Radiation is energy that transmits in the form of wave or light-speed particles. It can be divided into non-ionizing radiation and ionizing radiation according to its energy.
Non-ionizing radiation is low energy radiation that cannot produces ionization, e.g. solar ray, light, infrared, microwave, radio waves and radar waves etc.
Ionizing radiation is high energy radiation that can produces ionization.
The term radiation generally refers to ionizing radiation. Gamma ray belongs to ionizing radiation. It has high penetrable ability and can only be obstructed by thick cement wall or lead plate.
The source of radiation can be divided into natural and man-made source.
Natural sources are those that exist in the nature, include Radon in the atmosphere, Uranium and Thorium in soil and rock, cosmic ray etc.
Medical source is the main man-made sources, e.g. X-ray and radiation treatment etc, others are from dust of explosion of nuclear weapons, nuclear reactors and daily living such as television etc.
Statistic information shows that 70% of the radiations received by human were from nature sources while 30% were from man-made sources. Only less than 0.1% of radiation comes from nuclear reactors.
The health effects of radiation can be divided into deterministic effect and stochastic effect.
When the dose of radiation receives by human body exceeds certain amount, the following symptoms such as tiredness, nausea, vomit, erythema, psilosis, decrease of lymph corpuscle in blood etc may occur. If one receives a higher dose, the symptoms may become more severe or even death. This effect is known as deterministic effect. Deterministic effect occurs only if the dose received exceeds a certain level.
Furthermore, the International Commission on Radiological Protection (ICRP) has proposed the following conservative assumption due to safety precaution:
Any amount of radiation, no matter how small, may have possibility to induce cancer and cause genetic effects. The possibility is proportional to the dose received. This effect is called stochastic effect.
The Macao Meteorological and Geophysical Bureau (SMG) measures the absorbed radiation dose of Gamma ray i.e. the average absorbed energy of ionizing radiation per unit mass. Its SI unit is Gray (Gy) and defined as follow:
1 Gy = 1 J / kg
Dose rate means the average absorbed radiation dose per hour. Its SI units are Gy/h, mGy/h or µGy/h.
According to the European standard, the dose rate below 0.35 µGy/h is considered as normal.