Do you have a source for the high radon in LA? According to this county source it is pretty low in LA county but much more pronounced in Ventura county (1% homes with high levels vs 14%). I imagine there is some potential for accumulation effects but this is probably much worse in markets where the homes actually have basements.
I found a map at some point. It was 5 years ago. I'm in the San Fernando valley area myself. I do believe there were hotspots in the hills here but perhaps it was mostly ventura county. I'd have to find the map.
http://www.publichealth.lacounty.gov/eh/safety/radon.htm