National University of Singapore

PC4199 - Honours Project in Physics

Quantifying Sources of Indoor Radon in OfficeEnvironments

Sam Koh Boon Kiat

A0140608M

Supervisor: Associate Professor Chung Keng Yeow

Acknowledgements

I am grateful to my Supervisor Associate Professor Chung Keng Yeow for giving me the opportunity to pursue

this Final Year Project, and for guiding me throughout the project. I am also thankful to the staff at Singapore

Nuclear Research and Safety Initiative (SNRSI) for their support in my project. In particular, Mr Chan Yi

Meng for assisting me in data collection for this project.

Contents

1 Introduction 4

2 Radon 5

2.1 Problem of Indoor Radon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Quantifying Radon Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Preliminary Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3 Emanation from Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3.1 Mass Balance Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3.2 Solution of the Differential Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3 Instrumentation 12

3.1 Interior of Instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2 Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.3 Reliability of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4 Results 16

4.1 Initial Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.2 Hard Surface Emission Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3 Bulk Emission Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.3.1 Results with Concrete Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.3.2 Results without Concrete Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.4 Dose of Ionising Radiation Received . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.5 Risk of Lung Cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5 Mitigating Factors and Future Work 35

5.1 Use of Sealant or Paint as a Protective Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

ii

iii

5.2 On Indoor Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.3 Improved Bulk Emission Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.4 Predictions using Formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

6 Conclusion 39

Chapter 1

Introduction

The United Nations Scientific Committee on the Effects of Atomic Radiation reports that over 50% of an

individual’s annual dose to ionising radiation originates from radon. Scientists around the world have conducted

research on the effects of exposure to indoor radon and the source of radon indoors.1 Many studies have reported

that concrete is a major source of indoor radon due to the radium present in the concrete aggregate. A highly

urbanised city like Singapore, sometimes also dubbed the concrete forest, would therefore have an abundance of

indoor radon sources. Despite the health risks that Singaporeans are exposed to on a daily basis, there are not

many studies conducted particularly targeting indoor radon in Singapore.

This report is one of the initial studies conducted on indoor radon in Singapore. In chapter 2, we start with the

generic properties of radon, before discussing the source of indoor radon in Singapore. In chapter 3, we discuss

the instrumentation of the measuring instrument used to measure radon concentrations. Then, in chapter 4 we

present some results based on experiments conducted at various locations in Singapore. Next, we discuss the

possible health effects, and how the numbers obtained in this study affects the average Singaporean. Finally, we

end off in chapter 5 with possible mitigation factors that may be adopted to reduce the concentration of indoor

radon.

4

Chapter 2

Radon

Radon is the sixth heaviest noble gas. Out of the different isotopes of radon, 222Rn has the longest half life of

3.8 days. This is the main isotope that would be of concern to us as it is within the timescale for it to escape

the soil after it is formed from the decay of radium. It is a decay product of the decay chain of Uranium 238,

and since it is unstable, it undergoes further decay in the form of alpha decay.

2.1 Problem of Indoor Radon

Figure 2.1. shows the decay chain of uranium, with radon as an intermediate decay product in the decay chain.

Out of the different intermediate decay products in the decay chain, only radon is in the gaseous state. Radon

present in indoor air can undergo decay, and the daughter nucleus (218 Po) can attach itself to aerosols present.

Aerosols with progenies of radon attached can be inhaled, and be deposited in the lungs. Natural aersols, such

as dust may have a high fraction of it being deposited in the lungs.2 Radon progenies would continue undergoing

a decay while in the lungs and releases alpha particles in the lungs, and biological tissues in the lungs are in the

direct path of the alpha particle. Ordinarily, alpha particles are not penetrating in nature, and sources of alpha

particles outside the body does little damage as the skin provides a sufficient barrier to prevent direct contact

with biological tissues. However, since the alpha decay took place in the lungs, this exposes biological tissues

directly to the highly ionising alpha particle, and it is the proposed mechanism in which radon increases the risk

of lung cancer for individuals. The alpha particle carries with it a large amount of energy, which can increase the

possibility of damage to DNA. It is this highly ionising property that is exploited in Nuclear Medicine therapy,

as it becomes possible to target individual cancer cells.3

The Environmental Protection Agency (EPA) in the United States of America reported that exposure to indoor

5

6

radon is the second leading cause of lung cancer, behind smoking.4 With this statistic in mind, it is therefore

important to quantify, identify, and reduce the sources of indoor radon in a highly urbanised country like

Singapore.

Figure 2.1. Decay Chain of 238U, with 222Rn as an intermediate decay product

In USA, many live in standalone houses, with possibly a basement in their houses. The levels of radon is the

highest at the bottom of the house, where the basement sits. The source of this radon has been traced to the

surrounding soil around the basement. The radon is produced when radium present in the soil undergoes a decay

and releases radon. The radon then enters through cracks on the walls of the basement. In addition, radon is

significantly denser than air. Any source of radon indoors can effectively cause radon to be accumulated in the

lowest point of the dwelling. Hence, while the results of the effects of radon exposure can be applied to the local

context, the source of indoor radon, and possibly, mitigating factors proposed by the EPA may not be applicable

at this point.

In land scarce Singapore, future developments have been planned to be downwards into the earth.5 When

such plans become a reality in the future, the indoor spaces underground will not only have concrete in the

surroundings, but also an abundance of soil. Long-lived nuclei above radon in the decay chain also leads to the

continual source of radon in soil. When terrestrial matter is used in the building materials, this may potentially

increase the amount of radon in such indoor spaces, and hence increase the risks associated with radon exposure.

7

This heightens the importance of a study to quantify the source of indoor radon in Singapore.

Furthermore, radon is a gas that is completely undetectable without the right instrument. It is colourless,

odourless, and chemically inert. Exposure of the general population to radon is therefore difficult to ascertain.

