Radon is a naturally occurring, odourless, colourless, radioactive gas produced by radioactive decay of radium-226.  Radium is naturally found in most soils and rocks, particularly in granite.  As radon gas further decays, a series of tiny radioactive particles are formed.  When either the gas or these particles are breathed into the lungs, some are deposited and will continue to emit radiation.  Life-long exposures to high levels of radon can cause lung cancer.

Why indoor radon level is potentially higher in Hong Kong?

Granite is widely used in concrete for building construction in Hong Kong. Significant levels of radon may be found in any floor of any building using concrete as a major building material.  As the radioactive half-life (the time taken to reduce the activity of a radioactive isotope to one-half of its original activity) of radium is about 1600 years, the level of radon contributed by building materials containing radium in a building is likely to be constant throughout the building’s lifetime and is expected to be about the same in old buildings as in new buildings.

How can we reduce radon exposure at home?

The simplest way to reduce indoor radon exposure is to improve ventilation.  At home, residents are recommended to open windows regularly to facilitate natural ventilation.  There is a common misconception about switching on the ventilation valve of the window-type air-conditioning unit and thinking that there is sufficient amount of fresh air coming into the room.  While it may help a little, the amount of fresh air drawn through the air vent of the window-type air-conditioner may not be adequate to reduce the radon accumulated when all the windows are shut for better air-conditioning.  Residents are advised to keep some windows open slightly while running the air-conditioner or keep windows open when the air-conditioner is not operating. 

How is radon managed on campus?

In offices, control of indoor radon relies on mechanical means.  The ventilation system on campus is so designed that fresh air is drawn through the Air Handling Unit (AHU) on top of each building zone and supplied to the fan-coil units located on each floor.  Different from the AHU, the main function of the fan-coil unit is to provide finer temperature regulation and better local circulation of indoor air in offices.  Depending on the location and layout of the floor plan, one fan-coil unit may serve one or more offices at the same time.  Turning on the fan-coil unit will not speed up the removal of radon unless the unit receives enough fresh air supply from the AHU.

All AHUs are controlled by a central, computerized system which operates on a fixed schedule whereas the fan-coil unit can be controlled, to a certain extent, by the control switch which occupants may find in their offices.  The control switch regulates primarily the speed of the fan of the fan-coil unit, and thus allows localized control of air circulation and temperature regulation.  However, the control switch is only functional when the fan-coil unit is activated.  The operating hour of the fan-coil unit is regulated by preset schedule as well.  The operating schedule of the fan-coil unit does not necessarily follow the same schedule that controls the AHU operation.  For majority of the offices, the AHU system starts 1 to 2 hours earlier than the fan-coil system, which helps to reduce radon accumulated over the night to a more acceptable level.  This is also where, from time to time, confusion had occurred as some occupants thought that the ventilation system was turned on only at about 8:15 am when they first heard the fan of the fan-coil units in their offices start operating.  In fact, the AHU has been turned on much earlier without the occupants really noticing it.

If I feel cold in my room, does it mean I am getting plentiful supply of fresh air?

Not necessarily.  As explained previously, the fan-coil unit is responsible for temperature control in an office but it does not have much influence on the amount of fresh air supplied to the office.  The fresh air that is needed to dilute radon in the office is supplied by the AHU which operates independently.

How is the operating schedule of the AHU determined?

The AHU system is designed to provide sufficient fresh air to satisfy the occupants’ biological needs as well as to maintain optimal indoor air comfort to the office population during regular office hours.  Depending on the size of the AHU, population and the activities in the zone, some AHUs may operate for longer operating hours than the others.  However, they all satisfy the performance target established by the Working Group for achieving an average radon level of 150 Bq/m³ or below during the hours from 8:00 am to 10:00 pm, from Monday to Saturday.  At the same time, other performance parameters such as fresh air input requirements, temperature and humidity controls, carbon dioxide level management, etc. are achieved.

As the ventilation system is not designed for operating 24 hours nonstop, some degree of radon accumulation is expected when the ventilation system is shut down. In the office environment, accumulation of radon usually rises to a peak level in the middle of the night and returns to a low level when the ventilation system starts operating again in the morning.  As the ventilation system in the office area is completely shut down on Sundays, higher radon levels are expected in offices on Sunday as compared to the weekdays.

Should I open windows to reduce radon level in office?

The office blocks are serviced with centralized air-conditioning.  To achieve optimal effectiveness, the office blocks should be maintained as an enclosed compartment. Open widows in offices will upset the air balance within the office block which in turn, will affect the effectiveness of the system and the indoor air quality of the office block as a whole.  Moreover, open widows in offices may introduce warm, moist air into offices and may cause condensation problems when the outside air mixes with the cool indoor air especially in a hot, humid summer day.  From previous experiences, indoor condensation may lead to indoor biological contamination problems, causing some acute health concerns which may include hypersensitivity pneumonitis, chronic fatigue syndrome, etc.

If all the offices are receiving centralized air from the main system, how is it possible that the radon level recorded in one office differs significantly from another office down the corridor?

The difference in level of radon recorded is a reflection of how much fresh air is delivered to a given office as compared to others.  AHU supplies fresh air to offices through a labyrinth of air ducts.  The amount of fresh air received on each floor and office is regulated by control valves located at various positions along the pipe duct.  The degree of opening of the control valve determines how much fresh air an office is receiving.  Balancing of the fresh air supply by tuning the degree of opening of the fresh air control valve usually helps to reduce the difference in amount of fresh air received in each room, thus reduces the difference in radon level detected.  The amount of time an office door is opened can also affect radon level.  There is also a tendency for offices located at the distant end of the air ducts receiving lesser amount of fresh air than those located at the front end partly due to loss of air momentum as well as imbalance of fresh air received in each room.  Adjustment of the fresh air distribution will be carried out by EMO colleagues as needed.