Dose and Sievert

The effect of radiation on any material is determined by the "dose" of radiation that material receives. To put all ionising radiation on an equal basis with regard to the potential for causing harm, a quantity known as effective dose is introduced. The effective dose is a single number broadly representing the risk to health taking into consideration the susceptibility to harm of different tissues and weighted for the harmfulness of different types of radiation. The effective dose is frequently abbreviated to dose. The unit of dose is sievert (Sv). This is a very large unit and submultiples, in particular, the millisievert (mSv -- one thousandth of a sievert) are commonly used.

The annual dose received by a member of the public in Hong Kong from natural background radiation is about 2 mSv. A radiation worker may legally receive up to 20 mSv per year. The effective dose received in a typical chest X-ray examination is about 0.05 mSv.