Radon Monitoring in Tourist Caves in South-West Western Australia

Bill Chandler, Western Radiation, & Peter Bell


As a result of reports in 'New Scientist' and 'The West Australian' newspaper, the operators of the tourist caves in the Augusta/Margaret River and Yallingup/Busselton regions of Western Australia decided to commission a study of radon gas and its decay products in their most popular tourist caves. The caves were first 'screened' for a 2-week period and follow-up monitoring was carried out in those caves that might be considered to present a possible hazard to the health of tourist guides or other cave workers.

The draft guidelines for radon exposure produced by the International Commission for Radiological Protection(ICRP) (Ref 1) were used as a basis for further monitoring. Being aware of Occupational Safety & Health legislation which requires that employers must exercise due care and diligence for the safety and health of their employees, the operators reported the results of the screening tests to the Western Australian Department of Health (Radiological Council) and requested advice on the health implications for their employees.

The Radiological Council (Ref 2) recommended monitoring and action levels as recommended by the ICRP be adopted by the operators of the tourist caves as an interim measure until the National Health & Medical Research Council's Australian Ionising Radiation Committee (AIRAC) considers the ICRP recommendations. Western Radiation Services were commissioned by the cave operators to carry out the studies required to provide sufficient information for assessment of employees radiation exposure and to recommend a method for future monitoring that would be both accurate and cost effective.

Monitoring Programme

An initial screening programme using E-PERMTM radon monitors was carried out at the Jewel and Mammoth Caves operated by the Augusta/Margaret River Tourist Bureau (A/MRTB) and at the Yallingup Cave operated by the Busselton Tourist Bureau (BTB). The Moondyne Cave (A/MRTB) was re-opened during the testing period and included in the monitoring programme.

Those caverns with average Radon concentrations greater than 400 Becquerels per cubic metre (Bq/m3) were monitored for monthly averages using E-PERMTM monitors and for Radon progeny using both grab sampling techniques and semi-continuous monitors. The grab sampling methods used were those of Kusnetz (Ref 3) and Tsivoglou (Ref 4) with the Rolle (Ref 5) method being used for semi-continuous monitoring.

At the request of the Radiological Council (Ref 2) of Western Australia the monitoring was extended to include Thoron (Radon-220) and its progeny. Thoron was monitored using E-PERMTM ion chambers with a permeation time of approximately 30 minutes to measure Radon-222 only and a rapid response ion chamber to measure the combined Radon and Thoron. Thoron progeny was measured using the method of Rock.

Fluctuations in gas and progeny concentrations were observed to occur diurnally and these were measured by operating the semi-continuous progeny monitor (Thomson & Nielsen Model TN-IR-21) for 24 hour periods in each of the caves of interest.

The initial programme was run from October 1992 to April 1993 and will be modified to operate on a seasonal basis for on-going dose assessment purposes.


The results of the screening tests are shown in Table 1 below.

Table 1: Results of Radon Screening Tests

Elec Ser. No.LocationRn Conc (Bq/cu.m)
$19204Jewel Cave Entrance673
$19373Jewel Cave Organ Pipe736
$19378Jewel Cave Camel Cavern802
$19387Moondyne Snow Flake340
$J0029Moondyne End Chamber524
ExposureJewel Cave 26-10-92 to 9-11-92
Moondyne 27-10-92 to 10-11-92
Elec Ser. No.LocationRn Conc (Bq/cu.m)
$19205Yallingup Cave Top Office448
$19407Yallingup Cave Main Chamber810
Exposure29-11-92 to 12-12-92 

Monthly monitoring and radon progeny checks were carried out and the results are shown below in Table 2 & 3.

Table 2: Radon and Radon Progeny Concentrations

LocationRadon Conc. (Bq/cu.m)Progeny Conc. (mWL)F
Jewel Camel Cavern75227.5K29.6T0.15
Jewel Organ Pipe729110K116T0.58
Yallingup Top Office4516.6K8.1T0.06
Yallingup Main Chamber1054101K102T0.36

Note 1:  K = Kusnetz, T = Tsivoglou (modified)
Note 2: 1 mWL = 1 milli Working Level = 20.8 x 10-9J/ m3
Note 3:  F = calculated equilibrium factor.

