Usually, when exposure to x-rays is compared with "a day in the sun" or "a day at the beach," the mistaken assumption is that the ultraviolet light from the sun is comparable to the penetrating photons of x-rays and gamma rays.

          That assumption is mistaken. Because ultraviolet radiation does not penetrate past the skin, it does not cause Cancer of internal organs such as the tongue, lungs, breast, pancreas, colon, etc. Ultraviolet light can cause Cancers of the skin (and probably some Cancers of the eye). Xrays can cause Cancer not only of the skin, but also of any internal organ exposed by the xray beam.

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          When the "day in the sun" analogy is meant to compare x-ray exposure to exposure by one day of natural background radiation, the comparison is still mistaken, even though natural background radiation has access to the body's internal organs. The reality:

          At sea-level, the typical dose from natural background (excluding the dose from inhaled radon/thoron) is about 0.1 rem per year. It is a little higher at higher altitudes (e.g., the comparable annual dose in Denver is about 0.12 rem).

          At sea-level, the natural background dose per 24 hours is 0.1 rem divided by 365 days, or 0.000274 rem per day (274 micro-rems/day). This refers to average INTERNAL organ-dose (not the surface or skin dose), and it refers to the WHOLE body. Approximately 20% of the 0.1 rem comes from radioactive potassium-40 which is in every cell. The dominant share of the 0.1 rem comes from external radiation. Below, we will approximate that none of the 0.1 rem is from internal potassium-40.

          A reasonable APPROXIMATION is that average internal organ-dose is about one-half of the surface dose. So the surface dose from natural background radiation is about 0.000274 rem times 2, or 0.000548 rem per day, on the average, at any place on the body.

          By comparison, what are the surface doses from common x-ray procedures? X-ray doses are rarely measured. The figures below, from limited samples, may be typical. Doses are much higher at some facilities than at others, according to limited surveys. First, we multiply the surface dose from x-rays by 0.33, because we will use the approximation that the typical x-ray procedure exposes just one-third of the body. Then we divide the product by 0.000548 rem (the whole-body surface dose per day, from NATURAL background radiation). The resulting ratio reflects approximately how many times riskier the x-ray procedure is, than one day of exposure to natural background radiation.

• Routine chest x-ray, per image: About 0.01 rem * 0.33 = 0.0033 rem. Ratio = 6.
• Many other routine exams, per image: About 0.3 rem * 0.33 = 0.099 rem. Ratio = 181.
• CT scan, per exam: About 4.0 rem * 0.33 = 1.32 rem. Ratio = 2,409.
• Fluoroscopy, per minute: 2 rems * 0.33 = 0.66 rem. Ratio for a minute of FLU = 1,204.