Ugh, this stuff is bad news.
It's caught a lot of people off guard in the last few years around the Los Padres NF as it has followed in the aftermath of all of our wildfires and at least initially, people didn't know what it was. Apparently it's endemic to Southern CA and the seeds can live dormant in the ground for years until given the opportunity to re-sprout. Seems to come up after major disturbances… like wildfires.
I've mostly noticed it at moderate elevations, say 3,000' to about 5,000'. I haven't seen much of it below or above that range locally.
I don't have any first-hand knowledge/experience with the skin irritation caused by it, but it's my understanding that the hairs on the leaves that give people the itchies. Treatment is similar to PO: calamine lotion and/or cortisone-based products.