Coupled with the fact that effects on one’s health only sets in in the long term, most would be unaware of the

risks of radon exposure.

2.1.1 Quantifying Radon Concentration

The SI unit that could be used to quantify the concentration of radon in indoor air is Bq m−3 (becquerel per

metre cubed). Other units that are used includes pCi L−1 (pico curies per litre), which is primarily used in the

United States and almost nowhere else. The curies is a historical unit named after Marie and Pierre Curie who

discovered radium in 1898. One curie is the activity from one gram of radium (37 billion decays per second).

Almost everywhere else, including the World Health Organisation (WHO), uses the metric system and measures

radon in Bq m−3. The conversion factor between the two unit systems can then be derived to be

1 pCi L−1 = 3.7× 1010 × 10−12 × 103 = 37 Bq m−3 (2.1)

The action level is the concentration of radon, above which, actions should be taken to actively reduce the

amount of indoor radon. The WHO has recommended the action level to be 100 Bq m−3.6 The EPA, however,

has fixed its action level at 148 Bq m−3, or, in the non conventional units, 4 pCi L−1. Various member states

of the WHO reported having action levels ranging from 100 Bq m−3 to 400 Bq m−3, when surveyed in the

International Radon Project in 2005.7 Most countries also responded that newer buildings have a lower action

level (more stringent requirements) compared to older buildings. Table 2.1. shows the various action levels of

some countries.

Table 2.1: Action Levels of Some Member Countries of the WHO.7

Country For Existing Buildings / Bq m−3 For New Buildings/ Bq m −3

China 1000 (Workplace) 200

Germany 100 100

UK 200 200

USA 148 148

8

2.2 Preliminary Results

A preliminary study was done to measure the level of indoor radon in typical residential and office spaces in

Singapore. The results have been verified, and the trends observed were accountable. Figure 2.2. shows the

variation of radon concentration with time in an enclosed meeting room in an office.

Figure 2.2. Concentration of radon as a function of time in an small meeting room inside

CREATE tower. Shaded out time intervals represents intervals when ventilation system was

switched off.

The preliminary results obtained shows that radon concentration indoors is not a constant, and that it fluctuates

with time. In particular, the trend shows a strong correlation with the ventilation rate of the room. Most studies

conducted have also established outdoor radon to be at very low levels, in the range of 5 - 25 Bq m−3. The graph

is explained by ventilation in the room. Radon accumulates when there is a lack of ventilation, and is removed by

the ventilation system when there is exchange of air with outdoor air, as seen in figure 2.2. Radon concentration

is highly dependent on the ventilation rate of the room. As soon as air was exchanged with outdoor air, the

amount of radon is reduced significantly. Newer, modern buildings built to be more energy efficient are more

well sealed against influx of outdoor air. This reduces cost of air conditioning for offices, and buildings where

the majority of indoor space is air conditioned. However, this also means that natural ventilation and exchanges

9

between indoor and outdoor air is reduced when the air conditioning system is not switched on.1,8–11

Based on the preliminary results, it also suggests that being in the office during office hours when the air

conditioning system is switched on is not a cause of concern. However, the same could not be said during

non-office hours, when the ventilation system is not switched on. The exchange of air is prevented, and radon

is allowed to accumulate in the office environment.

2.3 Emanation from Concrete

Concrete has been identified as the major contributor to indoor radon.12 Concrete is made up of three main

raw materials- cement, aggregate (gravel and sand), and water. The aggregates present in the concrete consists

of gravel and sand in a large proportion. Even though most indoor environments in high rise buildings are

not surrounded by gravel and sand, it is kept inside the concrete and is all around us. The radium present is

primarily responsible for radon being emanated from the surface. This motivates the attempt to describe the

rate of change of radon concentration in a particular room, due to radon emanating from concrete.

2.3.1 Mass Balance Analysis

Consider a source of radon enclosed in a vessel. The rate of change of the concentration of radon (C) may be

attributed to a several factors. These are outlined and shown in the following mass balance equation.

dC

dt=AE

V− λC − q

V(C − Cair)−DC (2.2)

where

• A is the area where radon can emanate, with units of m2,

• E is the emanation rate of radon (Bq m−2 h−1),

• V is the volume of the system, (m3)

• λ is the decay constant of radon (h−1)

• q is exchange (ventilation) rate of air between the vessel and its surroundings, which may be of the form

of leakage (m3 h−1),

• Cair is the concentration of radon in the background (Bq m−3)

• D is the back diffusion coefficient (h−1).

10

The first term represents the contribution of radon from a concrete wall with area A that is exposed for radon

emanation. The second term represents the natural decay of radon. The third term refers to radon removed via

ventilation, or leakage from the vessel. Due to the air exchange, radon present in ambient air (air outside the

vessel) may also be introduced to the vessel. This is accounted for by the fourth term. Finally, the fifth term

represents a back diffusion of radon into the concrete walls.

2.3.2 Solution of the Differential Equation

The differential equation can be rewritten to group terms linear in C together, and terms independent of C

together.

dC

dt= −

(λ+

q

V+D

)C +

AE

V+qCair

V(2.3)

Before solving the differential equation, we can make the assumption that the back diffusion coefficient is

negligible, compared to the other terms.13,14 This value has been determined by other researchers to be much

lower compared to the other terms. With this simplification, the differential equation becomes:

dC

dt= K1 +K2C (2.4)

with

K1 =EA+ qCair

V, K2 = −

(λ+

q

V

)

Using the following integrating factor

exp

(∫−K2 dt

)(2.5)

the differential equation is solved to give the following solution:

C(t) =

[EA+ qCair

V λ+ q

]+

(Cair −

EA+ qCair

V λ+ q

)exp

(−(λ+

q

V

)t)

(2.6)

Equation (2.6) is the main equation that would be used in this research. The results obtained in this project

can be fitted to a curve of the equation described above using MATLAB.