Table 3: Radon and Radon Progeny Concentrations

LocationRadon Conc. (Bq/cu.m)Progeny Conc. (mWL)F
Jewel Camel Cavern58369.2K69.2T0.44
Organ Pipe1540140K141T0.34
Flat Roof Entrance 159K173T 
Narrows 64K67T 
Moondyne Lower Chamber52457K54T0.38
Moondyne Snowflake34039.4K36.8T0.40
Walkthrough 79K  

Note: K = Kusnetz, T = Tsivoglou (modified)

Further monitoring to determine the contribution of Radon-220 (Thoron) to the measured radon concentrations was carried out at the request of the Radiological Council of W.A. The results are shown below in Table 4

Table 4: Radon & Radon Progeny Concentrations - Yallingup Cave

LocationRadon Conc. (Bq/cu.m)Progeny Conc. (mWL)F
River Bed Crawl1270118K119T0.35
Pinch Gut Sand Flat1410103K107T0.28
Main Chamber1130R + Th950R55K60T0.16

Note 1:  K = Kusnetz, T = Tsivoglou (modified)
Note 2:  R = Radon-222; Th = Radon-220 (Thoron)
Note 3:  Equilibrium factor F is based on total radon concentrations

Table 5: Radon & Radon Progeny Concentrations - Moondyne Cave 20-3-93 - Testing using Thomson & Nielsen Monitor and Manual Sampling

TimeLocationRolleKusnetzTsivoglouRock (Th)
1140End Cham64   
1200End Cham73   
1430End Cham65   
1500End Cham689711911
1620Snowflake 30308

Note: The 1500 measurement for Thoron was based on the filter used for the Rolle count. The relatively high Kusnetz and Tsivoglou readings for the same time and location may have been caused by the higher volume manual sampling pump operating in close proximity to the T&N instrument.

Radon & Radon Progeny Concentrations - Jewel Cave 21-3-93 - Testing using Thomson & Nielsen Monitor and Manual Sampling

TimeLocationRolleKusnetzTsivoglouRock (Th)
0910Camel Cave22   
0940Camel Cave2932.235.78
1010Camel Cave23   
1030Camel Cave272628.510
1110Organ Pipe90899511
1130Organ Pipe84   
1200Organ Pipe54   
1315Organ Pipe29   
1530Organ Pipe54  8
1400Flat Roof 911008
1440Narrows 65746

The data from Tables 5 and 6 are shown graphically in Figure 1. It is noticeable that the data from the Moondyne Cave on the previous day closely follows the variations measured at the Jewel Cave Organ Pipe between 1100 and 1530.

The 24 hour measurements taken in the Jewel and Yallingup caves are shown graphically in Figures 2 & 3. The variations with time can be clearly seen and amply demonstrate that spot measurements are not sufficiently accurate for hazard assessment. It is also noticeable that each of the caves has an individual pattern of variation in the radon progeny concentrations. Since the variations occur during normal operating hours of the caves, it is suggested that an overall mean of the radon progeny concentrations be related to the long-term mean of the radon concentrations to provide an average equilibrium factor that can be used for personnel exposure calculations.


The screening tests showed that the Jewel, Moondyne and Yallingup caves were within the range of radon gas concentrations that require further monitoring based on the draft recommendations of the ICRP. These recommendations were aimed at workplaces and dwellings and suggested that an equilibrium factor (F) of 0.4 would be acceptable for such locations. Information on cave atmospheres from the USA suggests that F may be greater than 0.4 during periods of very low air exchange rates.

Data from Jewel and Yallingup caves showed that the F varied with time of measurement, so 24-hour monitoring for radon progeny was carried out in both caves to determine the variation pattern and to obtain an average F that could be used for personnel exposure calculations.

Table 7: Yallingup Cave Radon Progeny

12:30117 00:00157
13:0092 00:30135
13:3099 01:00170
14:0080 01:30160
14:3079 02:00166
15:0070 02:30160
15:3064 03:00153
16:0074 03:30165
16:3080 04:00150
17:0077 04:30163
17:3069 05:00144
18:0074 05:30123
18:3084 06:00114
19:0080 06:30129
19:3057 07:00162
20:0063 07:30151
20:3070 08:00103
21:0071 08:30118
21:3080 09:0092
22:0096 09:30104
22:30105 10:00124
23:00184 10:30118
23:30147 11:0092

Average: 112

Table 8: Jewel Cave Radon Progeny

14:3032 02:30101
15:0051 03:0059
15:30112 03:3075
16:00132 04:0081
16:30151 04:3069
17:00128 05:0060
17:30125 05:30151
18:0099 06:0062
18:30127 06:30111
19:00117 07:0000
19:30100 07:3080
20:00108 08:0099
20:30106 08:3084
21:00100 09:0077
21:30102 09:3084
22:00106 10:0001
22:30124 10:3092
23:00111 11:0080
23:3080 11:3072
24:00105 12:0092
00:3093 12:3056
01:0097 13:0044
01:3080 13:30440
02:00110 14:0042


Using average radon concentrations of 1100 Bq/m3 for the Jewel Cave and 1130 Bq/m3 for Yallingup we obtain an F factor of 0.32 and 0.37 respectively. This factor appears to be reasonable when compared to earlier spot measurements.

