If our bodies are more transparent to X-rays than to visible light, why is it that X-rays are bad for you, whereas light isn't? A clue comes from the fact that X-rays photons carry much more energy than those making up visible light. Although this energy is far too much to be absorbed and bump an electron up an energy level (as with visible light), it is enough to do something far more destructive. X-rays carry enough energy to completely eject electrons from atoms, turning the atom into a positively charged ion.
Actually, it isn't only X-rays that have this potential. Anything to the 'high energy' side of the ultraviolet (UV) spectrum onwards (i.e. to the left in the diagram above) can also ionize matter - some UV rays, and all X-rays and gamma rays. For this reason, such radiation is often called ionizing radiation.
You may be asking yourself: what's so bad about making ions out of a few atoms? Well, the problem is fundamentally that in ionic form, atoms will undergo unscheduled chemical reactions. Often, this distorts or destroys the structure of the affected molecule, and (since structure is everything in biology) this is as good a way of messing up our bodies as any. Although almost any molecule may be affected, one of the most worrying cases is when DNA gets damaged. This can easily lead to serious malfunctioning or cancer. Ultimately, ionization is the reason why you can get skin cancer from the sun's UV light (and therefore another reason to protect the ozone layer that acts like a gigantic sunscreen for the earth), and why X-rays can harm you in sufficient doses.
How much danger is there from X-rays? Obviously, this depends on the dose you receive. First up, you get a certain amount from the sun and planet every day, so there's no way to be free of receiving X-rays (short of some special underground containment facility). I asked one of our consultant radiologists to quantify the danger to me and he put it this way:
- The average chest X-ray exposes you to the equivalent of one extra day of normal UV radiation.
- The average CT scan (sometimes called a CAT scan) exposes you to about one extra year of normal UV radiation. However, this varies quite a lot, depending on how much of your body is being imaged, and how thin the 'slices' are.
- MRI scans don't use ionizing radiation at all.
I'll try to confirm these figures with you all, but if they are true, then there doesn't seem much point worrying about conventional X-rays, provided you aren't doing hundreds! (This is why radiologists and radiographers have to wear those lead aprons, but you don't usually.) CT scans offer a slightly more significant risk, but it's still quite small in the greater scheme of things. Most importantly, though, the risk of not doing a CT scan is usually much, much, much greater than the risk of doing one. If you're still not reassured though, speak to your radiologist before the procedure to discuss the issue. Sometimes (but not always), the more expensive MRI option is equally likely to diagnose whatever problem they are looking for, and it is free of ionizing radiation.
[Update: I've done more research on the above figures, and they should now be replaced by these ones.]
There are two excellent online sources that I've principally used for this X-ray feature: How Stuff Works' page on X-rays, and this page on the Interaction of Radiation and Matter. In particular, take a look at this wonderful summary diagram.