Interesting FAQ thing there too. Q: Wait a moment... why are Montana readings quite a bit higher than in California? A: SF Bay Area is a coastal region, while

Interesting FAQ thing there too. Q: Wait a moment... why are Montana readings quite a bit higher than in California? A: SF Bay Area is a coastal region, while the other location is up in the mountains - which means that there is less atmosphere to stop cosmic rays. The difference in measurements is quite substantial, around 40%. If that freaks you out, consider that a commercial flight easily exposes you to about 300 µR/h!