Wow, that's lucky that you never had a reaction! I honestly don't see poison ivy around here - though I could just be being inobservant. A quick google implies that it is more common in the southern half of the state, and less so in the north. Denver is right in the middle, so maybe like many things, we're right on the border of natural regions. We're just to the east (by about an hour's drive? Less?) of the continental divide (where the water runs to the Pacific on the west side and to the Atlantic on the east side); my tree books park the east/west divide of plants practically right on the north-south highway that runs through town; the SW desert pretty much officially stops here, we're considered "high desert" being right on the northern border of it; tornadoes almost always hit east of that north-south highway but almost never west of it; wildfires pretty much stay west of it and don't cross east of it. Ahhh, sitting right in the shadow of a big ass mountain range! It's the convergence of everything and simultaneously kinda a buffer zone. LOL