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The dose of radiation received from diagnostic X-ray procedures is quite small (go here) and the risk of this causing a fatal cancer (go here) or causing genetic problems (go here) is quite small. For every total body rad (or 1cGy or 1000mRem or 1Rem   or 10mSv) the odds of dying of cancer go up only by 1/2500 (0.04%) which is quite small when you consider every one already has a 23% risk of dying of cancer and about a 40% risk of developing cancer (go here.)

The risk of severe genetic defects in first generation offspring due to occupational radiation exposure to one or both parents is estimated to be  4 in 1,000,000  (0.0004%) per mSv. Under NRC guidelines (go here) the limit exposure dose for occupational workers is 5 rem/ year (50mSv) and the general public 0.5 rem (5mSv). (For comparison, the exposure from a total body CT scan is 12mSv and the  dose to Japanese atomic bomb survivors was 20mSv.)

Background radiation everyone is exposed to is about  3.6mSv or 360mRem/year (see here). For another discussion of the health risks of CT scan go here and here. A recent review of risks from medical imaging here.

 There is
web site calculator that will calculate the cancer risk from diagnostic imaging here, using data here, here ,here, here, here, here and here

 PET scanning is non-invasive, but it does involve exposure to ionizing radiation. The total dose of radiation is small, however, usually around 11 mSv. This can be compared to 2.2 mSv average annual background radiation in the UK, 0.02 mSv for a chest x-ray, up to 20 mSv for a CT scan of the chest. A policy change suggested by the IFALPA member associations in year 1999 mentioned that an aircrew member is likely to receive a radiation dose of 4–9 mSv per year.

 

(the units of radiation dose are: Gray (Gy) is the SI unit of absorbed dose. One gray is equal to an absorbed dose of 1 Joule/kilogram (100 rads). Rad is the special unit of absorbed dose. One rad is equal to an absorbed dose of 100 ergs/gram or 0.01 joule/kilogram (0.01 gray).Rem is the special unit of any of the quantities expressed as dose equivalent. The dose equivalent in rems is equal to the absorbed dose in rads multiplied by the quality factor (1 rem=0.01 sievert). Sievert is the SI unit of any of the quantities expressed as dose equivalent. The dose equivalent in sieverts is equal to the absorbed dose in grays multiplied by the quality factor (1 Sv=100 rems). So basically 1 rad = 1 rem = 1cGy = 10mSv or  100 rad = 100 rem = 1Gy = 1Sv (More on definitions go here.)

The radiation risks to a pregnant woman cause considerable concern. There is general agreement that fetal exposure of less than 5 rads is not considered teratogenic. It cannot be stated that there are no risks associated with lower doses, however. There are numerous sites that discuss this topic in detail: a, b, c, d, f, g,). Somewhat comic 1950"s propaganda film about radiation here and here.