system
Given the following post, respond as if you're commenting on Reddit:
human
What specific effects does radiation have on your body?

With the scare of radiation leaks in Japan, I realized I don't know how/why radiation is harmful to the human body - I just know that it is.
gpt
Dangerous radiation is the kind that ionizes.  "Ionizing radiation" just means that it is capable of separating electrons from their atoms.  The direct consequence of radiation exposure is that you are left with several free electrons and several ionized atoms.

These charged particles are chemically reactive.  You see an increased amount of free radicals along the particle's path.  These are very reactive chemicals like H3O, H2O2, etc.  If these free radicals happen to be close to a DNA molecule, they may react with the DNA backbone and break it. 

If this break isn't repaired, several things can happen.  First, the cell could be non-functional, and die.  Second, it could be unable to divide, also causing it to die.  These two effects are relatively benign at low levels of exposure, but at very high doses can have macroscopic effects on biological systems.  For instance, your bone marrow could be destroyed, and you can no longer produce blood cells.  Or the lining of your small intestine becomes so damaged that you cannot absorb nutrients.  Both of these effects take a LOT of radiation.

The final effect is mutations.  Your DNA could be changed in such a way as to allow the cell to continue living, but with slightly altered function.  This change in function is passed to that cells progeny.  If enough specific functional changes accumulate in a cell, it can become cancerous.