Uncertainties in the evaluation of the dose coming from radon in
tourist caves
Carlos Sainz Fernández*, Ismael Fuente Merino, Luis Quindós López, Jose Luis
Gutierrez Villanueva, Jose Luis Arteche García, Luis Santiago Quindós Poncela
Department of Medical Physics, RADON Group
Faculty of Medicine, University of Cantabria
c/ Cardenal Herrera Oria s/n, 39011, Santander, Cantabria, Spain
*Corresponding author: e-mail: sainzc@unican.es
Abstract
Indoor radon poses a health risk confirmed by a variety of studies. High concentrations
of this gas can be found in closed enclosures depending on several factors like activity of source
term, permeability of the source/indoor air inter-phase, as well as the interchange rate of
external air. Low ventilation rates together with the presence of 226-Ra in rocks, make the
tourist caves a place where medium and high radon concentrations can typically be found. With
the incorporation of EURATOM basic standards for radiological protection, into the national
European legislations radon have been recognised as a health risk to be controlled in
workplaces. The transfer of EURATOM standards to the Spanish legislation came out on Title
VII (BOE 178/2001) where radiation coming from natural sources has an analogous role as
radiation emitted from artificial ones.
The most significant exposure to radon in show caves is received by tourist guides who
work providing tours for the general public. The implementation of an efficient radiological
protection system for this kind of workplaces must take into account that in some circumstances
forced ventilation may alter the humidity inside the cave affecting some of the formations or
paintings. For this reason the best option to protect workers from radon is a system based on
limitation of exposure by restricting the amount of time spent in the cave. For doing so, the
knowledge of main uncertainty sources in dose evaluation is of main importance.
The principal variability sources in dose assessment are the integration intervals for
radon measurements and the aerosol conditions related with the unattached fraction of radon
progeny. The first factor takes into account the daily and monthly variations in radon
concentrations and can not be fully addressed in caves with the seasonal correction factors
proposed for the determination of radon doses in houses. On the other hand, mainly due to the
low particle concentration usually present inside the caves, the unattached fraction of radon
progeny can be higher than in other workplaces. This factor is significant when dose assessment
is approached from the dosimetric respiratory track model of ICRP.
In this work the results of radon measurements carried out monthly in different points
of 7 caves located in the region of Cantabria (Spain) as well as estimations of the dose received
by workers approached from different integration intervals are presented. Also the results
concerning particle concentration and its relationship with unattached progeny fraction together
with their implications in dosimetric calculations are discussed for some of the caves.

KEYWORDS: radon, dose, tourist caves

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1. Introduction
When we breathe in air with a concentration of radon and radon progeny, several
alpha radioactive decays take place inside the lung. This irradiation is responsible of
about half of the annual average effective dose received by the human due to natural
sources of radiation. [1]. Outdoor radon does not represent a significant health hazard
because high concentrations are never reached. However, it becomes a problem when
released into a closed or poorly ventilated enclosures like dwellings, buildings and also
caves and mines. Indoor concentrations of radon and its short-lived progeny depend on
several factors mainly related with the entry or production rate from various sources and
the ventilation rate.
Many workplaces both above and below ground may be affected by high radon
concentrations. On its 1990 publication, the International Commission of Radiological
Protection, ICRP recommended that exposure to high radon levels should be considered
as occupational exposure and remedial actions have to be taken in such situations [2].
Concerning the Spanish situation regarding radiation coming from natural sources, in
2001 the Spanish law incorporated EURATOM basic standards for radiological
protection, which include a request at the EC Member States to determine the working
places on which exposure to natural radiation is significant. On Title VII (BOE
178/2001) radiation coming from natural sources plays the same role than radiation
emitted from artificial sources.
Tourist caves represent a type of workplace with particular environmental
conditions that might be affected by high radon concentrations [3, 10]. In these places in
which guides host visits for the general public, typical remedial actions like forced
ventilation, sealing or reducing pressure in the source rock can not be used for
conservation reasons. For example, forced ventilation could alter the humidity inside the
cave thus affecting the paintings or geological formations that attract the tourists. So in
most of the cases the only way to reduce radon exposure to guides and other workers is
to apply a radiation protection system based on restrictions in the amount of time spent
in the cave.
The information needed for carrying out the abovementioned protecting actions
is related with the specific characteristics of the cave concerning the behaviour of radon
and it decay products. In order to perform a precise effective dose calculation, factors
like unattached progeny fraction (fp), equilibrium factor (F) and particle concentration
(Z) are of main importance. One of the specific characteristic of the caves is the high
unattached fraction due to a particle concentration far below from the usual value in
dwellings. The fp values can be higher than 0.1, for places with low ventilation and
without additional aerosol sources, with Z < 4 103 particle cm-3 .
In the present work, the results of radon measurements carried out monthly in
different points of 7 caves located in the region of Cantabria (Spain), showed in Fig. 1,
as well as estimations of the dose received by workers and general public are presented.
Radon exposure values are calculated from different measurement integration intervals
in order to stress its importance as a source of variability in dose assessment.
Additionally the results concerning particle concentration and its relationship with

