KFKM992-36/K -REPORT

É.BELEZNAY, M. OSVAY

LONG-TERM CLEARANCE OF ACCIDENTALLY INHALED *°Co

AEROSOLS IN HUMANS

Hungarian Academy of Sciences CENTRAL RESEARCH INSTITUTE FOR PHYSICS V B U D A P E S T

KFKM99246/K REPORT

LoDf-term Clearance of Accidentally Inhaled

Go Aerosols in Humans

Éva Beleznay Central Research Institute for Physics

Atomic Energy Research Institute H-1525 Bttdapest 114, P.O.B. 49, HUNGARY

* • Margit Osvay

Institute of Isotopes H-1525 Budapest, HUNGARY

HU ISSN 0368 5330

yt MOsvay: Long-term Clearance of Accidentally Inhaled "Cfr Aerosols in Humans KFKM992-36/K

ABSTRACT

Long-term body retention was measured on six workers inhaling "Co aerosols incidentally during with a high activity *°Co source. An improved whole body counter technique and calibration

provided good conditions to follow the body clearance over 4 years. A two-d^ector profik sauining arrangement was med to measure the activity distribution "in vivo" over the lung region.

The observed whole body retention followed a two-exponential time function between 10-1200 days. Bt sed on the profile measurements the shorter exponent ranged in 20-70 days was associated with activity leaving the pulmonary region, while long-term exponential should be interpreted к the clearance of the slowest component of systemkaDy distributed cobalt with biological luif-lrves of 6t)0-10Wd^ys. These experiences dass£ed fresh air-bora cobahcom^umng aerosols úuotlwúihabüon class "W".

The consistency of the measured retention pattern with ICRP inhalation model was investigated i—Ti-g different aerosol size distributions characterized by AMAD - 0.25 /tm, 1/im and 4jim. For the sake of abetter comparison all calculated curves and measured data were noniulized to the whole body content on'he 10th day. It was found that ICRP model with AMAD • 4/um could describe reasonably the retention of our investigated

A possible extension of ICRP lung model is also proposed to take into account the actual pulmonary lung clearance determined individually.

Векаюу Éva, Osvay Margit: Véletlen beidegzett Со-60 aeroszolok hosszú idejű kiürülésének vizsgalata az emberi szervezetben KFKI-1992-36VK

KIVONAT

Hat felnótt férfi egésztest retencióját követtük egésztestszámlálóval ~1500 napig, akik nagyaktmtású Co-60 sugárforrással végzett munka során radioaktív kobalt aeroszolokat lélegeztek be.

A15. és a 80. napokon végzett profil scanning mérés kimutatta, hogy az aktivitás egyenletesen deponálódott a két tüdólebenyben és onnan közelítőleg 50 cap felezési idővel ürült Feltételeztük, hogy a - 300-400 °C hómétsékletű sugárforrásból a dolgozók kobaltoxidokat és fémes kobalt részecskéket inhaláltak.

összehasonlítottuk a mért egésztest retenáót az ICRP 30 "W" -osztályú inhalációra javasolt dozimetriai modelljével, AMAD - 0.75 цт, l / ш és 4/im aeroszol méreteket feltételezve. A mért egésztest értékeket a 10-ik napi értékekre normáivá azt találtuk, hogy az ICRP dozimetriai modell igen jól leírja a hosszú idejű kiürülést és a dózis szempontjából 10%-nál kisebb eltérést okozott, hogy milyen aeroszol méretet tételeztünk fel.

CONTENTS

ABSTRACT

INTRODUCTION

CASE-HISTORY

METHOD AND MEASUREMENTS

RESULTS

INTERPRETATION OF THE RESULTS ON THE BASIS OF ICRP 30

DISCUSSION

CONCLUSION

APPENDIX A

APPENDIX В

REFERENCES

1

INTRODUCTION

During manipulation with % high activity "Co source six people were exposed accidentally. Artough one day later the whole body measurements showed that the inhaled activity was less then a thousandth of ALI (Annual Limit of Intake) given by ICRP, it means there was no health hazard from this low incorporation, the workers cooperated voluntarily to measure their long term body retention.

Inhalation is one of the main routes of incorporation of radioactive materials into the human body. When the limits for intakes of radionuclides by workers were calculated (ICRP.1979), a lung model was developed for general application in radiological protection, and a simplified dosimetric model was recommended for describing metabolism of a radionuclide reaching the body fluids and calculating the dose in organs. It was emphasized that this ICRP model can serve as a guide for the control of intakes and it is not intended to describe the unique behaviour of every incorporated radioactive compound. To asses the dose to workers using these suggested models, many complex parameters have to be considered, such as size of aerosol particles, solubility of inhaled compounds, etc. and generally in accidental circumstances one has no reliable information about them. In spite of these difficulties the Task Group of ICRP encourages the laboratories to give 'intelligent' interpretation on measurements of actual human exposures in those cases where the intake is substantial enough to justify such an evaluation.

There are double reasons why investigation of cobalt incorporation cases have importance. The radioisotopes of cobalt produced by neutron activation are usually present in reactor coolant system from fuel element claddings or structure materials via corrosion, and frequently revealable in maintenance workers. On the other hand cobalt isotopes are widely used in clinical and research application because of their high specific activity. During manufacturing such sources there is a risk of incorporation as it happened in our laboratory. Still uncertainties in the conditions of incorporation may cause that only few numbers of cases heve been published.

