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Difficulties in the dose estimate of workers originated from radon and radonprogeny in a manganese mine

Norbert Kavasi a, Janos Somlai b, Tamas Vigh c, Shinji Tokonami d, Tetsuo Ishikawa d,Atsuyuki Sorimachi d, Tibor Kovacs a,b,*

a Social Organization for Radio Ecological Cleanliness, POB 158, 8201 Veszprem, Hungaryb Institute Radiochemistry and Radioecology, University of Pannonia, POB 158, 8201 Veszprem, Hungaryc Mangan Ltd., 8409 Urkut, Hungaryd Environmental Radiation Effects Research Group, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inageku, Chiba 263-8555, Japan

a r t i c l e i n f o

Article history:Received 1 January 2008Received in revised form30 January 2009Accepted 15 March 2009

Keywords:RadonMineDose assessmentManganese mineRadiation protection

a b s t r a c t

The regulation of radon concentration at workplaces has gained an accentuated importance in allcountries. Nevertheless, at this time there is no globally accepted workplace protocol that sets out saferadon concentration values.In this study the radon concentration measured during the working hours and the equilibrium factor,which are necessary for the exact estimation of the radiation dose originating from radon and effectingmanganese miners are determined in a Hungarian manganese mine. In order to determine the exactcommitted effective dose, workers wore track-etched radon.The annual average radon concentration measured at the different points in the mine that was inves-tigated may vary by �30% relative to the average of the total of the measurement points. The influence ofthe measurement period is even more significant as the difference related to the annual average may be�30% with a measurement period of 6 months. The radon concentration level measured during workinghours was approximately half of the average measured during the whole period. The measured equi-librium factor was 50% greater than the value given in literature. Disregarding any of these factors maylead to significant errors for the estimation of dose to workers.The average value of the estimated effective radiation dose was 3.7 mSv year�1 based on the personalradon dosimeters.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Research on health risks caused by the naturally occurringradioactive noble gas radon and its progenies has gained greatattention, in particular those aspects related to controlling highradon concentrations (ICRP65, 1994; USEPA, 2004) and to regu-lating the air quality in workplaces (SSI, 1999; Gooding and Dixon,1992; Risica, 2001). Since workers spend an average of approxi-mately 2000 h per year at their workplaces, the radiation exposurefrom radon in the workplace could be significant in cases when theradon concentration is relatively high (Scivyer and Gregory, 1995;Dixon et al., 1996). The ICRP provides guidance to regulatoryauthorities on the radon action levels in its publication of ICRP65

(1994). ICRP65 suggests that workers who are not regarded asbeing occupationally exposed to radiation should be treated in thesame way as the general public. Consequently, considering the ratioof time spent at work versus at home and applying the commonlyused equilibrium factor (0.4) and dose conversion conventions,the suggested action level is somewhere between 500 and1500 Bq m�3. In Europe, fifteen countries have set radon referencelevels for underground workplaces. The majority of countries haveexpressed this reference level in terms of radon concentration, andsome (Germany, Sweden, Belgium) in terms of exposure, as illus-trated in Table 1 (Synnott and Fenton, 2001).

In Hungary, the action level for workplaces during workinghours is an average radon concentration of 1000 Bq m�3

(Hungarian Regulation, 2000). This legislation was actually imple-mented in January 2003. In order to set this action level an occu-pational exposure of 2000 h per year and an equilibrium factor of0.4 were used.

* Corresponding author at: Social Organization for Radio Ecological Cleanliness,POB 158, 8201 Veszprem, Hungary. Tel./fax: þ36 88 427 681.

E-mail address: kt@almos.vein.hu (T. Kovacs).

