Radon Mitigation in a Typical Underground Car Park in Hong Kong

Experiments were conducted to investigate the equivalent dose to the lung (lung dose) due to radon in an underground car park with four levels. Size distribution of aerosols and activity size distribution of radon progeny loaded aerosols from 9 nm to 10 µm were measured and their relations to car movement and emission were investigated. Unattached fraction, equilibrium factor and PAEC of radon progeny were monitored and were used to predict the lung dose by adopting the ICRP Human Respiratory Tract Model. The mean annual (i.e. 2000 h) lung doses at the four levels, B1, B2, B3 and B4 during daytime were 1.5, 2.4, 2.0 and 2.8 mSv respectively. The lung dose for each car park level at night were 2 times higher. Mitigation methods were investigated and it was found that increasing the ventilation rate only resulted in a few per cent reduction in the lung dose rate during daytime though a larger reduction in PAEC (50% at b1 and 38% at B4) was observed at night. Cleaning the fresh air filters and washing the car park floor with water also resulted in a 21% and 12% reduction in the lung dose rate respectively during daytime.