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unattached progeny fraction together with their implications in dosimetric calculations
are discussed.

2. Material and methods
Radon measurements were carried out within 7 caves located in the region of
Cantabria in the North of Spain (Fig. 1). Radon detectors were exposed monthly along
2008 and placed inside of each cave following criteria related with most probable risk
situations for workers. The analysed points were those in which guides usually spent
longer periods giving explanation to the public.
CR39 track-etched detectors were used for integrating measurements. Every CR39 detector was fastened under the cap of a cylindrical polypropylene container 55 mm
high and 35 mm diameter which prevents radon decay products and also 220Rn from
entering. Then, only alpha particles from radon that has diffused into the container and
from the polonium produced inside can strike the detector. After the exposure time an
etching process is performed, and the radon concentration determined by counting the
tracks in a given area. Accuracy and precision of this method has been tested in a
National intercomparison exercise [11], as well as in a recent European intercomparison
with good results (not yet published).
Also particle concentration was also measured by means of a condensation
particle counter (CPC ISI 3007). In this device, air is pumped at a rate of 100 cm3 min-1
and passed throughout a porous wick containing liquid isopropyl alcohol. After the
exposure of the sample to the alcohol vapour, particles grow by condensation and can
be detected optically with a laser light and a detection unit. With this system particle
concentrations in the range of 0 to 500,000 particle cm-3 can be detected.
Mean annual effective doses coming from radon inhalation have been estimated
by using ICRP65 dose assessment methodology [4]. The dose conversion factor (DCF)
used for radon exposure was 5 mSv per WLM at work and 4 mSv per WLM for the
general public, and the dose assessment is usually performed by assuming an
equilibrium factor of 0.4 and indoor occupancy 2000 hours per year. In the special case
of workers in tourist caves it must be taken into account that the occupational time is
significantly lower than 2000 hours per year. For this reason, doses have been estimated
from the real distribution of time spent by tourist guides inside each cave. On the other
hand, the singular atmospheric conditions characterized by high indoor humidity and
very low particle concentrations makes more appropriate the use of an equilibrium
factor of 0.6.
From an alternative approach, the effective dose can be determined using the
respiratory track model of ICRP 66 [5]. For doing so, the measurement of unattached
fraction fp is essential. The dependence of the fp as a function of particle concentration Z
can be approximated by the semi-empirical equation [6]:
fp = 400/Z (cm-3)

(Eq. 1)

on the model of ICRP 66, DCFu for inhalation of the unattached short-lived
radon progeny in mSv per WLM can be calculated from equation:

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DCFu = 8.4 + 64 * fp

(Eq. 2)