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High radon concentrations can easily build up in undergroundareas, mines (Fathabadi et al., 2006; Kavasi et al., 2004), caves(Szerbin, 1996; Kavasi et al., 2003), and cellars (Wang and Ward,2002), due to the high radium-/thorium-content in the rockssurrounding the air-spaces, and due to the large emanating surfacesrelative to the air-space volume in underground environments. Themeasurement of the radon concentration within enclosed air-spaces is essential, as radon emanating from rock layers, even layerswith an average radium-content, can accumulate to significantlevels, greatly increasing the radiation dose of those working there.Several researchers have already drawn attention to the difficultiesin estimating the radiation dose originating from radon to peopleworking in underground air-spaces (Kavasi et al., 2006).

However, at this time there is no globally accepted workplaceprotocol that sets out safe radon concentration values. Among theabove-mentioned fifteen countries (where have set radon refer-ence levels for underground workplaces) only 5 countries havemade statements concerning measurement techniques and proto-cols for radon in air in workplaces. The duration of measurementvaries between 1 and 12 months, however measurement intervalsof 1–3 months are the most frequent (Scivyer and Gregory, 1995;Synnott and Fenton, 2001). The number of measurement points,sampling times and the determination of the concentration of theprogenies, i.e. the necessity of determining the equilibrium factorare only included in the requirements of some countries.

This work presents some of the difficulties encountered indetermining the average radon concentration, as well as the radi-ation dose originating from radon to the workers in a manganesemine in Hungary. Manganese oxides, due to their structure, havegood adsorption characteristics, and are therefore widely used innuclear measurement techniques as well as for the selectiveenrichment of different radionuclides (Moore and Reid, 1973). Themine has been in operation since ancient times, first in painting andafter 1924 it was mined for use as an alloy in steel production. Thereare two main types of mined manganese oxide, the rich clay slatefinely stratified in micron-sized rhodochrosite grains, the so-calledcarbonate manganese ore, and the oxidic manganese ore thatoccurs in the different forms of manganese oxide that is rich in oxi-hydroxides (Polgari et al., 2000).

During our work the changes in average radon concentrationwere surveyed in relation to the different measurement locations,the seasons, more exactly the different months, and the workinghours, and also the equilibrium factor was examined. In order to

Table 1Radon reference level in fifteen European countries.

Country Radon concentration [Bq m�3]

Czech Republic 1000Denmark 400Finland 400Greece 400Hungary 1000Ireland 400Italy 500Slovenia 1000Switzerland 3000United Kingdom 400Belgium 800a

Germany 2000a

Sweden 2500a

a Radon exposure [kBq m�3 h�1 year�1].

Fig. 1. Location of the sampling points in the manganese mine in Urkut.

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establish the appropriate measurement method, the differencebetween the radon concentration of the workplace air, and theaverage radon concentration determined by personal dosimeterswas studied. Furthermore, the influence of these parameters on thedose estimation of the workers was also investigated.

2. Measurements and methods

2.1. Radon concentration measurements

2.1.1. Spatial and long term temporal variations of radonconcentration determined by track-etched detectors(passive method)

In order to assess the dependence of radon concentration withthe location of the measurement point within the mine, severalsampling points were selected. Some of these points were locatednear new sections of the mine (Mb1–7), whereas other points werelocated in the water pumping chamber (Mb9) and in the mainmining tunnel (Mb8). The main mining tunnel (Mb8) is where theair from the different passages in the mine mixes and hence,average values for the air in the mine are expected. In this latterarea continuous radon concentration and equilibrium factormeasurements were carried out. A total of 9 measuring points wereestablished in the mine, as sketched in Fig. 1.

Solid state CR-39 type TASTRAK nuclear track-etch detectorswere used, for the measurements. Detectors were placed at a heightof 1.5–2 m from the ground. The height of the tunnels is 3.2 m withsteel support, and 2.6 m with wooden support. The cross-section isapproximately 10 m2 in both cases.

After 1 month of exposure time, the detectors were etched in6 M NaOH solution at 80� 0.5 �C for 2 h. The track-etch detectorswere calibrated in a Genitron EV03209 calibration chamber using

a PYLON RN 2000A radon emanation source (with 226Ra activity:105� 0.4% kBq, radon emanation: 100%).