The graph of the data for the Yallingup Cave (Fig. 3) shows a clear day/night breathing pattern for the cave, with the radon progeny concentrations reaching a peak at 11:00pm and gradually reducing to a low plateau at about 2.30pm the following day. The pattern for the Jewel Cave (Fig.2) shows two distinct peaks at 5:00am and 5:00pm with the low level plateau being of much shorter duration. The major difference between the caves is that Jewel is apparently linked to at least two other caverns (Easter and Moondyne caves) whereas Yallingup appears to be a single cave system.

Short term monitoring at the Jewel and Moondyne caves (Fig. 1) seems to show a similar pattern for radon progeny concentrations in the Camel section of Jewel and the Moondyne cavern with a time offset, but this will need to be confirmed by further 24 hour measurements at both locations. If (as believed) the caves are connected then air movement would occur between the caverns as well as to and from the surface. This would give rise to multiple peaks and troughs as seen on Fig.2. It can be seen that although many caves are similar in appearance and appear to be similar in atmosphere, the air movement in the caves can be extremely complex, necessitating that monitoring for radon gas and progeny be individually carried out for each cavern. This is true even when a cavern is part of a multiple cave system.

The health hazard associated with exposure to radon gas has been determined by the ICRP and the units used here (mWL) can be related to breathing rate of personnel and time of exposure to arrive at a Committed Effective Dose Equivalent (CEDE) per annum. The current limits for personnel not classified as radiation workers are set at 1 mSv/a for 50 years exposure. Where personnel are not full-time long-term employees, the lifetime exposure of 50 mSv can be apportioned at a maximum of 5 mSv/a for up to 10 years.

Assuming a working year of 2000 hours and continuous exposure during working hours a radon progeny concentration of 8 mWL would reach the limit of 1 mSv/a and 40 mWL would reach 5 mSv/a. The actual figure for maximum exposure would be lower than 40 mWL since there is also a gamma radiation exposure associated with the decay of radon that will need to be added to the radon progeny CEDE to obtain total exposure.

In order to comply with Occupational Health and Ionising Radiation exposure regulations it will be necessary for some cave operators to take action to minimise the exposure of personnel. Since it is not a viable option (in most cases) to ventilate efficiently the caves the most effective method of exposure control is to limit the time spent in the caves by employees. Record keeping of employee working hours becomes very important when it is necessary to monitor their exposure levels, with careful recording of actual times spent in the various locations.

Once the parameters for a particular cave have been determined for daily, seasonal and annual concentrations of radon gas and the equilibrium factors for radon progeny, it should be acceptable for operators to monitor only radon gas on a seasonal average basis and apply the equilibrium factor to obtain an exposure assessment for their personnel. This would be the most economic method of ongoing monitoring in those caves where it is required. The ICRP recommend that monitoring should be carried out where radon levels exceed 400 Bq/m3 and action should be taken where the levels exceed 1000 Bq/m3.


1.   Protection Against Radon in Buildings and Workplaces: Report by an ICRP Task Group: August 1992

2.   Letter from Radiological Council of W.A. to Cave Operators. March 1993

3.   Solomon S. et al: Monitoring of Airborne Radioactivity (radon, thoron and daughters: radioactive dusts) (1987) Ch. 5 of "Course Notes for Radiation Safety Officers" Australian Radiation Laboratories, Melbourne

4.   Leach V.A. and Lokan K.H.: Monitoring Employee Exposure to Radon and its Daughters in Uranium Mines. 1979.ARL/TR011 Australian Radiation Laboratories, Melbourne

5.   Rolle R.: Rapid Working Level Monitoring. Health Physics, Volume 22, pp 233-238. Pergamon Press March 1972

6.   Rock R.L.: Sampling Mine Atmospheres for Potential Alpha Energy due to the Presence of Radon-220 (Thoron) Daughters. US Dept. of Interior, MESA, Denver Colorado

7.   Yarborough K.A.: Alpha Radiation in Natural Caves. Radiation Hazards in Mining Ch.97. Kingsport Press, Kingsport Tennessee 1981

8.   Carson, B.C.: Summary and Findings of the Radon Daughter Monitoring Programme at Mammoth Cave National Park, Kentucky. Radiation Hazards in Mining Ch.98. Kingsport Press, Kingsport, Tennessee


The authors wish to acknowledge the assistance and co-operation of all the personnel at the Augusta/Margaret River and Busselton Tourist Bureaus and to thank Mr Keith Tritton and Mr Barry Brown for permission to publish this paper.