A cross-country plane flight is about the same dose as a chest x-ray (or it least it was in the past, maybe they have lowered the dose for x-rays now.) Many natural rock formations such as granite have a fairly high background emission. I often came across situations in which people had built structures using materials that had considerable radioactive emissions. In Texas, a farmer proudly showed us his springhouse built over an area where geologists in the 50's had gotten an extremely high reading. The family built the springhouse there on the theory that the radiation would be good for arthritis.
We were once doing some exploration in the Rockies, and came across a hippy-type family who had built the the fireplace in their home out of local stone. Our counters went off the scale. I would not have lived in that house, myself.
Is it dangerous? Well, sunshine is dangerous. It gives you cancer. But it's just kinda the way things are. But it's often useful to keep things in perspective to realize that we get low-level radiation all the time.
Here in New Mexico, some areas have levels of radon gas emission much higher than the EPA recommendation. Here's what the
state says about radon: "Radon emanates from soil and water supplies, and collects in homes and work places because of its gaseous properties. When radon is inhaled, the lungs can be potentially damaged by radiation energy released by the radioactive decay process. Radon itself delivers only 1% of the radiation dose to the lungs, while the other 99% comes from the decay of radon daughter products. Because of its ubiquitous nature and because it delivers radiation dose to the lungs, radon (and its daughters) is considered the second leading cause of lung cancer."
Oddly enough, New Mexico has an unusually low rate of lung cancer compared to the rest of the country. So what to make of it all? ;) I had a system installed in my house that theoretically keeps radon from coming up through the foundation, but does it work? Dunno!