switched on only after office hours. The remaining time there is only low levels of natural air exchange with most windows closed. Differences in ventilation patterns could also be the reason for the observed mean radon activity concentration value over 100% higher in offices than in homes, the former using predominantely closed air-condition system.
Another potentialy important parameter is the age of the building. In dwellings constructed between 1985 and 1990 the mean radon activity concentration is more than 400 % higher than in constructions built prior to 1980. The working hypothesis is that this is caused by the increasing use of industrial waste material (e.g. ash from wal-find power-plants) as construction material.
No correlation was found between indoor radon values and location of the
site investigated inside the building.
5.2. LEVELS
A pilot survey of the gamma dose rate indoors and outdoors (81 sites) reveiled that the average annual effective dose equals 1.40 mSv, i.e. 200 % above the equivalent value for China (P.R.).
The weighted mean radon exhalation rate (10 sites) from the most common concrete wallsand covering materials in Hong Kong is approximately 16 mBq/m2,s. Individual values covered a wide range (from below limit of detection to 60 mBq/m2,s) and are generally higher in new buildings than in old buildings. This mean value is higher than the corresponding value for China by 500 % respectively higher by 800 % compared to the global representative value (UNSCEAR).
Several pilot radon surveys (typically 100-200 measurements per survey) have been conducted, using short-term and long-term integrating radon detection devices. These studies were made during spring, summer or winter months, respectively. Either charcoal canisters were exposed for several days, or electret- based or track-etch detectors for several months. The latter method, indicative of long-term statistically significant average values, reveiled a summer-mean value of 72 Bq/m3 and a winter-mean of 155 Bq/m3. The maximum individual radon activity concentration measured was 1460 Bq/m3 (store room). In a survey covering 99 indoor- and 15 outdoor sites almost 60 % of the radon measurments were higher than 150 Bq/m3 and 30 % above 300 Bq/m3.
A pilot survey was carried out on radon- and thoron decay product levels indoors (1 grab sample each at 13 sites). Results are indicative of the potentially significant contribution by thoron decay products, since the ratio of annual
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