Radiation has unique characteristics in terms of travel and penetrating through other substances. The ability to penetrate varies between radiation types (Fig. 2.1). Alpha and beta particles only travel for a very limited distance. Alpha particles do not penetrate much (travel only a few centimeters in the air) and can be blocked with a single sheet of paper. Beta particles are more penetrating than alpha particles; however, they can be blocked with a few millimeters thick of aluminum sheet. Although they have relatively weak penetrating features, alpha and beta radiations give high amounts of energy to tissues resulting in serious damage if incorporated into the body.

On the other hand, gamma rays are very penetrative and easily travel through thin lead aprons commonly used for protection from X-ray energies used in medical imaging. In Fukushima (especially at the damaged nuclear power plant), there are places with extremely high gamma radiation levels which gamma rays accounted for. A thick concrete block or lead wall is required to block gamma rays. Neutrons are extremely penetrating; they can only be stopped by thick masses of concrete, water, or paraffin. In addition, neutrons have a unique feature of inducing radioactivity when absorbed by stable materials. Neutrons are produced mostly in a specific condition called nuclear fission which takes place inside nuclear reactors in operation or in a nuclear detonation. In the JCO Co. Ltd. Tokai Plant accident, three workers were exposed to very high levels of neutrons which were produced in a nuclear fission induced while they were dissolving U₃O₈ (18.8 % enriched) and pouring nitrate solution into a precipitation vessel using a funnel. External exposure was the major health problem (IAEA 2000).

The toxicity of a radioactive material depends on the type of radiation emitted, physical and biological half-life, target organs, and toxic natures of the substance itself. Toxicity of radiation is modified by linear energy transfer (LET), which is a measure of how much energy is transferred from ionizing radiation to tissue. Alpha particles and neutrons have high LET, whereas gamma rays and X-rays have low LET. For example, ¹³¹I has a short half-life of 8 days. However, ¹³¹I has a strong affinity to the thyroid gland, staying in the tissue while emitting beta particles which may result in later development of cancer. Another example is ¹³⁷Cs that has a relatively long half-life of 30 years; however, the biological half-life is 9 days in small children and approximately 109 days for adults. Cesium acts like potassium, spreading throughout the body after incorporation and is excreted into urine. Plutonium-239 (²³⁹Pu) inhaled into the lung moves to the bone and liver, resulting in damage to these organs through the effects of alpha particles over many years. The effects of radiation are broadly divided into two types: external exposure and contamination (Fig. 2.2).