From the perspective of radiation protection, effective dose is used to assess the impact of ionizing radiation on human body, with the unit being Sievert (Sv). Effective dose cannot be measured directly, but is calculated through the following concepts:

When ionizing radiation passes through a substance, its energy is absorbed, which is defined as the absorbed dose. The unit is Gray (Gy), equivalent to joules per kilogram (J/kg). Absorbed dose can be measured directly by instruments. For example, in the Observatory's current radiation monitoring network, high-pressure ionization chambers are used to measure the dose rate of environmental gamma radiation absorbed by air.

Different types of ionizing radiation produce different levels of impact on human body, even when the absorbed dose is the same. To use calorie as an analogy: fat can provide more energy than protein of the same weight. Similarly, alpha particles have a greater impact on human organs or tissues than gamma radiation under the same absorbed dose. To assess the radiation impact on individual organs or tissues due to different radiation types, different radiation weighting factors (Table 1) have to be considered. The equivalent dose is calculated by multiplying the absorbed dose by its relevant radiation weighting factor.

When multiple organs or tissues are irradiated simultaneously, individual organs or tissues have different levels of sensitivity to radiation-induced biological effects. Therefore, different tissue weighting factors (Table 2) have to be considered to evaluate the total impact on a human body. The effective dose is the sum of the products of equivalent doses of all irradiated organs or tissues and the respective weighting factors. This value quantifies the overall health risk of ionizing radiation to a human body.

When a patient undergoes a medical radiation exposure, for example X-ray or Computed Tomography (CT) scan, professionals can calculate precisely the effective dose from the type and intensity of the radiation used, as well as the irradiated organs and tissues to evaluate the associated risk. While the natural background radiation is present everywhere and influenced by environmental factors, scientists can still estimate the dose due to the natural background radiation using hypothetical models by analyzing various exposure pathways and people’s living habits. On global average, for an individual in a year, around 80% of received dose is contributed by the natural background radiation, with the remaining mainly attributed to the medical sources^[2].