11

If we are just considering the initial rate, then the exponential may be expanded to give the following expression,

which is an expression linear in time.

C(t) ≈ Cair +

[EA+ qCair

V− Cair

(λ+

q

V

)]t (2.7)

Chapter 3

Instrumentation

The field of radon concentration measurement is a highly specialised one. There are companies that provide

radon concentration measurement instruments as part of their huge catalogue of other equipment. The instru-

ment available for this project was obtained from a company specialising in radon concentration measurement.

Durridge is a company based in the United States that manufactures the RAD7 Radon Detector and maintains

them. This is the instrument that was used for this project, and radon concentrations are consistently reported

in Bq m−3.

The RAD7 is an instrument that allows direct sample analysis. It takes in samples of air and detects the

number of alpha particles that arises from the decay of radon and its progenies. Due to the short-lived nature

of radon, it is also not practical to collect a sample of air back for measurement in a laboratory. There are also

other detectors that measures radon concentration. However, such detectors are generally used for long term

monitoring of radon (about 3 months to a year) and not suitable for the purpose of this project.

3.1 Interior of Instrument

The sample holder inside the RAD7 consists of a hemispherical charged shell with volume 0.7 L. The hemisphere

is positively charged, and pushes the positively charged alpha particles and decay products upwards to where a

detector is positioned. The frequency at which the air is sampled from the environment can be set on the RAD7

itself. The RAD7 has an internal pump which takes in air at the specified interval from the surroundings.

12

13

Figure 3.1. Interior of the RAD7.

The hemispherical shell is positively charged so that daughter nuclei produced in the decay of radon will be

repelled upwards towards the detector. Radon progenies are then stuck on the detector, and can undergo further

decay in which the alpha particles are detected. The detection process is not 100% efficient, and the manufacturer

have taken this into account during the calibration process.

The presence of moisture in the interior will adversely affect the efficiency of the detector. As such, a desiccant

is interfaced to the RAD7, in order to dry the air before it enters the chamber. If there is high humidity, it

reduces the efficiency of repulsion of decay products to the detector, which hence reduces the counts of alpha

particles detected. In such an event, the software provided by the manufacturer attempts to correct this, and the

reported radon concentration is corrected to give a higher reading. This is particularly the case over long-term

monitoring, when the desiccant can become depleted.

Just before the air is allowed to enter the RAD7, it is filtered to prevent other particulates from entering the

14

RAD7 to ensure that the counts of alpha particles in the desired energy ranges are solely contributed by the

progenies of radon. This prevents false positives from being recorded in the RAD7, and ensures that counts are

solely due to contribution from radon gas.

3.2 Detection

The RAD7 uses a solid state alpha detector that allows it to perform alpha spectroscopy within the unit itself.

This feature allows the parent nucleus to be identified, and the amount of that parent nucleus can then be

quantified. The part of the decay chain that is relevant for detection within the RAD7 is as follows.

222Rnα−−−−−−→

5.49MeV

218Poα−−−−−−→

6.00MeV

214Pbβ−−→ 214 Bi

β−−→ 214Poα−−−−−−→

7.69MeV214 Pb

As radon decays, the positively charged daughter nuclei are propelled towards the detector. As they undergo

further decay, the alpha particle arising from such decays have a 50% probability of entering the detector and

producing an electrical signal proportional to its energy. After about three hours, when 214Po reaches secular

equilibrium with 222 Rn, this count is also taken into account when computing the concentration of radon.

The end result from a given run is presented in a spectrum, in which a plot of counts of alpha particles at a

given energy is shown against the energy of alpha particles. To facilitate the use of the spectrum provided at

the end of the run, the RAD7 divides the energy axis into different channels, each one depicting a certain range

of energies of alpha particles. Each channel is labelled with the alphabets, and a sample spectrum is shown in

figure 3.2.

Figure 3.2. Sample Alpha Spectrum obtained in a test run.

15

Channel A provides the counts of alpha particles arising from the decay of 218Po, and channel C provides the

counts of alpha particles arising from the decay of 214 Po.

The counts per minute of decay products measured by the RAD7 can then be used to compute the concentration

of radon, which is pre-calibrated by the manufacturer of the RAD7.

3.3 Reliability of Results

Numerous runs were performed using the RAD7, before a second RAD7 was made available for the course of

this project. To ensure that results obtained are independent of which instrument was used, both RAD7s were

switched on simultaneously to measure the radon concentration in the same room, placed slightly less than a

metre apart. The results obtained from this test run are shown below.

Figure 3.3. Data obtained from both RAD7s. The two instruments are placed slightly less than

a metre away from each other, and are sampling the air in the same room.

From the comparison of the data obtained using both RAD7s, it tells us that the measurements obtained are

consistent with each other. The average percentage difference between the two instruments is 7 %.

Chapter 4

Results

Once we are confident the RAD7 is giving us usable data, radon concentrations at various locations, subject to

different conditions can then be measured. In this chapter, we present the initial measurements performed using

the RAD7s and some observations made. Then, we look at the measurements obtained using accessories, and

evaluate if these accessories are fit for its intended purpose.

4.1 Initial Measurements

The initial measurements performed using the RAD7 were mainly for the purpose of familiarising with the

instrument. The initial measurements were conducted at a residential location, and at an office in CREATE

tower. Radon in residential locations appear not to be a problem due to the consistently low values of radon

measured. One of the measurements taken from a residential unit is shown in Figure 4.1. The residential unit is

a HDB unit in Sengkang. During the measurement process, the room was enclosed as tightly as possible. That

is, windows were all closed, and the door was closed for majority of the time during the measurement process.

The gap between the door and the floor was slightly covered during this process.