2.1.2. Measurement of daily changes of the radon concentration(active method)

The measured radon concentration during working hours mayin many cases differ from the average radon concentration, espe-cially if work is not on a 24 h basis in the workplace. This is becausethere may be significant changes in the radon concentration due to,for example, changes in the intensity of ventilation, the time of day,the season, among others. In the case of this manganese mine, therewas a single work shift, and hence the ventilation fans were notconstantly working.

Continuous measurements were carried out using AlphaguardPQ 2000 radon monitor to see the effect of ventilation. Measure-ments were carried out in the main air-course (Mb8) for 16 days inAugust.

2.1.3. Radon concentration measurements with personaltrack-etched detectors

In order to make our dose estimation more precise, personaldosimeters were distributed among the workers (actually theforemen). The above-mentioned solid state track detectors wererequired to be worn on the outside of their working clothes duringworking hours. After working hours the detectors were stored ina room in a controlled low (about 12 Bq m�3) radon level envi-ronment. Within the evaluation period detectors were supplied tothe three teams working at the face of the mine and to one teamworking in the pumping chamber.

2.2. Determination of the equilibrium factor

The equilibrium factor also plays an important role in estimatingthe radiation dose and it was determined in the main mining tunnel(Mb8). A Pylon WLX Radon Daughter Element ConcentrationMeasuring device was used, with radon concentration measure-ments carried out with the above-mentioned Alphaguard Pro 2000device.

The equivalent equilibrium concentration was calculated usingthe equation:

CRn;EEC ¼ WL$3700: (1)

The equilibrium factor (F) was calculated with

F ¼CRn;EEC

CRn: (2)

CRn: the radon concentration measured [Bq m�3], CRn,EEC: equivalentequilibrium concentration deriving from the WL values [Bq m�3].

2.3. Calculation of radiation exposure

Having the average equilibrium factor (F) from the above resultsand the measured radon concentration (CRn) the committedeffective dose could then be calculated using the equation:

Maximum

Minimum

0

200

400

600

800

1000

1200

Mb1 Mb2 Mb3 Mb4 Mb5 Mb6 Mb7 Mb8 Mb9

Averag

e

Sampling sites

Rad

on

co

ncen

tratio

n [B

q m

-3]

Fig. 2. Annual average radon concentration values at the given measurement points.

Table 2Natural radioactivity of the various manganese mine products.

Sample name 40K [Bq kg�1] 226Ra [Bq kg�1] 232Th [Bq kg�1]

Average Min. Max. Average Min. Max. Average Min. Max.

Mn ore carbonate master lode (10 pcs) 654 183 1369 8.1 2.8 23.4 11.9 5.6 27.1Mn ore oxidic (8 pcs) 175 162 189 8.9 8.4 9.3 6.4 6.0 6.9Under-, Upperlayer black shale (8 pcs) 286 82 769 10.9 3.6 21.9 14.8 5.5 28.9Underlayer limestone (8 pcs) 100 8.5 162 2.9 2.3 3.4 6.1 5.0 6.7

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E ¼ CRn � F � t � K: (3)

E: committed effective dose [mSv], CRn: average radon concentra-tion [Bq m�3], F: average equilibrium factor, t: time spent inside inthe investigated period [h], K: dose conversion factor from ICRP[mSv m3 Bq�1 h�1].

3. Results

3.1. Sampling intervals and the influence of representativemeasurement points on radon concentration

The annual average radon concentration for the entire mine was817 Bq m�3 (Fig. 2), being 575 and 997 Bq m�3 the minimum andmaximum values that were measured at the different samplingpoints.

In the main mining tunnel (Mb8) the annual average was756 Bq m�3, which is very close to the average measured for all thedifferent locations of the mine. Therefore, in this particularmanganese mine, the locations of the measurement points have nosignificant influence, and thus the main mining tunnel (Mb8)would be a suitable location for measurements.

