The radiation hazards to human beings arise from
exposure of the body to external radiation
ingestion or inhalation of radioactive matter
The effect of radiation depends on the nature of the radiation and the dose received.
Alpha: The hazard from alpha particles is slight (unless the source enters the body) since they cannot penetrate the outer layers of the skin. (If the source enters the body, it can be fatal as well. See: http://en.wikipedia.org/wiki/Poisoning_of_Alexander_Litvinenko)
Beta: Beta particles are in general more penetrating than alpha particles. Most of their energy is absorbed by surface tissues and a few milimetres of aluminium is sufficient to provide adequate protection.
Gamma: Gamma rays are highly penetrating. They can penetrate deeply into the body and may require a few centimetres of lead or concrete shielding.
Radiation can cause immediate severe damage to body tissue such as radiation burns. Delayed effects such as cancer and eye cataracts may appear many years later. Hereditary defects may occurs in succeeding generations due to genetic damage.
When radiation passes through living tissues, it can damage the structure of molecules leading to the malfunction or death of living cells. Some cells can recover, others cannot and the effects on tissues are cumulative.
Chromosomes are particularly sensitive to ionising radiation at the moment of cell division resulting in genetic mutations which are probably harmful. Such genetic mutations can lead to birth defects if an unborn child and/or the mother is exposed to radiation.