Radiation refers to a process through which energetic particles move from one place to another in the environment. Different types of radiation fall into two basic categories: ionizing and non-ionizing radiation. Ionizing radiation contains high enough energy to break the ties between electrons and atoms, according to the World Health Organization. Non-ionizing radiation does not contain as much energy, but can still cause damage. Human tissue reacts in a variety of ways to both types of radiation.
Repaired Damage
The human skin is under near constant bombardment from ultraviolet radiation when outdoors during the daylight. Ultraviolet radiation, a type of non-ionizing radiation, comes in three types: UV-A, UV-B and UV-C. UV-C is completely blocked out by the ozone layer of the earth, and never comes into contact with skin. Small amounts of UV-B and a large portion of UV-A reach the skin, and are responsible for sunburn when exposure is too long. In addition to sunburn, skin damage occurs on a molecular level. The radiation from the sun causes breaks and mutations in deoxyribonucleic acid, DNA. Fortunately, human skin cells are equipped with DNA damage repair mechanisms that travel along the DNA fixing errors and damage from sources like the sun. In almost every case, the DNA repair mechanisms do their job and repair damage before the damaged DNA gets passed on to new cells.
Unrepaired Damage
In some cases, radiation damage to DNA overwhelms the ability of the repair mechanisms. Whether due to a large amount of DNA damage or DNA damage directly affecting a gene that produces DNA repair enzymes, sometimes DNA does not get repaired properly. If a DNA-producing cell is damaged but the cell is still viable, it can continue to divide, creating many copies of the damaged or improperly functioning cell.
In normal cases, a new cell goes through a life cycle in which it divides into new cells through mitosis, and then dies off. This keeps a constant supply of fresh, new cells in the tissue. In cancer caused by radiation, the part of the cell cycle which tells the cell to finish its life cycle and die is damaged, and the cells continue to divide uncontrollably. Both non-ionizing and ionizing radiation can cause DNA damage, but ionizing radiation is much more potent, albeit much less commonly in contact with human tissue under normal conditions.
Cellular Death
Environmental health and safety documentation at Princeton University notes that when cell damage to a cell or group of cells in a tissue due to radiation is extensive, the damage can simply kill the cells immediately with no chance for repair. In this situation, there is no opportunity for cellular repair mechanisms to get started, much less to repair any damage and maintain a healthy tissue.