As this was a preliminary test that was conducted, the settings on the RAD7 were different compared to the other

tests. In this case, each cycle time was 20 minutes. The sample of air is kept in the RAD7’s interior chamber

for 20 minutes before it is refreshed. Nevertheless, the results of this test shows that radon in a residential unit

is relatively low. It is mostly below the action level set by the EPA, and hence, it would not be the focus of this

project.

In this project, at least 5 other runs were conducted with the room less sealed, and the concentration of radon

measured were at or below the levels as shown in Figure 4.1.

16

17

Figure 4.1. Data obtained from initial measurements. This was conducted in a resident unit in

Sengkang.

Additionally, a preliminary study on indoor radon in residential areas in Singapore arrived at the same conclusion,

at other residential units.15 This reinforces the fact that residential radon is not a cause of concern within the

units we have sampled.

The movement of radon after it emanates from concrete walls was also investigated as a preliminary study.

Both RAD7s were placed in adjacent rooms, with the door open. This allows radon to diffuse freely across

the boundary of the two rooms. The two RAD7s were placed approximately 5 metres apart from each other

in an office in CREATE tower. The concentration gradient would drive the diffusion process. The underlying

hypothesis is that diffusion of radon across rooms would be registered, and when the concentration of radon in

one room decreases, the concentration of radon in the adjacent room would increase. The results from this run

is shown in Figure 4.2.

18

Figure 4.2. Concentration of radon placed inside and outside of a meeting room in an office.

The door joining the two adjacent room was kept open during the measurement process.

The results obtained from this run shows that for the 2 rooms system, the concentration of radon does not seem

to differ significantly, and the diffusion coefficient of radon in air is so high that on the time scales at which

measurements are taken, the radon concentration profile is at a steady state. This allows us to proceed with a

zero dimensional assumption in the mass balance analysis equation, and for even smaller systems, we will assume

the concentration of radon to be the same throughout the system.

The variation of radon concentration with time in an enclosed environment was shown earlier in section 2.2-

Preliminary Results. The measurements provides a good and clear gradient during the accumulation of radon

process. This could also be used to determine the emanation rate of concrete from the room, given the dimensions

of the room, and under certain assumptions. The assumptions that we would make are:

• Radon emanates from the 4 vertical walls. Even with furniture placed in directly in front of the wall such

as cupboards, this would not affect the flux of radon into the room.

• There are no other sources of radon in the room apart from the walls.

• Treating the meeting room as a ‘big box’, Cair is taken to be the concentration of radon in the adjacent

19

room, which was shown to be the same as the meeting room itself. This modifies the equation describing

the concentration of radon to be:

C(t) =EA

V λ+

(C(0)− EA

V λ

)exp(−λt) (4.1)

where C(0) is the initial concentration of radon.

Since in the measurement process, only the initial rate was detectable and no saturation concentration was

achieved. This approximates the solution to

C(t) = C(0) +

(C(0)− EA

V λ

)(−λ)t (4.2)

With these assumptions, the curve fitting could be performed, and the emanation rate obtained. By considering

the slopes, and obtaining the physical dimensions of the room, the emanation rates obtained over the course of

three days in the particular room of interest are 21.8 Bq m−2 h−1, 18.7 Bq m−2 h−1, and 25.0 Bq m−2 h−1,

with an average of 21.8 Bq m−2 h−1. Slight variations were expected in the determination of the emanation

rate, given that there are uncertainties in the measurements of radon concentration.

4.2 Hard Surface Emission Chamber

With the initial measurements, and the primary source of radon identified, the hard surface emission chamber

was used to measure the emission from a hard concrete surface. The hard surface emission chamber is also

manufactured by the same manufacturer. It interfaces with the RAD7, and allows for the concentration of

radon in the closed system to be measured. The results obtained was then fitted to the equations in chapter 2,

in order to obtain the empirical constants such as the emanation rate. The hard surface emission chamber was

fixed to the wall using a Caulking cord, also provided by the manufacturer. This seals the hard surface emission

chamber to the wall, and air exchange is kept to a minimum. As the hard surface emission chamber is fixed to

the wall directly, it allows for direct, on-site sample analysis. The chamber can be fixed to the concrete wall and

subsequently removed when the experiment is completed. However, this technique proved to be less than ideal,

as the emission chamber appears to be more suited for a horizontal surface, rather than a vertical wall.

Figure 4.3. shows a schematic of the closed system with the hard surface emission chamber interfaced.

20

Figure 4.3a. Schematic of Hard Surface Emission Chamber attached to a Concrete Wall.

Figure 4.3b. The Hard Surface Emission Chamber used in this project.

The hard surface emission chamber was attached to a concrete wall based in physics department in NUS. The

resulting graph of concentration of radon against time in hours is shown in Figure 4.4.

21

Figure 4.4. Concentration of radon as a function of time obtained by attaching the hard surface

emission chamber to a wall. The chamber is then interfaced to the RAD7.

From the results, it is clear that concrete is a significant contributor of indoor radon. In the absence of a large

ventilation rate to exchange air with the surrounding air, radon can accumulate to a high level, well above the

action level of any country, or the action level set by the WHO. The saturation concentration of radon achieved

in this set up is approximately 1550 Bq m−3.

As the RAD7 measured the concentration of radon in time units of hours, the following calculations will be

done in the corresponding units. For instance, the decay constant of radon will be given as ‘per hour’, with the

numerical value of 0.00756 h−1.