Cofidd (1963) reported in vivo gamma spectrometry measurements on three men who had suffered lutig irradiation following a failure of the protective cap­sule on a M C o source. The lung clearance graphs indicated a two exponentials type function, measured from the first day till 35, 147 and 290 days, respec­tively. He could deduce for the short clearance an effective half-time of about 10 days, whereas within the period of 60-120 days suggested a half-time of about 90 days.

2

Sill et al. (1964) encountered more frequently "°Co and "Co nuclides during their "in vivo" whole-body counting programme. They identified radiocobah incorporation on approximately 400 individuals where in 5 cases the measured whole body activity was more then 10 kBq each . Whole body retention in four cases individually followed for about two months showed half-lives ranging from 70 to 177 days. The highest burden was received by a worker who removed his respirator momentarily while working inside one of the large cashes used for the shipment of radioactive materials, here an effective half-life of 2 years was found over a period of 11 months.

An inhalation case was investigated by Jordan et al. (1965) in a very sophis­ticated manner. The incident occured during a two days maintenance operation of a high intensity ^Co 4т irradiator. They followed the body retention till about four months in calibrated chair geometry. In addition to whole body and chest measurements, 3" x 3" Nal(TI) scintillation detector and flat field collimator were used to obtain the scans of activity distribution along the body. Interpretation of the early results was difficult because of the evidence of considerable external contamination. From analysis of clearence curve, the cobalt disappeared from the lungs with a biological half-life of 1.6 days from the second to seventh days after the exposure, and with a 42 days half-life from the seventeenth to fiftieth days. This removal time gradually increased to about 110 days three months after the initial exposure.

Newton and Rundo (1971) reported long term behaviour of ^Co inhaled ac­cidentally in five men for time periods up to 2000-4000 days. The whole body retention and the clearance from the thorax were measured in calibrated geom­etry. In one case following a minor reactor incident 92% of the inhaled activity disappeared from the body within the first few days, and the rest 8% followed a single exponents I retention of 1 year biological half-life. Unfortunately in their other cases they had no data at times earlier then about some months after assummed inhalation of radioactive dusts, the estimated half-lives for the long-term clearance were in the range of 5-7 years. The longest retention time was found in that worker who was contaminated during series of cleaning operations in a contaminated cell. In this case half of the total ^Co whole body content was d finitely in the chest region measured on the 1250th day, and periodically thereafter.

Riel, Jarvela and Richmond (1977) reported promt and repeated analysis of body burden, urine and fecal activity concentration of three workers exposed dur­ing a scheduled refueling action. Contrary to the instructions, the workers used wire brushes and no respirators during the cleaning operation. Accomplishing initial decontamination 336 kBq. 13 kBq, 14 kBq S 8Co and 8.2 kBq, 1.1 kBq, 1.3 kBq M C o activity were determined by whole body counting analysis after 4-7 hours post exposure. Unfortunately the measurements were not followed over 29, 7, 8 days, respectively.

3

Hedge et al. (1979) studied long-term clearance of inhaled m C o in five BWR power station workers up to periods ranging from 120 to 450 days after the first follow-up detection. They confirmed that the "Co activity was confined to the chest. The average of the long-term clearance gave an effective half-life of 493 days. The corresponding biological half-life was calculated to be 664 days.

The most common chemical forms of cobalt oxides wer* investigated by Barnes et al. (1976) in beagle dogs with AMAD's between 1.5-2 /im aarosol size range. They found the higher solubility of the cobaltous oxide (CoO) formed at 1400°C relative to cobaltosic oxide (C03O4) formed at 850°C. Generally, aerosols formed at higher tempratures are less soluble than aerosols formed at lower tem­peratures in the early postexposure period. In their experiment urinary excretion exceeded the fecal excretion after the fifth day. In both oxide types the effectiv half-time of lung clearance were in the range of 38-86 days.

With our inhalation studies we wish to contribute to the collection of human retention data and compare our results to recommendation of ICRP 30.

CASE-HISTORY

A working group of six people prepares regularly sources for clinical and research uses. During such a manipulation with a high activity м С о source (2x370 TBq) in a hot box they became contaminated accidentally. Ten minutes after starting the action the monitors detected an increased radiation level in the working area, the operator turned back tightly the covering lid of the container and the whole team left the contaminated room. They returned into the contam­inated room putting on protective clothing and respirators to stop the infiltration of the hot cell and decontaminate the working place. It took 3 hours on that day. After changing their clothing and having bath there were no measurable activity on the surfaces of the workers' bodies.

One day later the whole body counting measurements revealed 2-10 kBq *°Co internal contamination at these six people, far below the recently accepted hazard by ICRP (ICRP.1979; О Р Д 9 8 8 ) .

In our cases relatively exact time of the inhalation was quite accurately known. We assumed it happened during those 10-20 minutes that they spent in the contaminated room without respirator and special clothing. For the aerosol characteristics, such as particle size, shape, density and surface properties and chemical form, we assume that it was probably a composition of metallic and cobaltic or cobaltosic oxide formed at 300-400°C on the surface of the cobalt wire of high activity.

