| The allergenic oil "urushiol" is in the resin of the plants. The resin is in canals throughout most of the plant and is released when the plant tissue is cut, such as broken leaves or stems. If any resin was transferred to the wood, it can last for 100 years if it is in a dry cool protected spot. Outside, it will harden and become non allergenic much-much quicker. In the summertime I would put a sprinkler on the wood pile and let the wood get wet. Wet and warm conditions will cause the allergenic oil to harden in about 24 hours if the conditions are just right. If there was a lot of resin escaping onto the surface of the wood, It will turn black and hard, but that does not mean it is not allergenic.
The Japanese, Chinese and Koreans have a tree that has almost the same chemistry as the allergenic oil of poison oak/ivy/sumac. THey harvest the resin from the tree (Toxicodendron vernicifluum) and make a lacquer that they paint on bowels and many other implements. They then cure the lacquer in a warm humid cabinet.
My credentials: I wrote "The Poison Oak & Poison Ivy Survival Guide."