>The worldwide average background exposure is about 250 millirems
>(2.5 millisieverts) a year.  People in Denver get about twice that
>because of the altitude.  Various places get more, some a lot
>more.  Granite is radioactive, so is coal and seawater so people in
>New Hampshire get a slightly higher exposure.
>
>A chest or abdomen CT scan delivers somewhat over twice the average
>yearly background dose, whereas a static chest xray adds only a very tiny dose.

For context, the allowed yearly industrial exposure for a
non-pregnant worker in the US is 5,000 millirems taken over the
entire body, or 50,000 millirems to any individual organ, or 15,000
millirems to the eye, or 50,000 millirems to the skin.

Pregnant women are limited to an industrial dose of 500 millirems in
addition to background radiation, and it's considered desirable that
members of the general public receive no more than 100 millirems of
industrial radiation per year.

There's an anomaly in that it appears that airline pilots who do
transpolar flights normally receive higher radiation doses than any
other industrial workers, but up until now their exposure is not
monitored.  I personally expect that will be changing.

Yours,
David