4

METHOD AND MEASUREMENTS

Whole body activity was measured by whole body counter system KFKI HY 2.1 in scanning-end-stop geometry using a 15 cm dia. x 10 cm thick Nal(TI) scintillation detector on the 1-2, 6.13.16. 33. 79-82.1009-1161.1429-1456th days after inhalation. Calibration of the measuring geometry has been performed for the isotope *°K with a series of bottle phantoms of weight in range of 70-100 kg. For calibration of "°Co a point source has been used in Presswood-phantom, when the activity efficiency was normalized to the 70 kg BOMAB type phan­tom's efficiency (Andrisi et al.,1986). Evaluation of the measurements has been done with an improved version of least square analysis using a computer program SKAL, which was able to compensate the small shift of standard spectra to the actually measured spectrum. The evaluation procedure was restricted to the en­ergy range of 950-2000 keV.ln 'his way we could eliminate the contributions of 1 3 7Cs and 1 3 4Cs isotopes originating from fallout of Chernobyl reactor accident in the first year period after the incorporation (see Fig. 1).

1 0 4

с с о JO о

1 0 3

Q.

(О

•ё io 2

о ü

10'

• 1st day (7710 Bq "Со! • 16th day 2560 Bq ">Co, * 79th day (1150 Bq "Co. О humon background

JL 50 100 150

Channel number 200 250

Figure 1. In vivo gamma-ray spectra1 from subject F.L. after incidental inhalation of60Co

*Date of the incident: 23 February 1988, the human background was measured one month before; energy calibration: 6.8 keV/channel; measuring time: 1293 a

Accuracy of the whole body activity in this geometry was better then 10% even for realistic situation of inhomogenous internal distribution (deposition into lungs, liver, intestines , etc.). The detection limit for whole body activity was 30 Bq °°Со at the end of measuring period, using 2586 s measuring time.

The measured data of *°Co whole body content of each person are shown on Fig.2.

In one of the subjects the distribution of the inhaled activity was studied with a computer controlled two-detector scanning arrangement using focused multisiit collimators (Andrisi et al.,1985). The collimated detectors were moving along the body by 2 cm steps and in every step the gamma spectrum was summed in the energy window of 1000-1400 keV. Following the longitudinal scanning measurement, on the 15th day a rectilineal scan was done at the position of the maximum of the longitudinal profile. The longitudinal profile scan was repeated on the 80th day. Counting profiles are presented on Fig.3 and Fig.4.

1 0 1 ! 1 1 1 1 0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0

Time after inhalation [days]

Figure 2. Measured whole body activity of the investigated cases and the calculated two-exponential function fit to these data (conf. Table 1)

6

350 Counts/60 s

350 F.L.ccise

300

250 / 1 \—151h day

200

150 |V \ - S 0 t h d a y 100

50 J ^ * ^ — N -0 1 j . — i - •

10 20 30 40 60 60 70 80 90 100 Longitudinal prollle scanning, cm

Ш^М

position of the rectilineal scan

Figure 3. Longitudinal profile scanning measurements of F.L. case on the 15th and 80th days after incidental inhalation of ̂ Co aerosols. (The arithmetic mean of the upper and lower detector response are plotted in every steps.)

7

CounbAtO •

- I -• w » I J» m ik Pecfllineol profil» scanning, cm

Figure 4. Rectilineal proße measurement of the same subject (F.L.) above the chest on the 15th day. (Central plane of the multislit collimator was turned to parallel with the body-axis.)

8

Table 1. Two-exponential function fit for retention of inhaled *°Co re­mained in whole body after 10th day of exposure.

1 Subject Age

W Body

weight [kg]

Parameters of the two exponentials Age

W Body

weight [kg] a, [Bq] T, [days] a 2 [Bq] T 2 [days]

I S,v* 57 88 4346±275 60± 7 534±192 8131185 I F'L' 43 89 2641+198 50± 9 401±111 925+108 | L.I. 34 74 1752± 75 77+ 5 234± 42 1014+139 | H.I. 45 85 1275+180 19+10 355+126 594±115 | K.F. 34 79 880± 92 25± 5 457± 26 511+ 20 | N.D. 36 85 182i 14 54±15 not applicable \

W The retention was described by a function of

R(t) = iate-^2' -^'je- 1 ^, where T i and Tj are the biological halt-times and physical half-time of *°Co To = 1925 days was taken into correction.

9

RESULTS

The longitudinal profile scanning measurements indicated that the major part of inhaled activity was deposited in chest region on the 15th day, and in this body region, associated with the lungs, a higher activity concentration is dearly revealable even on the 80th day (see Fig.3). F.L.'s whole body activity was found to be 2650 Bq on the 15th and 1100 Bq on the 80th day. Using ICRP dosimetric model the activity content in lungs was estimated to be 2200 Bq ( ~ 700 Bq/I) and 840 Bq (~ 300 Bq/I) respectively, moreover activity in the liver was calculated to be 45 Bq ( ~ 30 Bq/kg) and 26 Bq (~ 17 Bq/kg) only, that is contribution from the liver content to profile spectrum in chest region is negligible. The measurements show increasing 'tissue-background' on the 80th day according to the expectance that some portion of activity was transferred from the lungs to body tissues.