The parameters that needs to be filled into the mass balance equation includes the volume of the setup, the area

of radon emanation as well as the ambient concentration of radon, Cair. The volume of the system is estimated

using values provided by the manufacturer, to be 1520.1 cm3. This was computed by taking into account the

volumes of all apparatus used. The volumes of the hard surface emission chamber, the RAD7, the desiccant, as

well as tubings used in the experiment were taken into account. Next, the area in which radon can be emanated

is computed using the physical values provided by the manufacturer, verified by measurements physically on the

hard surface emission chamber, to be 0.036644 m2. Finally, the last parameter was independently measured by a

second RAD7 running parallel to the one attached to the hard surface emission chamber. The purpose of this is

to measure the concentration of radon in the ambient air, ideally right outside the hard surface emission chamber

22

simultaneous to the RAD7 interfaced to the hard surface emission chamber. To ensure that the concentration

of radon measured in the ambient air is a good representation of the air surrounding the hard surface emission

chamber, the inlet to this RAD7 was placed as close as possible to the hard surface emission chamber. Figure

4.5. shows the background obtained in this run. There are some fluctuations in the values recorded, but the

average value can be taken as the value for Cair. The average value was obtained to be 21.4 Bq m−3.

Table 4.1 summarises the parameters that would be substituted into the curve fitting equation.

Table 4.1: Parameters substituted into Curve Fitting Equation.

Parameter / (units) Value

Volume of system / m3 0.0015201

Area that radon can emanate through / m2 0.036644

Cair / Bq m−3 21.4

Figure 4.5. Background concentration of radon measured by the second RAD7.

The initial rate was first used to obtain the emanation rate. The linear equation (2.7) was used for this purpose.

The first 6 points on the graph was used to fit the linear equation. Using the initial points, the gradient was

determined to have a value of 50 Bq m−3 h−1. This gives an equation for the gradient of the graph.

Gradient =

(Cair −

EA+ qCair

V λ+ q

)(λ+

q

V

)= 50 Bq m−3 h−1 (4.3)

23

This forms one equation. To solve for the unknown parameters E and q, the steady state data has to be used.

This is the saturation concentration of radon as recorded by the RAD7. Due to fluctuations in values recorded

of radon, an average value of 1550 Bq m−3 was adopted as the saturation concentration of radon in this set up.

This forms the second equation.

C(∞) =AE + qCair

V λ+ q≈ 1550 Bq m−3 (4.4)

Solving the two equations simultaneously, the emanation rate and ventilation rate was obtained.

Table 4.2: Parameters obtained using Curve Fitting

Parameter / (units) Value

Emanation rate, E / Bq m−2 h−1 2.1

Ventilation rate, q / m3 h−1 3.8× 10−5

The ventilation rate enters into the equation for the effective decay constant of this set-up. The two terms that

contribute to decay of radon in the system are the natural decay constant, λ, as well as the ventilation of the

system. Based on the curve fit, the following shows the ratio of the decay constant to the removal of radon via

ventilation.

λ

q/V=

0.0076

0.025= 0.304 (4.5)

The ratio implies that the fraction of effective decay constant due to leakage from the system is significant.

The experimental procedure was replicated in a small room in the office of CREATE tower. The hard surface

emission chamber was attached to a concrete floor, and the concentration of radon measured as a function of

time. Ideally, the results from this test would have a lower value of leakage, as it is on a horizontal surface, which

would ensure the system is sealed more tightly. Similar to the previous run in NUS Physics office, the second

RAD7 was measuring the concentration of radon in the ambient air simultaneously. The results obtained from

this is shown in figure 4.6.

24

Figure 4.6. Graph obtained in a small room with hard surface emission chamber attached to

concrete floor. The second RAD7 was monitoring the ambient concentration of radon

simultaneously.

Here, we make the following observations on the trend of data obtained.

• The concentration of radon inside the hard surface emission chamber is consistently higher than the

concentration of radon of the ambient air. This suggests that there is some form of accumulation inside

the chamber, relative to the ambient air.

• There is a correlation in the trend of radon concentration inside the hard surface emission chamber and

the concentration in the ambient air. This suggests that there must have been significant exchange of air

between the interior of the hard surface emission chamber and the ambient air.

The second observation would lead us to the conclusion that the effective decay constant of the hard surface

emission chamber set up in this case is much larger compared to the previous experimental set-up. Due to the

larger effective decay constant, no saturation concentration was achieved. Equilibrium with the ambient air was

attained between measurement times such that each measurement records an already saturated concentration.

This is supported by the observation that there is a strong correlation between the two graphs. This means

that the effective decay constant of this system is so large that the equilibrium, or saturation concentration, is

attained in the order of less than an hour. A closer inspection of the hard surface of concrete studied revealed

that there are cracks on the surface. This would explain the large decay constant observed in this set-up.

25

This is a significant revelation because it might render future tests using the hard surface emission chamber

less valid, and extreme care must be taken to account for such defects in the concrete matrix. For other

tests conducted during this research using the hard surface emission chamber, leakage that was observed could

therefore also have been due to the presence of such inevitable defects in the concrete. Such defects may go

beyond just the surface, and might not be visible. This limits the use of the hard surface emission chamber, and

the use of a bulk emission chamber is proposed to overcome the limitation of cracks present in concrete. The

use of the bulk emission chamber is discussed in the next section.

4.3 Bulk Emission Chamber

Another possible way to measure the emanation rate from a sample of concrete is by means of a bulk emission

chamber. This is a self-made chamber made up of PVC plastic, with two holes drilled into the sides. This allows

the tubes for airflow to be connected to the RAD7, and the concentration of radon in the air of the closed system

to be measured. However, compared to the hard surface emission chamber, this is a more indirect method to

measure the emanation rate from concrete, because a portion of the concrete has to be removed and placed

inside the bulk emission chamber.

Figure 4.7a. Schematic of Bulk Emission Chamber where a cube of concrete is enclosed in it.

26

Figure 4.7b. Bulk Emission Chamber where a cube of concrete is enclosed in it.

This method is considered a destructive method. A sample of the building material is required for this purpose.

