Three forms of radioactivity are known: alpha radiation, originating from relatively big particles, which can only travel a few centimetres and are stopped by the skin. Only when they are inhaled or ingested do they get stored in the body and irradiate the surrounding tissue. Alexander Litvinenko (remember?) was poisoned by alpha radiation, which cannot be detected by normal survey instruments.
Beta radiation is caused by small particles, which can travel a few meters and can penetrate into the deeper layers of the skin. Ingestion and inhalation is problematic. Those types of radiation can remain in the body lifelong, depending on their half-life and lack of excretion.
Gamma radiation is caused by electromagnetic waves, which can travel many meters and penetrate many materials. They are thus a danger to the body, mainly as an outside source of radioactivity. Gamma radiation can accompany alpha and beta radiation to a lesser degree. X-rays are similar to gamma radiation.
The way radiation causes damage to the body is through the ionization of atoms. This removes electrons from the atoms that make up the molecules of the tissue. The removal of the electrons break the bond between the atoms and thus, the molecule inside the cell falls apart, causing damage. This is the basis for understanding the effect of radiation.
When radiation interacts with cells, it may or may not strike an important part of the cell, the chromosomes certainly being the most critical part, since they contain the genetic information and instructions for the cell on how to function. Damage can often be repaired by the cell, but the repair may be incomplete and the cell functions incorrectly or may then be damaging to other cells. These cells may be unable to reproduce themselves or may reproduce at an uncontrolled rate, thus causing cancer. If a cell is extensively damaged by radiation it will die.
Radiation damage to cells depends on how sensitive the cells are to radiation. In general, cells which divide rapidly (blood, skin, digestive system, cancers) are more sensitive than cells that divide slowly (brain cells). The effects also depend on how much and how fast radiation is received.
Acute damage is due to high levels of radiation over a short period of time and as it affects the rapidly dividing cells, you expect internal bleeding, severe infections, diarrhoea, vomiting, abdominal pains, hair loss and later infertility. Chronic damage is usually low-level radiation over a long period of time. It is considered a lesser risk, because the repair mechanisms of the body have time to do their work.
The assessment of the magnitude of that type of damage is difficult, because it can take decades for the damage to occur. It results in certain types of cancer, cataracts and to a lesser extent, genetic mutations in the following generation.
Embryos are likely to suffer from growth or mental retardation and later from childhood cancers. Medicine uses the acute effects of high radioactivity for cure and modern technology now has been able to deliver it so exactly, that damage to healthy tissue is being kept to a minimum. Diagnostically a weak, very shor-lived radioactive substance, which only accumulates in certain organs, is applied, taking advantage of radioactivity being visualized similarly like x-rays.