Understanding Radiation

Health Effects of Ionising Radiation on People

Measuring Radiation Dose

The health effect of ionising radiation can vary with the type and energy of the radiation, duration of exposure and bodily tissue that is radiated. The effect is referred to as effective dose and measured in a unit called the Sievert (Sv). Since one Sievert is a large quantity, radiation doses normally encountered are expressed in milli-Sievert (mSv) or micro-Sievert (µSv) which are one-thousandth or one millionth of a Sievert, respectively. For example, one chest X-ray will give about 0.05 to 0.1 mSv of radiation dose. The risk of exposure to radiation can also be substantially lower if the radiation dose is low and/or is delivered over a long period of time. Therefore, it is often useful to express the rate at which the dose is delivered (dose rate), such as µSv per hour or mSv per year. The International Atomic Energy Agency (IAEA) limit for public exposure to ionising radiation is 1 mSv per year, excluding what a person normally receives form natural background radiation.

Exposure Pathway

A person can be exposed to radiation by external or internal means. External exposure occurs when the radioactive source is outside of the body. X-rays and gamma rays can pass through your body, depositing energy as they go. Internal exposure occurs when radioactive materials gets inside the body by ingestion (eating, drinking or breathing) or injection (medical procedures). This may pose a serious threat to health if significant quantities of the radioactive material enter the body.

Radiation Sickness

When a human body receives a high radiation dose of more than 1 Sv over a short duration, acute radiation effects such as nausea, vomiting, fatigue and loss of hair may occur. This is commonly known as “radiation sickness”. When exposed to a whole body radiation dose of above 10 Sv, death is likely even with medical treatment. It takes a very high radiation exposure to cause radiation sickness, equivalent to getting 18,000 chest x-rays distributed over your entire body in a short period. 

Present evidence from scientific studies shows that the damaging effects of ionising radiation are greatly reduced when the radiation dose is delivered in small amounts and spread over a long time period. This is because there is a greater likelihood of the body repairing the damage. However, even exposure to low levels of radiation can increase the risk of developing long-term effects such as cancer over a lifetime. This is known as stochastic effect.

Common Sources of Radiation and Effective Dose

A chart depicting the levels of ionising radiation exposure is appended below.

Effective Radiation Dose
For more in-depth information on the sources and effects of ionising radiation exposure, please refer to the following reports published by the international expert community on ionising radiation:
a) 2008 United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) Report
b) 2007 International Commision on Radiological Protection (ICRP) Report