In this part, we used the following parameters for the equation. The concentration of radon in the laboratory

is lower compared to the experiment conducted using the hard surface emission chamber. The environment in

the laboratory is always controlled, and the ventilation system is switched on. This promotes the exchange of

indoor and outdoor air, so the concentration of radon is consistently lower compared to the on-site test with the

hard surface emission chamber.

Table 4.3: Parameters substituted into Curve Fitting Equation.

Parameter / (units) Value

Volume of system / m3 0.019304

Area that radon can emanate through / m2 0.06

Cair / Bq m−3 7

There were two main experiments that were performed using the bulk emission chamber. The first involved

a concrete block being placed inside the chamber, and the corresponding radon concentration is measured as

a function of time. The second experiment removes the concrete block from the chamber, and measures how

the concentration of radon will decay with time. Both experiments will be able to determine a value for the

ventilation rate, q. In an ideal situation, the q value obtained from both experiments should agree with each

other.

27

4.3.1 Results with Concrete Block

The concrete block was placed inside the bulk emission chamber, as shown in Figure 4.8. The experiment was

allowed to run for 12 days, in a controlled environment. In the laboratory, the ventilation system is always

switched on, and at no time was the ventilation system being switched off during the measurement procedure.

The data obtained from this gives a significant accumulation of radon in the chamber.

Figure 4.8. Data obtained from a concrete block sealed inside the bulk emission chamber.

By performing a curve-fitting, the equation obtained that describes this curve is

C(t) = 1553.70− 1537.49 exp (−0.009799t) (4.6)

The effective decay constant allows for q to be determined immediately. Subsequently, substitution of the

parameters as shown in table 4.4. yields the following fitted parameters.

Table 4.4: Parameters obtained from curve fitting

Parameter / (units) Value

Emanation rate, E / Bq m−2 h−1 4.88

Ventilation rate, q / m3 h−1 4.244× 10−5

Although there is a range of values for the emanation rate from literature,9,16 the values obtained in this project

are at the same order of magnitude.

28

For the bulk emission chamber, the ratio of the natural decay constant to the decay constant due to ventilation

is larger compared to the ratio obtained from the hard surface emission chamber.

λ

q/V=

0.0076

0.00220≈ 3.5 (4.7)

This implies that the natural decay of radon is more significant compared to the leakage due to ventilation.

Although these two values are at the same order of magnitude, the natural decay term is now larger compared

to the ventilation decay term- an improvement from the hard surface emission chamber. By modifying the fitted

equation such that the ventilation rate, q, becomes zero, an ideal situation can be attained. This allows us to

predict the variation of the concentration of radon in an ideal scenario where there is no leakage from the system.

This is shown in figure 4.9.

Figure 4.9. Comparison of data points obtained vs ideal scenario of no leakage.

4.3.2 Results without Concrete Block

In this experiment, the source of radon was removed by opening the lid, and physically removing the concrete

block before measurements were made. The concentration of radon was allowed to attain a steady state before

the block was removed. This gives a decay curve of radon in the bulk emission chamber, due to both the natural

decay of radon and the ventilation. The curve obtained in this part is shown in Figure 4.10.

29

Figure 4.10. Concentration of radon as a function of time in the bulk emission chamber. The

concentration of radon was allowed to reach saturation levels before the concrete block was

removed.

In this case, the equation to be used for curve fitting was modified slightly due to a differing boundary condition.

Previously, the boundary condition set by the constants of integration assumed the initial radon concentration

to be the same as the ambient concentration, Cair. This is not the case in this experiment. After adjusting for

the modified boundary condition, the following equation was used for the curve fitting instead.

C(t) =qCair

q + V λ+

[C(0)− qCair

q + V λ

]exp

(−(λ+

q

V

)t)

(4.8)

We first examine that the correct limiting cases are obtained.

• The initial concentration is C(0).

• In the long time limit, the limiting concentration is given by

qCair

q + V λ

which, in the limit of large ventilation rate, the limiting concentration is given by Cair, and in the case

where there is no ventilation, all the radon in the bulk emission chamber eventually decays to give a

30

concentration of zero.

Fitting the curve gave the following equation for the experiment conducted without the concrete block in the

bulk emission chamber.

C(t) = 381.1 exp (−0.01028t) + 1.82 (4.9)

With more data points providing the characteristic decay curve, the effective decay constant was first determined,

which allows the value of q to be determined.

Table 4.5: Parameters obtained from curve fitting

Parameter / (units) Value

Ventilation rate, q / m3 h−1 5.154× 10−5

Figure 4.11. Limiting cases together with data points shown.

By adjusting the parameters, we can verify that the decay relation is correct.

Examination of the curves in figure 4.11. we observe that in the absence of any ventilation, i.e. air exchange with

the surrounding ambient air, the decay curve will be less steep, and take a longer time to reach the concentration

of radon in the ambient air. The graph will tend to the value of zero, as all radon present in the chamber will

decay away. The converse is true when the ventilation rate is increased. When the ventilation rate is artificially

31

increased by a factor of 10, the decay slope is much steeper, and the equilibrium with the ambient air is attained

at a much faster time scale. Curves obtained by running the experiments using the same set up will be in

between these two limiting cases. In the data points shown, it can be seen that the leakage is not as significant,

as the curve obtained is closer to the case when q = 0.

4.4 Dose of Ionising Radiation Received

In this section, we compute the dose of ionising radiation received by an individual due to indoor radon. The

generic method to compute the annual dose is in the following equation.

Dose received = EF ×OF × t×D × C (4.10)

where

• EF is the equilibrium factor (assumed to be 0.4 for indoors17),

• OF is the occupation factor,

• t is the total time in a year given in hours,

• D is the dose conversion factor,

• C is the average concentration of radon.

