From the experts:
Until 2003, the preservative most commonly used in residential pressure-treated lumber was chromated copper arsenate (CCA), an extremely toxic chemical. During a fire, emissions of chrome and arsenic would make their way to the catalyst and especially arsenic, is a major catalyst poison.
Although the “post-2003” formulations no longer contain arsenic, your local home store or lumberyard is now selling lumber treated with less toxic alternatives... amine copper quat (ACQ) and copper azole (CA). Both of these contain copper, which is a catalyst masking agent. So the cats would last a little longer with the newer pressure treated, however eventually, the copper would mask the catalyst surface. The arsenic and copper are “inorganic”, they won’t volatize or burn like organic compounds will. The inorganic additives to make the pressure treated resistant to rot and vermin, do not dissipate over time. They don’t leach out much either.