The problem with using modern pressure treated wood for outdoor furniture is less any cancer risks like with the old CCA treatments, and more that it's just a bad choice for a bunch of different reasons.

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.

You can buy normal wood, then apply treatments afterwards, then stain or what not.

Still trouble when the chair is at end of life, and you burn or bury it.

With a product this simple to make and prices of wood and finishing chemicals what they are I don't know if I'd even bother with finishing, but just remake.

In many parts of the world, an untreated wood outdoor chair would only survive a season or two before atmospheric moisture ruined it.

Basic weatherproofing treatments for basic lumber ? There's tar, which nobody does at home. There's wood oil. What else is there ? Do I have other options ?

Varnish like polyutherane.

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.

"wood oil plus polyurethane" sounds interesting. NSo you've had no problems with the two reacting badly to each other ? I'll be drilling screws in, so maybe the wood oil adds extra protection where screws penetrate the polyurethane layer.