Yikes! Gamma rays would be far more dangerous than x-rays -- but are also commonly blocked using lead.
Linky That's the general problem with WHATEVER the x-ray vision is. If it's high-energy enough to go through most substances, it's also high energy enough to cause significant damage, especially with frequent exposure. (Poor Lois!)
As for what x-ray vision is, I haven't heard a single good explanation. Someone did tell me, however, that the further you go along the spectrum, the more likely it is that a given type of radiation will pass right through an object. Visible light will bounce off most things, but x-rays will go through just about anything less dense than lead. Go even further down the spectrum, and you find nutrinos, which tend to go through entire planets without stopping. ("Nutrino traps" are designed to slow them down enough to study. Basically, they take extremely sensitive equipment and put it in a chamber with extremely thick concrete walls surrounded by thick lead walls surrounded by ... all of which is located underneath a mountain. Every once in a while, a single nutrino, passing by with tens of thousands of others, will have been slowed enough by all this that the sensor will be able to detect it.)
And
neutrinos aren't on the E-M spectrum. They have mass (
probably ). Hence they can be slowed down.