There are two international committees committed to the study of radioisotpes and its effect on public health.

The two oragnisations are the United National Scientific Committee on the Effects of Atomic Radiation (UN-

SCEAR), and the International Commission on Radiological Protection (ICRP). Each organisation published

a value for the dose coefficient for inhaled radon. UNSCEAR published this value to be 9 nSv (Bq m−3 h)−1,

and maintained in 2016 that this value is a realistic and practical value.17 Revised in 2018, the dose coefficient

published by ICRP for inhaled radon takes the value of 6.7 nSv per Bq m−3 h.18

In our calculations, we use the higher value for calculating the dose of ionising radiation received due to indoor

radon. We start by partitioning the general population into two- one that works only during office hours, and the

other that works beyond office hours. An assumption we will be making is that ventilation systems in the office

are switched on only during office hours, and that it does not cater for workers in the office working overtime.

A realistic estimate of the number of hours of a ‘normal’ office worker is 8 hours. The average concentration of

radon in the air inhaled by normal office workers is taken to be the average value during office hours, 9 am to

32

5 pm. This takes a value of 20 Bq m−3. We first compute the dose of ionising radiation received by a normal

office worker.

The occupation factor is computed by accounting the number of hours in the office in a year. In a year with

365 days, assuming that there are 104 (2 × 52) days of the weekend, there are 261 days in the office. With 11

public holidays in a year, this reduces the number of days in the office to be 250 days. From this, the occupation

factor factor can be computed.

OF =8× 250

24× 365= 0.228 (4.11)

The corresponding annual dose of ionising radiation received by a normal office worker is therefore

Dose received = 0.4× 0.228× 8760× 9× 10−6 × 20 = 0.144 mSv (4.12)

This is the dose that a normal office worker would receive under such conditions at the workplace only, and it

is considered on the lower end of the dose received from radon world wide.

For overtime workers, the concentrations of radon in the air they inhale are higher, and also there is a higher

occupation factor. We adjust the occupation factor by making an assumption that they work for 12 hours a day,

say from 9 am to 9 pm. As the dose was received under different conditions, for a more accurate representation,

we consider the following:

Total dose = Dose received during office hours + Dose received during overtime hours (4.13)

In terms of equation 4.10, this is expressed as

Total dose = 0.4× 8760× 9× 10−6(OF (normal)× C(normal) +OF (overtime)× C(overtime)) (4.14)

And with the earlier assumptions, the occupation factor due to overtime work is

OF (overtime) =3× 250

24× 365= 0.0856 (4.15)

The average concentration of radon after the ventilation systems are switched off is higher, but as it has not

accumulated significantly, this value was measured to be 30 Bq m−3. The total dose an overtime worker received

from inhaled radon is therefore given by

33

Total dose = 0.4× 8760× 9× 10−6((0.228× 20) + (0.0856× 30)) = 0.225 mSv (4.16)

Table 4.6. Summarises this set of results for the dose received.

Table 4.6: Annual dose received by normal office workers and overtime office workers.

Annual dose due to inhaled radon / mSv

Normal office worker 0.144

Overtime office worker 0.225

For workers who return to office on a weekend, with even higher radon concentrations, the dose would be

increased further. Depending on the nature of work, the number of weekends a worker would return to office

differs greatly. We will make a reasonable assumption that an overtime worker would return to his workplace

for 5 hours per weekend for one year. The average radon concentration over a particular weekend in the office

sampled was 240 Bq m−3. The annual effective dose received by this worker due to the additional time in office

over the weekend is:

Annual dose received due time in office over weekends = 0.4× 5× 52

24× 365× 8760× 9× 10−6 × 240 = 0.225 mSv

(4.17)

4.5 Risk of Lung Cancer

Health effects of ionising radiation are well known and studied at high doses. The effect of low dose ionising

radiation of 100 mSv and below, however, is not well understood. Low dose ionising radiation can be defined

to be doses of 100 mSv and below. One such model to predict the effects of low dose ionising radiation is

known as the Linear No Threshold model (LNT model). In this model, effects of high dose ionising radiation

are extrapolated to predict the effects of low dose ionising radiation. There is debate on the validity of the LNT

model. The EPA have adopted the LNT model in their research to claim that lung cancer is the second leading

cause of lung cancer in the USA.19 Adopting this model used by the EPA, some calculations will be performed

based on the data obtained in this project.

In the report by ICRP publication 103 published in 2007, with an assumption of the LNT model, the combined

detriment due to excess cancer and heritable effects remains unchanged at around 5% per Sv.20 Translating

this into the probability of cases per million population, it corresponds to a probability of 50 cancer cases per

34

million population per milli-Sievert. Singapore has a population of 5.612 million as of 2017, and we assume that

2 million Singaporeans work in indoor offices. Based on the dose received due to inhaled radon and its progenies

by normal workers, this gives an occurrence of 58 lung cancer cases per year in the workplace.

Number of excess cases of lung cancer = 0.144 mSv× 2× 106 × 50× 10−6mSv−1 ≈ 14 (4.18)

In comparison, the Singapore Cancer Registry’s Annual Registry Report on the Trends in Cancer Incidents in

Singapore for the years 2009 to 2013 published in 2015 reported that the total number of lung cancer cases over

this 5 year period was 6612.21 On average, this amounts to 1322 cases per year.

The number calculated is solely due to exposure to indoor radon by a normal office worker, and the value used

is an internationally recognised value. In reality, the number may vary as no statistics on the actual number of

office workers in Singapore is available, and the conversion factor may be disproved by new research methodology.

Chapter 5

Mitigating Factors and Future Work

Is indoor radon therefore inevitable? Concrete, being essential in the construction of buildings, will continue to

emanate radon. Due to the presence of long-living parents of radon, concrete can continue to emanate radon for

a long time. Some possible methods to reduce the level of indoor radon are proposed.

