|Poison oak, ivy, sumac, and other members of the Rhus family grow in all portions of the North American continent (except Nevada, Alaska, and Hawaii) at elevations below 4,000 feet with a rainfall above 8 inches per year. These plants are a significant cause of worker's compensation claims in the United States. They are also the source of considerable distress to outdoor recreationists and hobbyists.
Got this from this site's general info link in case you want to look at it in more detail. It shows a U.S. map of p.i.
(Look in the right corner)