From readings taken at various locations in Hong Kong during the period 1987-2009, the background ambient gamma radiation levels may vary between 0.06 and 0.30 microgray per hour.

The extent of health effects to a person as a result of exposure to radiation is expressed by a physical quantity called equivalent dose, in unit of sievert.   The health effect of ambient gamma radiation (usually expressed in microsievert or one millionth of a sievert) is numerically equal to the ambient gamma dose expressed in microgray.

Protective action may be required if there is an increase in the ambient gamma dose rate above background such that the total additional dose is expected to exceed 5000 microsieverts in a relatively short period of time.

For example, a dose of 5000 microsieverts may be reached when the ambient gamma dose rate is 1000 micrograys per hour (several thousand times above background) and lasts for 5 hours.