In any case, radon concentrations are too large to be explainedsolely by the natural radionuclide concentration values of the rocksthat constitute the walls of the tunnels, as illustrated in Table 2(Kavasi et al., 2004). Further studies are indeed necessary tounderstand the high levels of radon concentration in the tunnels ofthe mine.

Fig. 3 shows radon concentration values measured at thedifferent measurement points, according to the month.

Radon concentration in the mine primarily depends on seasonalfactors, as shown in Fig. 3, where differences in radon concentra-tion values in summer and winter months exceed one order ofmagnitude. Therefore, the length and time of the measurementperiod may be a decisive factor. Table 3 summarizes the minimumand maximum values of the averages of measurements for 1, 3 and6 months and the average for 12 months, at the 9 measurementpoints, and the total average for the whole mine.

Dispersion of the data for 1, 3 and 6 months sampling intervalsdecreases with the length of the measurement time span. In thissense, dispersion could reach up to 60% for 3-month long

measurements and go down to 30% for 6-month long measure-ments. The average values estimated from the 1- and 3-month longmeasurements should be applied with great caution. As a conse-quence of these results, for similar workplaces, 12-month longmeasurements are strongly recommended. If that is not possible,a minimum 6-month period should be chosen.

The effect of ventilation is illustrated in Fig. 4, where the radonconcentration is significantly larger when the ventilation system isshut down during the night and weekends. During working hoursradon levels are considerably lower. Average radon concentrationvalues within the examined period were 1061 Bq m�3. Duringworking hours (06:00–14:00) it was 659 Bq m�3, while outsideworking hours (14:00–06:00) it was 1296 Bq m�3. So, during theshort working hours, almost 60% of the average radon concentra-tion was measured relative to the whole period.

3.2. Radon concentration values measured withpersonal dosimeters

The average radon concentration during the working shift ofminers was 412 Bq m�3 (Table 4), being this value approximatelyhalf of the average radon concentration in the mine, 817 Bq m�3.This difference is the result of the ventilation system that reduces

0

500

1000

1500

2000

2500

3000

3500

4000

Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec.Months

Rad

on

co

ncen

tratio

n [B

q m

-3]

Mb1 Mb2 Mb3Mb4 Mb5 Mb6Mb7 Mb8 Mb9

Fig. 3. Monthly average radon concentration at the 9 measurement points in the manganese mine in Urkut.

Table 3Minimum–maximum radon concentration for 1, 3 and 6 months measurementintervals and the total average radon concentration.

Measurement point Radon concentration [Bq m�3]

1 Month 3 Months 6 Months 12 Months

Min. Max. Min. Max. Min. Max. Average

Mb1 147 3484 330 2251 303 1633 968Mb2 329 2168 700 1317 709 1068 794Mb3 263 1586 339 1008 441 756 575Mb4 355 1749 564 1238 941 1027 984Mb5 231 2137 571 1665 721 1364 997Mb6 357 1140 471 935 597 913 718Mb7 368 2339 504 1532 652 1294 970Mb8 229 2017 424 1373 532 1027 756Mb9 118 1427 427 1030 285 901 593

Average 365 1968 471 1343 571 1064 817

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radon concentration during working hours. Radon concentrationsmeasured by personal dosimeters in the pump chamber are larger,since working periods are longer there.

3.3. Equilibrium factor

Continuous measurements of the equilibrium factor werecarried out in the main mining tunnel (Mb8), during 13 days of

March and at a sampling rate of 1 h (Fig. 5). Following Eq. (2), theaverage equilibrium factor was 0.57, recalling that this value shouldbe taken with caution due to the short duration of measurementsand the need to extend this measurement to other locations withinthe mine. In any case, this result shows that it is necessary tocalculate equilibrium factors for each mine individually.

3.4. Radiation dose to the workers

Table 5 shows the estimated radiation doses based on themeasurements taken from the personal dosimeters, when calcu-lated with the equilibrium factors of 0.4 as found in scientificliterature, and 0.57 as measured.

