During the early years after the discovery of X-rays by W. C. Roentgen in 1895 and radioactivity by H. Becquerel in 1896, we became acquainted with ionizing radiation and became aware of its harmful effects.

All living organisms are made of the fundamental unit of life, i.e., the CELL. Cell nucleus contains the chromosomes made up of Deoxyribonucleic Acid (DNA) and other basic proteins. The DNA molecule, which is the most radiosensitive, constitutes the genetic material of the cell and it is most important target molecule for the radiation induced damage in the cell.

When the ionizing radiation passes through living tissues, causes ionization, which eventually affect molecules that are the building blocks of tissues and organs in the body. These biological effects, in cellular or tissue level, can be detrimental to health. Such effects depend on the type of radiation, and the total amount of the energy deposited per unit mass of the tissue –radiation dose, and the rate at which the dose is delivered.

The biological damage in cells either by directly ionizing the DNA molecule or indirectly by producing chemically toxic species, such as hydrogen peroxide or free radicals by radiolysis of water molecule. The radiation induced damage may manifest in the breaks in the DNA molecule, called chromosome aberrations. It is found that the aberrations increase with dose. The increase in the frequency of the chromosome aberrations, mainly of dicentric type, in the human lymphocytes (white blood cells) is used as a biological dosimeter to determine the exposure of persons exposed higher radiation doses of the order of 100 mSv can be detected by this method. However, this magnitude of exposure is not normally received by the radiation workers.

The biological effects can be classified as: stochastic effect and deterministic effect (or tissue reactions). In stochastic effect, the chance or the probability of the effect occurring is statistical in nature. The examples are cancer and genetic effects. In the deterministic effect, group of cells or tissues is extensively damaged due to exposure to high radiation doses, the biological effects may appear with in a few hours to a few weeks after exposure to ionizing radiation. The effect has a threshold and it manifests only if the radiation dose received is above the threshold level. Cataract of the eye is an example of such effect. Human conceptus (embryo/fetus) during the initial pregnancy period has been shown to more radiosensitive to radiation. Hence, radiation exposure of pregnant women is restricted to the level of exposure limit to the members of public.

Radiation sickness refers to the early response to high levels of radiation exposures (acute exposures - high doses in short time) in excess of 1 Gy or 1 Sv. Acute radiation exposures of the whole body to doses over 2 Gy can cause death as a result of severe damage to blood forming organs. About 50% of the exposed people may die within the first 60 days for a dose of 3 – 5 Gy. This is called the Lethal Dose 50/60 (LD50/60).

Health effects at exposure levels above 100 mSv dose has been well established. The risk of health effects at lower dose levels are estimated based on linear extrapolation from experimental and epidemiological data obtained at higher doses. Stochastic risk is estimated by linear extrapolation without threshold, with certain uncertainties in the estimation due to biological effects from the background radiation itself, and due to intake of other pollutants. The risk factor is the estimated stochastic risk per unit sievert of dose received. Deterministic effects do have a threshold, and the effect increases with the dose. The effect can be clinically detectable.