|Year : 2015 | Volume
| Issue : 3 | Page : 115-119
Nasal swab reference levels for plutonium based on aerosol characteristics and breathing patterns of individuals
PD Sawant, SP Prabhu, DP Rath, RK Gopalakrishnan, DD Rao
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
|Date of Web Publication||10-Nov-2015|
P D Sawant
Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai - 400 085, Maharashtra
Source of Support: None, Conflict of Interest: None
In plutonium (Pu) handling facilities, there is a potential for internal exposure of radiation workers along with external exposure. Nasal swabs (NS) taken rapidly, on site, help in not only providing an early estimate of internal dose due to inhalation of Pu but also in decision making for medical intervention. However, the Committed Effective Dose (CED) computed based on the observed NS activity and that estimated from bioassay measurements of individuals may vary significantly. One of the reasons would be the use of default particle size (5 µm) for computing the CED based on the activity on NS. Other parameters like breathing pattern and levels of exercise would also influence the NS reference activity level and hence their impact needs to be assessed. This study indicated that observed NS reference levels had a direct dependence on all these parameters and use of default parameters for evaluation of internal dose based on NS activity would result in large uncertainties in the dose assessment.
Keywords: Annual limit on intake, nasal breathing, nasal swabs, oronasal breathing, Pu
|How to cite this article:|
Sawant P D, Prabhu S P, Rath D P, Gopalakrishnan R K, Rao D D. Nasal swab reference levels for plutonium based on aerosol characteristics and breathing patterns of individuals. Radiat Prot Environ 2015;38:115-9
|How to cite this URL:|
Sawant P D, Prabhu S P, Rath D P, Gopalakrishnan R K, Rao D D. Nasal swab reference levels for plutonium based on aerosol characteristics and breathing patterns of individuals. Radiat Prot Environ [serial online] 2015 [cited 2017 Dec 16];38:115-9. Available from: http://www.rpe.org.in/text.asp?2015/38/3/115/169391
| Introduction|| |
Different compounds of plutonium (Pu) are encountered in fuel reprocessing, as well as in fuel fabrication plants. Even though engineered safety features and safe work practices in nuclear facilities are aimed at preventing radiation exposures, there exists a potential for internal contamination of the radiation workers. In an exposure scenario, nasal swabs (NS) are immediately taken and analyzed as the primary indicators to initiate action and estimate the internal dose due to Pu. The presence of alpha activity on these NS provides a confirmation, as well as an indication of possible Pu inhalation. Results of NS are useful because of their early availability; however, confirmation of intake is always based on more definitive individual monitoring techniques, such as in vivo measurements with lung monitor and bioassay measurements with urine or feces.  However, both these techniques have certain limitations viz. lung monitoring is not effective for measurement of pure 239 Pu, whereas urine, as well as fecal analysis, is time-consuming. Thus, for a decision regarding prompt administration of a chelating agent, rapid dose estimation using NS method is very important although large uncertainty is associated with this method. , However, discrepancies may be observed between internal doses estimated based on NS activity and those based on follow-up individual monitoring. One of the reasons is the use of default physicochemical characteristics of the inhaled aerosol. Other parameters viz. breathing pattern and levels of exercise would also influence the reference activity levels for NS and hence, their impact needs to be assessed. This paper reports the results of a study undertaken to investigate the dependence of NS activity reference levels on particle size and breathing pattern of individuals.
| Monitoring techniques for radiation workers|| |
The individual monitoring procedures in case of special monitoring include a collection of NS followed by lung and fecal/urine monitoring [Table 1]. Each of these standard procedures is described in more detail below.
|Table 1: Recommended methods for routine and special monitoring of Pu after inhalation |
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Collection of nasal swabs (wipes)
NS are taken as soon as the occurrence of an incident is known/suspected or when the contaminated worker reports to the health physicist. It is taken before any facial decontamination is performed and if the worker has already tried to wash his face, this fact is noted while taking the NS and recorded. Extreme care is taken to ensure that NS collected are from the nostrils and do not have any cross-contamination from the facial skin or external regions around the nose. The procedure for collection of NS is as follows: A filter paper (or a soft tissue sheet) is wrapped around the little finger and rotated a couple of times inside the nostril. Normally, NS are taken from the left and right nostrils separately, dried under an infrared lamp, and alpha activity is measured using a ZnS (Ag) detector. The NS are then preserved with all relevant details for subsequent analysis to know the isotopic composition, as well as the physicochemical characteristics of the inhaled aerosols. 
