Stick; virtually all the P.T. lumber available up here is ACQ . My understanding is that here in Canada they can't sell CCA treated product at retail. I've been told it's still available to industrial users; power poles etc.
" Chromated Copper Arsenate (CCA-C), Alkaline Copper Quat (ACQ-C, ACQ-D, ACQ-D Carbonate), Micronized Copper Quat (MCQ), Copper Azole (CA-B & CA-C, μCA-C) and Sodium Borates (SBX/DOT).
These treatments are often referred to by trade names such as: Wolmanized Natural Select™ (Copper Azole), Preserve and NatureWood® (ACQ), MicroPro™, Smart Sense™ (MCQ), and Advance Guard® (Borate). Each preservative usually has a number of variations available so care should be exercised when specifying treated wood."
I've never seen a 4x 4 that's been saturated right through. The point of treating is to create a total envelope that prevents organisms from penetrating through to the good stuff. If the installer doesn't do a thorough job of treating any and all cuts and bored holes, well then it was a waste of money buying P.T.
As to the hazard in playgrounds, MILLIONS have been spent/wasted doing remediation of playground equipment and the ground around the equipment, with little or no evidence that it actually presented a serious hazard. (But giving your kid a cellphone that emits frequent RF is OK?!)
ACQ does not contain Arsenic. And if it did, so what? Arsenic occurs naturally in the soil. As long as the little rugrats don't eat it, how much are they going to ingest?