We could get more detailed information about the distribution of inhaled cobalt in the lungs from the rectilineal scan of chest region on the 15th day. The intensity distribution on Fig.4 shows two separate peaks according to the deposited activity into the right and left parts of lung lobes. Practically the same counting-profiles were got for both the upper and lower detector, which means a more or less homogeneous distribution in the whole lung volume. We assume that the higher counts on right lobe side proportional to the higher volume of right lobe as compared to the left ones. The well defined valley in the vertical body-axis proves that there was no considerable deposition in the bronchial tree after two weeks of inhalation.

Assuming single exponential clearence from 15th till 80th days a half-time of about 50 days lung retention was estimated for F.L.'s-case from the net counts summing in lung region of the longitudinal profile spectra (tissue-background was deducted). The early slope of F.L.'s whole body clearance curve was in good agreement with 50 days lung removal time estimated from profile scanning measurements.

INTERPRETATION OF THE RESULTS ON THE BASIS OF ICRP 30

The question is, how consistent are these measurements with inhalation and metabolic models proposed by ICPP to calculate internal dose for insoluble com­pounds of cobalt.

At the starting hypothesis in the report of ICRP Task Group on Lung Dynam­ics (ICRP,1966) for oxides and hydroxides of cobalt were determined to belonging to inhalation class "W, based on findings in animal experiments with clearance rates from pulmonary region between 10-100 days. This early arbitrary

10

Fraction oí Inhaled activity l e "to

class'W AMAD • 1 jam

0.01 —*

0.001

GI

0 10 20 30 40 50 60 70 80 90 100 Time (days)

Figure 5. Activity distribution and retention after inhalation of moder­ately soluble compounds of90 Co, assigned to inhalation class "W" (calculated on the basis of ICRP dosimetric model)

representation was reconsidered in ICRP 30 taken into account long term ex­periences reported by Newton and Rundo, who found in men longer than one year clearance from lungs (ICRP.1979). Recent ICRP Publications have kept to assign oxides and hydroxides of cobalt into the much more pessimistic inhalation class "Y" (ICRP.1982; ICRP.1988).

To demonstrate the differences between inhalation classes " W or "Y" ac­cording to activity distribution and retention in humans, Fig.5 and Fig.6 show the calculated early retention curves of the whole body as well as of the main body regions, which have importance in cobalt incorporation such as lungs, gas­trointestinal tract (Gl) and tissues. Here 'lungs' include trachea, bronchial tree, pulmonary lobes and lymph nodes. The upper nasal part was omitted, since its dose contribution is usually very small in comparison to the doses received by

11

Fraction ol inhaled activity

0.01

0.001' L

0 10 20 30 40 50 60 70 80 90 100 Time (days)

Figure 6. Activity distribution and retention after inhalation of poorly soluble compounds of*°Co, assigned to inhalation class "Y" (calculated on the basis of ICRP dosimetric model)

other respiratory regions. On the other hand, in scanning measurements it is well separated geometrically from the chest region.

From the figures we can see tha' during the first week a considerable amount of inhaled insoluble aerosols can be found in the gastrointestinal tract as a con­sequence of the mechanical pathway of particles along the ciliated airroutes to stomach by swallowing. This could influence the rapid whole body retention.

It is assumed that half of the cobalt entering the body fluids goes directly to a rapid urinary excretion. One-tenth of the other half is assumed to be transferred from body fluids to. K m and the remainder is assumed to be uniformly distributed among all other organs and tissues of the body. Fractions 0.6, 0.2 and 0.2 of cobalt deposited in liver and tissues are assumed to be retained with biological half-lives of 6, 60 and 800 days, respectively.

12

Comparing the two inhalation classes in respect of these measurements, from Fig.5 and Fig.6 one can see that after 10th day the whole body retention is practically determined by the speed of lung clearance in the first three months. From these model predictions one can conclude that while in the case of class

' " V more than 95% of deposited activity is present in lungs and the clearance is very slow, on the other hand in case of moderate lung retention (class "W") it is more rapid , and a higher portion of cobalt absorbs into the tissues.

In the light of the ICRP model we can distinguish three phase in the whole body retention following inhalation of radiocobal dust. During the first days dominant portion cf whole body activity can be found in the gastrointestinal tract. After this rapid short-term excretion the speed of the total retention seems to be governed by the lung clearance within 10 to 100 days. The long-term retention should be determined by the longest component of tissue clearance.

For the sake of comparison with ICRP model prediction, a two-exponential function was fitted to our measured data obtained after the 10-th days only to eliminate the individual uncertainties coming from the incíefinite conditions of the incident. A least squares method with logarithmic weighting was applied from the computer program package G E N P L O T . The results are summarized in Table 1. The calculated retention are drawn in Fig.2. The data have fitted to a two-exponential curve better than -f 3%, in H.I.-case the differences were about 15% between 30-180 days.

Our whole body measurements show that the esimated half-lives for the short-term whole body clearance are in the range of 19-77 days (see Table 1) and we may suppose that values are "lose to the actual removal time of ^Co from the lungs. The profile scanning measurements confirm assumption of class "W" type inhalation.

