Lion Rock is a landmark of the skyline of Hong Kong and also a symbol of resilience for Hong Kong people. But to scientists, this iconic peak tells another story—one written in atoms. This story involves environmental radiation: the ionizing radiation naturally present in our environment.

While we head to the hills for fresh air and exercise, most of us never expect the rocks beneath our feet to play a role in our environmental radiation levels. About 35% of the territory of Hong Kong (including Lion Rock) is composed of granite [Figure 1a], an igneous rock formed from cooled magma deep underground [Figure 1b]. Granite is naturally rich in elements like uranium, thorium, and potassium. These elements contain radioactive isotopes; as they decay over millions of years, they release energy in a decay chain, for example:

Uranium-238 → … → Radium-226 → Radon-222 (Gas) → … → Lead-206 (Stable)

Different types of ionizing radiation are released during the decay process. Alpha and beta particles have weaker penetrating power and are easily blocked by a few centimetres of rock; therefore, most of their energy is absorbed within the granite. In contrast, high-energy gamma rays can travel through the bedrock (i.e., solid rock beneath the soil) to the ground surface and air, contributing to the environmental gamma radiation levels. A unique intermediate step also occurs where the solid element radium-226 decays into gaseous radon-222 (radon), which can seep through microscopic cracks in the stone and drift into the air. Radon and its progeny also emit alpha and beta particles as they decay, and can pose a health risk if excessive amounts are inhaled. Proper ventilation in enclosed spaces (such as basements or rooms that remain closed for long periods) to avoid the accumulation of radon is thus recommended.

While the environmental radiation levels range from 0.06–0.3 microsievert per hour (μSv/h) in Hong Kong, the levels can vary significantly from place to place, depending on the geological environment. For example, "Black Sands” in Guarapari of Brazil are composed of high concentrations of heavy minerals, namely monazite ^[3]. The radiation levels on these beaches can reach 10–30 μSv/h [Figure 2].

While geology defines the foundation of our city, it is only one ingredient in our background radiation environment. In addition to ground irradiation and radon, our total exposure also includes cosmic radiation from outer space to the Earth, as well as trace isotopes found naturally in food and water from dietary intake. The Observatory has maintained a comprehensive Environmental Radiation Monitoring Programme to closely monitor the variations in environmental radiation levels of Hong Kong. The results are published regularly for the public’s reference.