In densely populated cities such as Hong Kong where people live and work in high-rise buildings that are all built with concrete, the indoor gamma dose rate and indoor radon concentration are not wide ranging. Indoor gamma dose rates (including cosmic rays) follow a normal distribution with an arithmetic mean of 0.22 +/- 0.04 microGy h(-1), whereas indoor radon concentrations follow a log-normal distribution with geometric means of 48 +/- 2 Bq m(-3) and 90 +/- 2 Bq m(-3) for the two main categories of buildings: residential and non-residential. Since different occupations result in different occupancy in different categories of buildings, the annual total dose [indoor and outdoor radon effective dose + indoor and outdoor gamma absorbed dose (including cosmic ray)] to the population in Hong Kong was estimated based on the number of people for each occupation; the occupancy of each occupation; indoor radon concentration distribution and indoor gamma dose rate distribution for each category of buildings; outdoor radon concentration and gamma dose rate; and indoor and outdoor cosmic ray dose rates. The result shows that the annual doses for every occupation follow a log-normal distribution. This is expected since the total dose is dominated by radon effective dose, which has a log-normal distribution. The annual dose to the population of Hong Kong is characterized by a log-normal distribution with a geometric mean of 2.4 mSv and a geometric standard deviation of 1.3 mSv.

Unlike detached or semi-detached houses, the main source of indoor radon in high-rise buildings is the building material. Radon released from the building material will be removed by ventilation, either forced or natural, so that its concentration, its progeny's concentration, and the equilibrium between the two will be different for different types of buildings and environmental factors. A number of surveys were carried out in buildings throughout the territory to look at the seasonal variation of indoor 222Rn levels; the dependence of indoor 222Rn concentration on the living environment; the indoor gamma dose rate and its relation to indoor 222Rn concentration; and the dependence of 222Rn progeny concentration and equilibrium factor on the environment in high-rise buildings.

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