To investigate the longer pattern of whole body measurements, all up to date data were normalized to the 10th day whole b_-dy content. With the help of this delayed normalization we thought to make discernible the general rules in the investigated cases. On Fig.7 the normalized data are plotted together with the calculated retention curves of inhalation class "W" with different AMAD values (viz. 0.25 /<m, 1 /<m and 4 /im). This figure suggests that the measured data points are spread around the time function of inhalation of aerosols of AMAD= 4 /im.

The retention curves could fit better to observations if we take into account the individual varieties of lung clearance. A possible extension of ICRP lung model parameters using logarythmic interpolation was adopted to this problem and the calculation was computed with our quasi-analytical method (see Ap­pendix A). Analogously to Fig.7, the whole body retention was calculated in cases of different pulmonary removal times which span the lung clearance range

13

measured individually. The calculation was done assuming aerosob characterized by AMAD=1 /im (see Fig.8) and by AMAD=4 /im (see Fig.9). This latter con­dition seems to describe in a quite good manner the trend of long term retention of our investigated cases.

Fraction of whole body activity

0.01 250 500 750 1000

Time [days] 1250 1500

Figure 7. Calculated long-term whole body retention of inhaled90 Co (class nW) in cases of different AM AD values together with the individual mea­surements ( To compare, all data were normalized to the 10th day.)

14

Ю р Fraction of whole body activity

reference time ( t«10th day )

0.1 :

0.01

О measured dato

calculated retention I M I

/ T p = 15 doys ' T p = 50 doys

T r = 70 dovs

0 250 500 750 1000 1250 1500 Time [days]

Figure 8. Calculated long-term whole body retention of inhaled60 Co taken into account the individual lung clearance rate (AMAD= 1 fim)

10 Fraction of whole body activity

0.1

reference time ( t«10th day )

0.01 i -_

О meacured data

calculated retention AMAD » 4 urn

T P = 15 doys T p я 50 doys T r = 70 dovs

250 500 750 1000

Time [days] 1250 150v

Figure 9. Calculated long-term whole body retention of inhaled90Co taken into account the individual lung clearance rate (AMAD= 4 цт)

15

DISCUSSION

The observed whole body retention of our cases followed roughly a two-exponential time function between 10-1500 days. Based on the profile measure­ments, the removal times in the range of 20-70 days have been connected to activity leaving the pulmonary region, while the long-term exponential should be interpreted as the clearance of slowest component of systemic burden with biological half-times of 600-1000 days.

It is worthwhile to compare these findings to the published values discussed earlier. Essential information on rewieved cobalt inhalation cases was summa­rized in Table 2. Here biological half-times observed by the other laboratories were tabulated into three columns according to time interval where those were estimated by fitting to the measured whole body data.

The first column represents very rapid processes after inhalation attributed to short-term lung clearance, urinary and fecal excretion. In addition, one can not preclude the possibility of presense of some external contamination within accidental circumstances in the beginning period of investigation. It is obvious that a single-exponential fit should not be enough to describe these complex metabolical processes, but many times it can be useful in calculating the dose.

Getting over a short-term period, our study suggests that whole body re­tention of 3-10 week half-lives may occure due to lung clearance during same months after intake. It was also found that the measured data fit remarkably well to a one-exponential retention function (see the subplot in Fig.2).

There have been some difficulties in comparing our measurements with data observed by others. It can be inferred from Table 2 that a dominant part of the reported events had happened during maintenance operation in contaminated areas and all in these cases any adequate information on the quolity of the inhaled dust were not obtainable, moreover the exact täte and duration of accidental exposures were also uncertain.

A rather homogeneous group of accidents occured during manipulation with high activity cobalt sources, here it is likely that the formation of radioactive aerosols proceeds more or less the same manner and the physicochemical prop­erties, such as density distribution, size distribution, or "in vivo" solubility, of the formed particles could be of the same sort. Earlier Cofield (1963), then Jordan et al. (1965) investigated human incorporation cases where the condition may have been similar to our situation. On Fig.7 whole body activities normalized to the 10th day after inhalation published in these reports are shown together with our measured data, and we can find that those measurements are in very good agreement with present experiments. In spite of the apparently existing coinci-

16

dence of time patterns, Cotield estimated somewhat slower body clearence rate than we have got for our subjects. We think that there is no real difference of the biological oehaviour of cobalt in all investigated cases, but a time period shorter thin one year seems to be not long enough to distinguish the lung clearence from the long term tissue clearence in practice.

Up to now the widest and most convincing study on the long-term behaviour of inhaled ^Co was dene by Newton and Rundo (1971) and deductions from these have been considerably taken into account by ICRP recommendations. They found that the measured activity being present in the body many years after incorporation, half of them was in the lungs, while the remainder was attributed to 'systemic burden'. They bad got indirect evidence of an unchanging distribution in the lungs instead cf activity translocation to pulmonary lymph nodes as it should be expected from ICRP lung model.

In the early period after intake they could perform measurements for two of the subjects only, and these data suggested an existing rapid clearance similar we found during the first year.

