Gamma and X-rays and neutrons are very penetrating and can readily pass through the body. Normal matter has no net electrical charge i.e. the positive and negative electrical charges are balanced. The interaction of ionising radiation with matter upsets the natural balance of positive and negative charges. The Sv is the modem unit of dose and has largely replaced an older unit called the Rem. One Sv is equal to one hundred Rems.
Human tissue is composed of cells.
Any problems from radiation damage to a somatic cell will affect only the irradiated individual. The risk that exposure to ionising radiation will cause a problem is proportional to the dose received. At the low doses that most of us might ever expect to encounter, the risk is very low indeed. However, there is no good reason to believe that there is any threshold level of dose below which risk is reduced to zero. It is presently estimated that if one million people each receive one mSv of whole body radiation, forty-five will develop a fatal cancer.