Bioassay (fecal/urine) monitoring is the most common method of assessing Pu intake. The total amount of Pu detected in fecal and urine sample of the exposed individual is usually used for final internal dose estimation, based on the excretion fractions given in the International Commission for Radiological Protection (ICRP).  Bioassay samples are collected by workers for a known duration of time (mainly overnight or for 24 h), away from the workplace and preferably, at home after taking a bath.  The method adopted for fecal/urinary analysis is described briefly as follows. Pu tracer ( 242 Pu or 236 Pu) is added to urine samples and equilibrated to determine the yield prior to any radiochemical separation. Urine samples are wet ashed; Pu is preconcentrated on calcium phosphate and allowed to settle overnight. The precipitate after centrifugation is dissolved in nitric acid, processed through an anion exchange resin column and separated Pu is electrodeposited onto a stainless steel planchette. The electrodeposited Pu is measured using a passivated implanted planar silicon (PIPS) detector. The minimum detectable activity (MDA) of this method for 239 Pu is 0.5 mBq/d for a 24 h counting time. The complete analysis of single sample takes nearly 35-40 h (4-5 working days). In the case of fecal samples, the procedure adopted is similar to that of urinalysis, except the initial processing which includes dry ashing in a muffle furnace at 450°C followed by acid leaching.
Assessments of pure 239 Pu in the lungs are made from measurements of the 17.2 keV and 20.6 keV X - rays of 235 U formed on alpha decay. The actinide monitoring system consists of a totally shielded room made up of mild steel of 20 cm thickness all around. The inner lining of the steel room has graded shield of Pb (3 mm), Cd (2 mm), and Cu (0.5 mm) which provides the maximum background reduction for the measurements of low energy photon emitters. The detecting systems used for contamination monitoring are phoswich detectors (sandwich of a thin NaI [Tl] 1.2 cm × 20 cm and a thick CsI [Tl] 5 cm × 20 cm) or arrays of three HPGe detectors.  However detection sensitivity for Pu X-rays (13.6, 17.2 and 20.6 keV) is poor and strongly influenced by the chest wall thickness of the subject. The MDA for pure 239 Pu retained in the lung is about 1000 - 2000 Bq. However, the presence of 241 Am in the material matrix considerably reduces the MDA to 10 Bq due to 60 keV gamma with 34% abundance. Following the detection of 241 Am in lungs, intake of Pu can then be assessed based on the Pu: Am ratio observed in the fecal or NS samples.
Nasal swab reference levels (NSRLs) for inhalation intakes of plutonium
In most of the incidental exposures, the physicochemical characteristics of inhaled aerosols are not known. Therefore, nasal swab reference levels (NSRLs) in India are computed based on the default values of particle size (5 μm activity median aerodynamic diameter [AMAD]); particle density (3 g/cm 3 ); and shape factor (1.5) as recommended by the ICRP.  While computing these NSRLs, it is assumed that only 10% of the fraction deposited in ET1-region of the lungs is collected on NS. These levels set for Pu intake are <0.1 annual limit on intake (ALI), 0.1-1 ALI and >1 ALI.  If the activity on NS indicates intake < 0.1 ALI, no trisodium salt of calcium diethylene triamine pentaacetic acid (Ca-DTPA) is administered to the worker, and internal dose is assessed based on follow-up bioassay monitoring. However, if the activity on NS indicates intake >0.1 ALI, Ca-DTPA is administered, and the internal dose is assessed based on stabilized urinary excretion after a period of ~ 100 days. ,, The influence of Ca-DTPA is stated to be nil after a period of 100 days of its administration. Internationally, the practice of administering decorporating agent is not followed for inhalation of Type S Pu,  however, in India one dose of Ca-DTPA aerosol/intravenously is recommended as a precautionary approach. However, use of default parameters for evaluation of internal dose based on NS activity would result in large uncertainties in the final dose assessment. Thus, the following study was taken-up to investigate the dependence of the NSRL on the particle sizes (AMAD) of inhaled aerosol and breathing pattern of individuals.