In our investigated cases we also attempted to test a possible dishomogenity in activity distribution years later after the incorporation. A longitudinal profile scanning measurement was performed ever the subject having the highest whole body content (S.V.) after 1160 days, if a substantial portion of whole body activity were cumulated in lymph nodes with in activity concentration of 10-100 Bq/cm 3, then it should have been seen on the profile spectrum undoubtly, but there was no significant sign of any pronounced activity localization. So, this result did not give arguments against the conclusion that the long-to .л retention was determined in our studies by biological process of systemicaliy absorbed cobalt.

Other reliable data on long-term tissue clearance have been available from "in vivo" measurements of hur.ians after intravenous or oral intake. Smith et al. (1971) reported 830 days and 770 days long-term biological retention for two volunteers after intravenous administration of C0Co-chloride. Letourneau et al. (1972) measured whole body retention of healthy male subjects following i.V. injection of carrier free 58Co-chloride and a 608 days long term component issued from fitting five exponentials to the average of data. In our laboratory we also observed 625 days whole body retention of a female volunteer after oral intake of s7Co-chloride with carrier (Beleznay,1993). All these human data have fallen in the same range that we got recently on subjects after incidental inhalation of м С о aerosols.

Newton's and Rundo's cases covered a large scale of routine working situ­ations where different insoluble matrices of radioactive cobalt could be présen; and this fact can offer an explanation for observed biological half-lives over the

17

Tabic 2. Summary of rt^oritd cobalt inhalation cases

N D . References Inhaled

i sotope Inhalation

condition* Investigated

cases Til»« period froo the assumed ciate of inhalation

Measure­ments

Whole body biological retention tiae, Ъ (days) fitted in time-interval after inhalation in

<10 day 10 - oO days >100 days 1 Cotleid,R.Е.

(1964) « c . failure of a high

activity source 3 М П 0-290 days whole body

lung 10 90

2 Sill et al. (196Í)

6 < J C o maintanance workers cleaning operation

4 »en 1 man

- 2 months -11 months

whole body whole body

70-177 -700

3 Jordan et al. (1965)

«°CO operation with a high activity 4* irradiator

1 nan 1-120 days chest U,F 1.6 42 115

4 Newton,0.; Rundo,J. (1971)

*°Co maintenance workers maintenance worker minor reactor incident

3 men 1 ЛйП 1 «an

-1Я0- >2000 20-20- ;ooo

0-1*0 days whole body

chest 500-1000

50 4000-6000 1 3601200

5 Barnes et al. (1976)

6 Ű C O CoO and C03O. oxides with AMAD's 1.5-2 цт

beagle dogs 0-140 days

lung,lymphs U,F, liver spleen, boi.es

2-10 34-84

6 Riel et al. Í1977 6 0 C o

mechanical cleaning ooeration

3 men Э- 8 davs 0-29 ^ays

whole body U,F

<2

7 Hedge et al. (1979)

6 0 C O SKR power station workers

5 men -120 Jays -45П days

chest 604 „,« € 0 C o manipulation with a high activity source

6 men 1-1500 days

vhole body chost

<6 15-70 600-1000

chest region from 1 year up to 17 years. The slow clearance from lungs could not account for the origin of the long term systemic burden, it may suppose occurance of inhalation of cobalt aerosols of a more soluble forms which can be detected in the early period only.

In ICRP Publication 54 there is reference to a paper of Hedge et al. (1978) who reported 664 days lung retention pattern as an average of five BWR power station workers. We believe that a critical treatment of individual experimental data could have given a more informative picture.

CONCLUSION

The improved whole body counter technique and calibration gave us a good chance to follow ^Co accidental incorporation over 4 years. The two-detector profile scanning arrangement proved to be very useful to measure the activity distribution "in vivo" and to give information about the activity deposition in lungs.

The ICRP dosimetric model could give valuable help in planning the schedule of the measurement and in the interpretation of the observed retention pattern.

Our actual cases represented only one group of incidents inhaling fresh airborn cobalt containing aerosols. Thi measured whole body activity fitted perfectly to a single-exponential fuction between 10-80 days . The estimated 19-77 days biological half-times were associated with pulmonary lung clearance definitely. These experiences classified cobalt into the inhalation class "W".

To get a better dose estimation we proposed a possible extension of ICRP lung model taking into account the lung retention measured individually. This seems to have importance especially in the cases of moderately soluble cobalt compounds.

Here the lung clearance was assumed to be proportional somehow to the physiological solubility of the inhaled dust, as well as proportional to the fractions transferred to body fluids from the different regions of the respiratory tracts that is determined by the aerodynamic size distribution, pulmonary clearance. In Fig.10 the calculated long-term tissue absorption for three different AMAD values are drawn as a function of pulmonary clearance, lp, together with our observed data from Table 1.

We think, the agreement between the calculation and the measurements is satisfactory assuming particle size distribution with AMAD'; between 4-10 цт.