Despite the fact that the radon concentration does not reach thelimit of 1000 Bq m�3, the radiation dose is considerable, especiallyif the recommended limits for exposure to artificial radionuclidesthat the general population (1 mSv year�1 total) is permitted, aretaken into account. It is also apparent from the table that theradiation dose estimated with the measured equilibrium factor is

0

500

1000

1500

2000

2500

3000

3500

Tue. Wed. Thu. Fri. Sat. Sun. Mon. Tue. Wed. Thu. Fri. Sat. Sun. Mon. Tue. Wed.Days

Rad

on

co

ncen

tratio

n [B

q m

-3]

After working hoursWorking hours

Fig. 4. Continuous radon concentration as a function of time in the main mining tunnel.

Table 4Radon concentration values measured with personal radon dosimeters.

Team Annual average radonconcentration [Bq m�3]

Min.–max. values ofmonthly averages [Bq m�3]

Team 1 371 241–783Team 2 409 241–853Team 3 457 205–984

Average 412 –

Pump operators 570 133–1495

0

200

400

600

800

1000

1200

1400

Wed. Thu. Fri. Sat. Sun. Mon. Tue. Wed. Thu. Fri. Sat. Sun. Mon.Days

Rad

on

co

ncen

tratio

n [B

q m

-3]

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

Eq

uilib

riu

m facto

r

C Rn F

Fig. 5. Equilibrium factor and radon concentration as a function of time in the main mining tunnel.

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almost 50% greater than that calculated with the value recom-mended in scientific literature. The uncertainty of dose estimationis increased by the applied dose conversion factor, which assumeseveryday activities and air parameters (e.g. office work). Therefore,the increased respiration performance of workers and the aerosolconcentration increase of inhaled air are not taken into consider-ation during its application.

4. Conclusions

This paper shows the possible problems that arise when tryingto determine the annual average radon concentration in anunderground workplace, and the radiation dose of workers, origi-nating from radon. The results of this study indicate that themeasurement protocols found in international literature today aredeficient in several areas. Based on the results it can be stated that:

Even with a low Ra-content in the rocks surrounding minepassages, a significant Rn-concentration may be generated inthe air-space of a mine.Radon concentration varies depending on the type of workbeing carried out. It would be therefore reasonable to carry outmeasurements in several places.Ideally, the length of the measuring period should be at least oneyear to avoid the large data dispersion for shorter measuringtime spans.A significant error would more than likely result if results areaveraged to 24 h, as radon concentration significantly increases(to twice the value) after working hours, due to the decrease inventilation.With the use of personal dosimeters, the average radonconcentration can be estimated much more reliably duringworking hours.

An accurate assessment of the equilibrium factor in the air-spaceof the mine is necessary as the use of the recommended value of0.4 may result in significant errors when dose estimations arecalculated.

The radiation dose to workers from radon and its daughterelements in this mine was 2.6 and 3.7 mSv year�1 for equilibriumfactors of 0.4 and 0.57 (calculated in this work), respectively. It doesnot reach the maximum permissible limit of 6 mSv, so thismanganese mine has not been classified as a radiation hazardworkplace.

References

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Gooding, T.D., Dixon, D.W., 1992. Radon in UK workplaces. Radiat. Prot. Dosimetry45, 187–189.

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ICRP (International Commission on Radiological Protection), 1994. ProtectionAgainst Radon-222 at Home and at Work. ICRP Publication 65. Pergamon Press,Oxford.

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Kavasi, N., Somlai, J., Kovacs, T., Nemeth, C., Szabo, T., Gorjanacz, Z., Varhegyi, A.,Hakl, J., 2006. Difficulties in radon measurements at workplaces. Radiat. Meas.41, 229–234.

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Table 5Committed effective dose estimated with the equilibrium factors of 0.4, as found inthe literature and 0.57, as measured.

Those wearingdetectors

Committed effective dose [mSv year�1]

F¼ 0.4 F¼ 0.57

Team 1 2.34 3.33Team 2 2.58 3.67Team 3 2.88 4.10

Average 2.60 3.71

Pump operators 3.59 5.12

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