Influence of particle size (activity median aerodynamic diameter) on nasal swab reference levels (NSRLs)
Regional deposition of inhaled aerosol in the respiratory tract strongly depends on the particle size which ultimately affects the NSRLs. Lung Dose Evaluation Programme (LUDEP) version 2.07 (NRPB, UK) was used to study the regional deposition of aerosol sizes in lungs from 1 to 10 μm AMAD (i.e. the respirable particle size). While computing the regional deposition for Type S Pu compounds, AMADs, as well as mass density was changed to 11.46 g/cm 3 (PuO 2 ), whereas for Type M Pu compounds default values of density (3 g/cm 3 ) was used. Fractional deposition in ET1 and TH-regions of the lungs with AMAD for Pu aerosols is given in [Figure 1] and [Figure 2], respectively. Based on these fractions, the ALI for Type M and Type S compounds of Pu was computed and is given in [Figure 3] and [Figure 4]. These values were then used for the computation of NSRLs [Figure 5] and [Figure 6]. It can be observed that the NSRLs corresponding to intake >0.1 ALI increase with an increase in AMAD mainly due to higher deposition in ET1 region of the lungs. This indicates that the use of default particle size (5 μm AMAD) can lead to underestimation in assessed internal doses based on observed NS activity if the actual particle of the inhaled aerosol <5 μm AMAD.
|Figure 1: Variation in ET1 deposition with particle size and breathing habit|
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|Figure 2: Variation in TH deposition with particle size and breathing habit|
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|Figure 3: ALI for Type M Pu compounds based on breathing habit and levels of exercise of a radiation worker|
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|Figure 4: ALI for Type S Pu compounds based on breathing habit and levels of exercise of a radiation worker|
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|Figure 5: NS reference levels corresponding to 0.1 ALI intake (Type M Pu) for different particle sizes and breathing patterns of individuals|
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Influence of breathing patterns of the individuals and levels of exercise on nasal swab reference levels (NSRLs)
It has been observed on a few occasions that for a given air activity of Pu in a working area, some individuals show detectable nasal contaminations while some show no detectable Pu activity on NS. In such situation, variation in NS contamination cannot be explained on the basis of particle size alone. The other probable reasons which needed to be studied in detail are the difference in breathing habit (nasal or oronasal) and levels of exercise of the individual workers. Thus, the following study was undertaken to address the impact of all these parameters on respiratory deposition, on dose coefficient (Sv/Bq) and on the corresponding NSRLs using LUDEP 2.07 (NRPB, UK).
Typically a radiation worker is assumed to be a nasal breather doing light exercise but there are few individuals who are habitual mouth (oronasal) breathers. It is also observed that for a nasal breather involved in the heavy exercise, the breathing pattern changes naturally from nasal to oronasal breathing to compensate for the increased demand of oxygen supply to the body. The fraction of air breathed through the nose for individuals varies with breathing habits and levels of exercise  [Table 2]. It can be observed that the fraction of air breathed through the nose for nasal breather involved in heavy exercise (0.5) and a habitual oronasal breather (0.4) is comparable. [Figure 5] and [Figure 6] also show the influence of breathing patterns of the individual on NSRLs. With the change in breathing pattern from nasal to oronasal, the fraction breathed to nose reduces, resulting in lower deposition in ET1 [Figure 1] and [Figure 2] and reducing the NSRLs.
|Figure 6: NS reference levels corresponding to 0.1 ALI intake (Type S Pu) for different particle sizes and breathing patterns of individuals|
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|Table 2: Fraction of air breathed through nose for individuals with different breathing habits and for various levels of exercise |
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[Table 2] also indicates the influence of exercise on the fraction of air breathed through the nose. It can be observed that the fraction breathed through nose does not differ significantly for oronasal breather involved in different levels of exercise, whereas there is a significant reduction (50%) for a nasal breather. Thus, levels of exercise (light and heavy) do not have much impact on the NSRLs for oronasal breathers but in the case of nasal breathers, the NSRLs are lower and comparable to that observed for oronasal breathers. For example, the NSRL for 5 μm AMAD, Type M is nearly seven times lower for oronasal breather as compared to nasal breather. Thus, use of default breathing pattern (nasal) may also lead to significant underestimation in Committed Effective Dose (CED) computed based on NSRL.
| Conclusions|| |
In the case of incidental exposures, getting information regarding physicochemical characteristics of the contaminant may be difficult. Hence, medical intervention is initiated based on NSRL corresponding to intake >0.1-1 ALI for Pu compounds based on the ICRP default parameters regarding particle size, density, and breathing patterns. This study shows that the parameters which had maximum influence on the NSRLs were the physicochemical characteristic of the inhaled aerosol and breathing pattern of the individuals. This work also indicates that assumption of default parameters would lead to an underestimation in CED computed based on NS activity not only for Pu but also for other actinides. The study also highlights the need to support and reassess the removal efficiency of the NS (assumed 10%) with more scientific findings.
The authors would like to acknowledge Dr. Pradeepkumar K. S., Associate Director, Health, Safety and Environment Group, BARC for his constant guidance and encouragement during the course of this work.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2]