3. Results
3.1 Radon concentration
It is known that radon concentration shows monthly variations in caves [8,12].
As an illustration, Fig. 2 represents the variation in radon levels at different sectors
inside the cave of Las Monedas. As it can be seen, seasonal variations are clearer at
specific sectors more influenced by external changes in meteorological parameters. In
each cave, from the set of values obtained monthly at different points, the annual
average radon concentration has been calculated. In order to assess the influence of the
integration interval in the estimation of the annual effective dose, average radon
concentrations were calculated from three and six month measurements.
In Table 1, a comparison of three-month integrated values with the annual mean
is showed. Taking as a reference value 1000 Bq m-3, which is the action level for radon
concentration in workplaces established by IAEA in 1996 [7] it can be observed that all
the studied caves present some points above this value, although all the annual means
are below. It can be found significant relative differences as high as 90 % with the
annual mean. In some cases, when the measurements are carried out in summer period,
the annual exposition to radon can be overestimated by factors up to 2. On the other
hand, mean values coming from winter or spring seasons can lead to an underestimation
of similar magnitude. Fig. 3 illustrates the abovementioned differences.
As it would be expected, integrated values obtained from six-month
measurements fit better to the monthly averaged annual radon concentration, as it can be
seen in Fig. 4. However, also in this approach, overestimations up to 75 % can occur
when the measurements are carried out during the second half of the year, and
underestimations up to 400 % can be observed when the measurements are performed
during the first half of the year.
3.1 Dose assessment
According to the above mentioned ICRP65 methodology and to the information
available about the real time spent by each tourist guide at different sectors of the
studied caves, the annual effective dose was estimated. As it can be seen in Table 2,
only three workers received doses above the general public’s limit of 1 mSv per year. In
the same way, Table 3 shows the dose coming from radon inhalation received by public
in one typical visit for each cave.
From ICRP’s human respiratory model point of view, the differences on aerosol
conditions can modify the dose conversion factors. For the most usual aerosol
conditions in homes of fp = 0.08 and equilibrium factor of 0.4, a DCF of 14 mSv per
WLM has been obtained by Marsh et al. [9]. This DCF can significantly increase in
caves, where particle concentration is very low and subsequently values of fp as high as
0.8 can be found. The uncertainties in the calculation of DCF can be high using the

326

dosimetric model because it involves the use of parameters like weighting factors for
alpha particles and lung tissues which are difficult to determine accurately. In spite of
this consideration, the great differences observed between the DCF’s obtained from
both models show the main relevance on unattached fraction of radon progeny in the
dose calculations.
In the present work, particle concentration was measured monthly at different
sectors inside the caves. Table 4 shows a summary of values together with the
corresponding dose conversion factors for the unattached fraction (DCFu) according to
dosimetric model. Particle concentrations as low as 100 cm-3 were found in some points,
which mean DCFu’s from 3 to 8 times higher than that obtained from epidemiological
approach. This would mean that in some work cases, radiological protection actions
should be taken because doses up to 6 mSv per year could be possible.

4. Conclusions
With this study the annual mean radon concentration have been determined from
monthly measurements at different points of several tourist caves. The results indicate
that for an accurate dose assessment for workers under realistic situations the most
detailed knowledge of the indoor radon levels evolution is needed. Approximations
made from usual longer integration intervals as 3 or 6 months can lead to over or
underestimations of the dose received by people working in the caves.
On the other hand, the extremely low particle concentrations found inside the
caves can lead to higher doses coming from radon progeny inhalation than those
received by people in workplaces with similar radon levels. Finally, in some cases
particle concentrations lower than 400 cm-3 have been found which would mean an
unattached fraction higher than 1. Maybe the relationship used in Eq. 1 can be
questioned under special environmental conditions like those present in some tourist
caves. This issue is now under analysis with new measurements of the unattached
fraction in the studied caves.