19

100 Absorption to tissues (%)

T, - Tp (days)

Figure 10. Correlaiion between the pulmonary clearance of inhaled **Co aerosols and the absorbed portion of cobalt ir, tissues estimated from long-term whole body retention (see Table 1)

20

APPENDIX A

A possible extension of ICRP lung model to fit individual inhalation cases in the range from 10 days to thousands of days of

long-term pulmonary clearance3

The ICRP lung model has been proposed by the Task Group of the ICRP for lung dynamics in the 1970-ies ([ICRP66],[ICRP73]), after an enormous amount of published material have been reviwed. This resulted in a general, simple, but adequate model of the structure and of the physiological processes of the respiratory system and in a set of descriptive parameters. The aim of this model was to provide a base for lung dosimetry and setting of limits for intakes of radionuclides to workers.

It seems to be reasonable to keep the main features of this model when those refinements are introduced which are able to describe individual inhalation cases.

According to deposition of dusts, the respiratory tract was devided into three regions, namely the nasopharyngx (N-P), the trachea and bronchial tree (T-B) , and the pulmonary parenchyma (P). Fundamental hypothesis of the model is that the deposition rate in each region is the function of the activity median aerody­namic diameter (AMAD) only. This assumption should be valid for any "poorly soluble" dust or aerosols . There are two main pathways for clearance from lungs: via body fluids or via the gastrointestinal tract. To consider the existing biological translocation mechanisms, every region in the model was divided into two or four subcompartments (see Fig.Al). Lung clearance was related to the chemical properties of the inhaled substances, this will affect both the long-term removal half time from the pulmonary part of lungs and the ratio between the two main pathways (via blood and via Gl tract) of the deposited aerosois in every region. Three inhalation classes (signed with letters D, W and Y) are defined and the characteristic removal half times and fractions for each compartments are given in ICRP 30 (see Table Al and Fig.Al). Here constants for long-term biological half times of 50 days and 500 days were chosen for inhalation classes W and Y, respectively.

In order to calculatt the individual internal dose burden from inhaled aerosols belonging to classes W or Y it is useful to base the calculations on such a model which is in harmony with ICRP recommendation.

In this paper we propose one possible solution to expand the model parameters 3This method was proposed by Doerfel in 1987 connected to adaptation ICRP 48 on

amerícium inhalation cases.

21

from 10 days to several thousands days while keeping the reference values of class Wand class Y.

The lung dose depends on the long-term pulmonary biological half Irfes ( T P ) above all, therefore our proposal in calculation of internal dose burden is to use the appropiate lung retention values measured individually for compartments (e), (g) and (h) in place of fixed constants of ICRF 30 (50 days for class W. 500 days for class Y) . Removal half times from (a), (b), (c), (d) and (f) compartments describe rapid clearance mechanisms, and in general on the basis of experimental data it is not possible to modify existing ICRP model clearance times. Similarly, without having quantitative very long-term clearance data about lymphatic sys­tem, there is no sense to change ICRP recommendation for biological half-time of compartments (i).(j)

It should be apparent from Table A l that the pathways from compartments (a) end (c), which determine the early uptake into the systemic distribution, are rather conditioned by the regional fraction of each. The clearance model pre­dicts about ten times higher amount of material absorbed in the blood in cases of breathing aerosols classified in W then Y. This implies the experiences that the early absorption is influenced by the same physico-chemical properties of inhaled material which governed the biological removal time(s) from the pulmonary re­gion. To modify the regional fractions ( F-factors in Table A l ) as a function of fp, log-log interpolation was adopted in the time range of 50 days to 500 days, simultaneously keeping the normalisation condition in every lung region, such as:

F. + Fb = 1, Fe + Fd=\,

Fe + FJ + Fi+Fk=\.

This consideration was extended to biological removal times less then 50 days and over 500 days. The share between the routes via compartments (i) and (j) wa-, calculated in similar manner.

22

Table AI. A possible extension of ICRP lung model parameter for mod­erately or poorly soluble compounds for pulmonary clearance from 10 days to 1000 days

Region Compartment ICRP 30 Extended model

class W class Y class W-Y T T T

[day} F [day] F [day] F

N-P a 0.01 0.10 0.01 0.01 0.01 i.V. Ь 0.4 0.90 0.4 0.99 0.4 i.V.

T-B с 0.01 0.50 0.01 0.01 0.01 i.v. d 0.2 0.50 0.2 0.99 0.2 i.v.

e 50. 0.15 500. 0.05 7> i.v. f 1. 0.40 1. 0.40 1. 0.40 g 50. 0.40 500. 0.40 1P 0.40 h 50. 0.05 500. 0.15 T> i.v.

i 50. 1.00 1000. 0.90 1000. i.v. j n.a. n.a. oo 0.10 oo i.v.

n.a. — not applicable i.v. — interpolated value (»ее Figure A.l)

23

Class W Class Y

0.011

0.001 10 100

Tp (days) 1000

Figure At. Calculated F-fractions in different lung compartments a-h as a function of pulmonary removal time, Tp

24

APPENDIX В

How does the aerosol size distribution affect on the effective dose?

In accidental circumstances an additional problem concerns the unknown physical properties of inhaled particles. We come to the conclusion from our mea­surements that the inhaled airborn dust should be characterized by AMAD ~ 4/tm.

To improve our understanding of the inhalation pattern, probable conse­quences were examined of this assumption about the realistic size distribution.

