Statistical studies show that residents of the Mississippi delta area get more cancers than people living at higher altitudes in Colorado. In the Mississippi delta, radiation levels - both naturally occurring and due to cosmic rays, are lower than areas such as Denver, where natural background radiation (Uranium, etc. in the granite) plus higher cosmic rays causes a higher level of radiation exposure.
Hormesis studies have been done ... and one accidental study was for a long-term low level exposure to lots of people. In Mexico City, a high rise apartment dwelling was built of steel that had a radioactive cobalt source that was melted/destroyed when the steel was mixed. The building was radioactive ... low level ... significantly above background, but not a high level (maybe 5 mRem/day ??).
Long term residents in the building ended up with significantly lower cancer rates after 5 years.
Hormesis is the concept used when someone with allergies gets allergy shots. Over time, the body's immune system learns how to better fight the problem. A body exposed to low level radiation may also learn how to better recognize and fight possible cancer cells.
Interesting concepts ... and probably worth more studies.
Mike