327

5. References
[1] United Nations Scientific Committee on the effects of Atomic Radiation UNSCEAR
2000 Report to the General Assembly with Annexes, New York, Vol. I: Sources, United
Nations Publication, Sales No. E.00.IX.4. New York (2000)
[2] Commission Recommendation of 21 February 1990 on the protection of the public
against indoor exposure to radon. (90/143/EURATOM)
[3] L. S. Quindós, P. Fernandez, C. Sainz, J. Gómez, Radon exposure in uranium
mining industry vs. exposure in tourist caves, Rad. Prot. Dos., 111-1, 2004, 1-4
[4] International Commission on Radiological Protection. Protection against radon-222
at home and at work. Oxford: Pergamon Press; ICRP Publication 65; Annals of ICRP
23 (2). 1993
[5] International Commission on Radiological Protection. Human respiratory track
model for radiological protection. ICRP, vol. 66. Oxford: Pergamon, 1994
[6] Porstendorfer J, Reineking A, Radon: characteristics in air and dose conversion
factors, Health Phys. Vol.76, 3, 1999, 300-305
[7] International Basic Safety Standards for Protection against Ionizing Radiation and
for the Safety of Radiation Sources, Safety Series No. 115, IAEA, Vienna (1996)
[8] Fernández P.L; Gutierrez I; Quindós L; Soto J,. Natural ventilation of the paintings
room in the Altamira cave. Nature, 321-6070, 1986, 586-588
[9] Marsh J, Birchall A, Butterweck G, Dorrian M, Huet C, Ortega X, Reineking A,
Tymen G, Schuler Ch, Vargas A, Vessu G and Wendt J. Uncertainty analysis of the
weighted equivalent lung dose per unit exposure to radon progeny in the home. Radiat.
Prot. Dosim. 102 (3), 2002, 229-248
[10] Radiation Protection against radon in workplaces other than mines. Safety Reports
Series No 33, IAEA, Vienna (2003)
[11] National Intercomparison Campaign. Spanish Nuclear Safety Coujncil, CSN,
Colección Documentos I+D. 12. 2004
[12] Lario J., Sanchez-Moral S., Cañaveras C., Cuezva S., Soler V., Radon continuous
monitoring in Altamira Cave (northern Spain) to assess user’s annual effective dose. J.
Env. Rad. 80, 2005, 161-174

Captions of figures and tables
Figure 1: Location of the studied caves in the Cantabria region in Spain

328

Figure 2: Continuous radon measurements in the Castillo cave during a 10 days period.
Figure 3: Continuous radon measurements in the Monedas cave during a 10 days period
Table 1: Integrated radon concentration and mean annual effective dose in several
places inside each cave.

Figure 1

329

Figure 2

330

Seasonal Comparison
1600

1400

1200
ANNUAL

Bq m-3

1000

AUTUMM
WINTER

800

SPRING
SUMMER

600

400

200

0

El Castillo

Las Monedas

Hornos de la
Peña

El Pendo

Covalanas

Cullalvera

Chufín

Cave

Figure 3
Semestral vs. Annual
1200

1000

800

Bq m-3

ANNUAL
1st HALF
600
2nd HALF

400

200

0

El Castillo

Las Monedas

Hornos de la
Peña

El Pendo

Covalanas

Cullalvera

Chufín

Cave

Figure 4

331

MEAN RADON CONCENTRATION (Bq m-3)
CAVE
ANNUAL

AUTUMN

WINTER

SPRING

SUMMER

El Castillo

571 (40- 1600)

643

500

534

595

Las Monedas

775 (170-1700)

804

863

995

438

Hornos de la Peña

539 (80-5300)

817

121

123

1096

El Pendo

707 (80-1700)

337

342

683

1465

Covalanas

350 (25-1110)

332

500

413

157

Cullalvera

397 (20-1600)

564

60

281

684

Chufín

179 (25-530)

256

121

88

250

Table 1

332

Workers
Tourist Guides

Dose (mSv y-1)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28

0.85
0.05
0.38
0.02
0.37
0.66
0.28
0.47
0.31
0.61
0.79
0.01
1.12
0.52
1.66
1.18
0.06
0.70
1.06
0.76
0.64
0.56
0.04
0.63
0.47
0.34
0.06
0.62

Cave
El Castillo
Las Monedas
Hornos de la Peña
El Pendo
Covalanas
Cullalvera
Chufín

Table 2

Dose to the Public
in one visit (mSv)
0.0027
0.0031
0.0088
0.0037
0.0021
0.0023
0.0015

Table 3

333

Cave
El Castillo
Las Monedas
Hornos de la Peña
El Pendo
Covalanas
Cullalvera
Chufín

Mean particle
concentration (cm-3)
868 (100-350)
1154 (600-3000)
3508 (440-19000)
1903 (400-5400)
1912 (370-16000)
3106 (950-12000)
3428 (1100-14000)

DCFu
(mSv WLM-1)
37,89
30,58
15,70
21,85
21,79
16,64
15,87

Table 4

334