5.1 Use of Sealant or Paint as a Protective Barrier

Figure 5.1. Measured radon concentration from hard surface emission chamber attached to

laboratory floor.

In one of the experiments to measure the emission of radon from a concrete floor coated with epoxy, the readings

obtained were not within expectations. No significant accumulation was observed. The results obtained in this

35

36

experiment is shown in figure 5.1.

The location chosen for this experiment was a laboratory in CREATE tower, that is underground. Figure 5.2.

shows the on site location of the laboratory.

Figure 5.2. Laboratory where hard surface emission chamber was fixed to the floor to measure

emanation from concrete floor.

The phenomenon of no accumulation was attributed to the presence of a sealant applied over the floor. Due to

the nature of laboratory work, the floor had to be additionally protected in the event of any chemical spills. This

layer of protection, however, acts as a diffusion barrier to radon, which explains why there was no accumulation

of radon in the hard surface emission chamber. Physically, due to this barrier, the increased diffusion constant

increases the amount of time needed for radon to diffuse through the floor. The result is that a greater proportion

of radon would decay before it is replenished with fresh radon from the concrete floor.

In indoor locations with elevated concentrations of radon, similar sealants could be used to prevent the emanation

of radon from concrete into the indoor environment, effectively preventing the build up of radon in a poorly

ventilated area.

37

For office spaces, the use of such sealants might not be feasible. It has also been proposed that wall paints may

be used to mitigate the amount of radon indoors.22 Future work includes investigating the effects of the thickness

of wall paint on the concentration of radon, and how different types of paint may affect the concentration of

radon indoors. Due to the limitation of the hard surface emission chamber as discussed earlier, it would be

more feasible to employ the use of the bulk emission chamber for this purpose. Concrete blocks or slabs may be

used, and the effect of the thickness of paint on the emanation rate of radon from the concrete can be studied

effectively. However, we also note that the best measurement would be a direct measurement of the vertical

surface of a wall on-site, compared to an indirect measurement of a sample of building material. Until the

limitations due to the defects present in concrete is effectively taken into account, the hard surface emission

chamber is not feasible to measure the emanation rate from concrete walls.

5.2 On Indoor Spaces

In order to give a better statement on the total dose one would receive from ionising radiation from radon and its

progenies, more indoor spaces should be sampled so as to obtain a better estimate on the average concentration

of radon one is exposed to. There is expected to be large variations in the concentrations of radon measured,

since radon concentration is highly dependent on the amount of radium present in the concrete. For example,

more offices could be sampled in order to obtain a more representative average value of radon concentration.

More studies on the risk association with indoor for Singaporeans should also be done. With more studies

performed, a more accurate value for the risk of lung cancer associated with indoor radon can be determined.

In this study, the value was adopted from studies performed overseas, and is one that is used internationally.

As shown in the preliminary results, areas with high ventilation rate shows lower concentrations of indoor radon.

Areas with significant build-up of radon should be ventilated actively when occupied by personnel. This will

reduce the concentration of radon present in that indoor environment. For buildings in the future, it may be

beneficial if there could be openings to allow air exchange with the outdoor air when the ventilation systems are

not switched on.

5.3 Improved Bulk Emission Chamber

In this research, there were limitations of the equipment set-up available. For instance, when allowing radon

to saturate in the bulk emission chamber and subsequently removing the concrete block, the concrete block

was removed by opening the lid of the box, and physically removing the block from the chamber. Although

radon is significantly denser than air, this process of removing the concrete block have reduced the saturation

38

concentration by more than half. In an ideal situation, a box containing the concrete block should be separately

constructed, and connected to the bulk emission chamber via tubings available. A pump may then be used

to circulate the air between both housings, so that the concentration is uniform in both. When saturation

concentration is attained, the box containing the concrete block can be detached from the bulk emission chamber

with minimal loss of radon. The tubes are connected to the bulk emission chamber via valves, which will only

open when a tube is connected. This process will be able to achieve a higher starting concentration for the

monitoring of radon concentration without a source.

Additionally, a ‘blank’ bulk emission chamber should be allowed to run so as to ascertain that there is no

contribution of radon from the bulk emission chamber. Although it is highly unlikely that the PVC box will

contribute a significant amount of radon, this should ideally be verified.

5.4 Predictions using Formula

A possible future work for this research topic would be to verify the equation that describes the concentration

of radon, scaled up to a room. Currently, the model has been assumed to work for a larger scale room. This

should be verified by actual measurements on-site. For instance, an initial measurement of a wall at the on-site

location could be done using the hard surface emission chamber, and the emanation rate of radon determined.

Subsequently, the location should be sealed and radon allowed to be accumulated. The emanation rate of radon

could then be determined once again using the whole room monitoring, and then compared to the emanation

rate obtained using the hard surface emission chamber.

Chapter 6

Conclusion

In summary, this project has employed a method to quantify sources of indoor radon. The emanation rates of

radon from concrete was measured on-site using a hard surface emission chamber, and the emanation rate of

radon from a concrete block was measured in the laboratory using a bulk emission chamber. The emanation

rate from a concrete block was measured to be 4.88 Bq m−2 h−1, and the emanation rate from two walls 2.1

Bq m−2, and 21.8 Bq m−2 h−1 respectively. The values obtained were compared to literature values, and agree

in the order of magnitude. Of the two methods, the bulk emission chamber provides greater control over the

experimental parameters, as the environment in the lab is controlled and the background concentration of radon

is generally low. However, this method is more destructive in nature. The more accurate measure is still to

measure the emanation rate on-site, although there is less control over the experimental parameters. The dose

to individuals due to indoor radon at the workplace was then computed, and the excess risk for lung cancer was

computed using values proposed by ICRP.

39

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