because interact much more closely with furniture than say a deck, any imperfections will be more noticeable.
Even then, it's not like P/T furniture isn't going to require ongoing maintenance in the future. At which point, you're better off with something like cedar or white oak. Hell, with a decent outdoor grade finish and proper care, even untreated pine is going to last for years without rotting away underneath you.
Still trouble when the chair is at end of life, and you burn or bury it.
Here’s what I use for furniture that I build:
- My planters use red cedar, often bare.
- My outdoor furniture use a wood oil plus polyutherane.
- My indoor furniture use just wood oil.
But that’s only in general. There are other considerations like impact resistance and spills that can stain the wood so you have to decide what combination of wood type and treatments work for your goal.
[1] https://www.instructables.com/How-to-Burn-Stain-Wood-Aka-Sho...
You can also add a sealant on top like polyutherane if you want it to last even longer at the cost of imparting a different texture.
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.
Wood is typically treated with nasty chemicals. For example, formaldehyde is extensively used in these applications, and it's linked with cancer.
https://www.cancer.org/cancer/risk-prevention/chemicals/form...
I was of the impression that PT lumber to modern standards involved relatively innocuous copper plus an organic biocide like an azole (for antifungal purposes), no?
And I thought the formaldehyde-heavy resins were regulated into obscurity back in like 2009, were they not?