Also we probably shouldn't tell them about background radiation.

For the curious a chest X-ray is about 0.02 mSv where your annual dose from background is about 2.4mSv, but this easily can be twice this if you live at high altitude or in an area with a higher level of radioactive minerals. Or if you are very lucky somewhere where both are a problem.

Hell airline crews are classified as radiation workers because the higher doses of cosmic radiation puts them over the threshold of on job exposure .