In the ICRP lung model a log-normal distribution is presun ed for all air­born dust. The deposition probabilities into the anatomical lung regions can be characterized by one parameter, namely the mass median aerodynamic diameter, MMAD. For radiation dose estimation it was useful to generalize these state­ments to be equally valid for a log-normal activity distribution, regardless of the relationship between particle size and activity, characterized by activity me­dian aerodynamic diameter, AMAD. As a noteworthy characteristic of this model was found its validity in wide range of standard geometric distribution a from 1.2 to 4.5. To illustrate this, calculated mass and count density distributions are shown on Figs Bl.a and Bl.b for MMAD = l/*m and MMAD = 3/iirt both in linear and logarithmic scale.

Unfortunately we did not find reports about airborn cobaltous aerosols. In­stead, several observed distributions for trace metals as Fe and Pb in airborn dust measured with cascade impactors are presented on Figures B2 and B3 (Davidson and Osborn, 1986).

In the case of Pb the MMAD varied from 0.3/jm to 3.8/jm with an average of MMAD = 0.53 ± 0.23fitn. In the literature it is generally accepted that airborn lead is derived primarily from the combustion of leaded gasoline, a process which produces mostly submicron particles.

Iron containing particles are associated mostly with a larger size range with an average of MMAD = 3.8 ± 2.0/tm. The airborn Fe are associated with soil dust following natural erosion, which mainly produces larger particles. There have been data about emission from nonferrous metal production into the atmosphere, viz. MMAD(Cu)= 1.5 ± 1.2/zm, MMAD(Cd) = 1.5 ± 1.3/im, MMAD(Zn) = 1.3 ± 0.8(tm and MMAD(Mn) = 2.4 ± 1.3/mv

Our findings for cobalt (AMAD ~ 4/tm ) suggest certain similarity to airborn iron containing particles.

25

The bulk densities of the cobalt compounds expected to exist in the inhaled air were calculated 6.5 g/cm 3 for CoO, 5.2 g/cm 3 for Coj0 3 and 6.1 g/cm 3 for C03O4, but little is known about the elemental density distribution of suspended particulates.

Barnes et al.(1975) used C03O4 and CoO aerosols for their animal experi­ments and determined 3.7 ±0.75 g/cm 3 and 6.0 ±0.94 g/cm 3 densities, respec­tively.

Sugimae (1985) presented the results of density distributions of atmospheric particulates (as Si, Fe, Mn, Zn, Cu), and his measurements indicated existing differences in elemental density distributions for the coarse ( > 3.3 /im ) and fine ( < 3.3 /im ) particles. The average density of submicron particles ranged from 1.1 to 1.7 g/cm 3 , while the densities of coarse were inthe broad range of 1.3 to > 3.3 g/cm 3.

There is a relation between the AMAD serves to characterize the deposi­tion probabilities and the observed particle distribution of density pp (Baláshizy, 1985):

AMAD = MMD*y/fp/x

where MMD(Mass Median Diameter) is a useful parameter of the aerosol, and the irregular shape of airborn particles can be taken into account by a factor of X = 1.33 - 1.67 (IAEA.1973).

To describe the inhalation of our cases we may assume with considerable caution 4-5 g/cm 3 average density and ~ 3 //m aerodynamic diameter, that means approximately 3 /*m mass median diameter.

In Tables B l , B2 and B3 dosimetric values are calculated for rem présen tat ive AMAD's and pulmonary clearance rates ( O P , 1979a).

26

Table BL Number of nuclear transformation over 50 years in source or­gans or tissues per unit intake of activity (transformations/Ba) tm'MCo

INHALATION CLASS W-Y

AMAD, /im 0.25 1.00 4.00 4.00 4.00 10.0 Tp, days 50 50 15 50 70 50

lungs xlö» 1.90 1.00 0.16 0.51 0.72 0.20 SI content xlO 3 6.50 690 7.80 9.90 10.3 11.8 ULI content xlO4 2.20 2.30 2.60 3.30 3.40 3.90 LU content xlO 4 3.90 4.20 4.70 5.90 6.20 7.10 liver xlO* 1.30 1.20 2.50 1.30 110 1.40 tissues xlO 8 1.20 1.00 2.20 1.20 1.00 1.80

27

Table BS. Ccmmitted dose equivalent in target organs or tissues per in­take of unit activity (HT9Sv/Bq) of*°Co

INHALATION CLASS W-Y

AMAD. pm Tp. days

0.25 1.00 4.00 4 00 4Ö0 10.U 50 *A 15 50 то 50

gonads 4.5» 394 690 4.59 4.39 6.47 breast 5 87 400 5.34 3.71 3.60 4.50 red marrow lungs Ш wall liver remainder

6.00 4.02 5.76 3.98 3.82 4.90 66 6 35.9 10.4 19.9 26.5 10.7 8.35 8.05 11.9 10.1 10.5 13.7 14.0 10.4 15.9 9.80 9.20 10.7 13.2 800 6.04 558 6.26 5.26

7ViWc B3. Weighted committal dose equivalent in target oryans or tissues per intake of unit activity (l<r9Sv/liq) ofmCo

INHALATION CLASS W Y

AMAD,/<m 0.25 1.00 400 4.00 4.00 10.0 TV, days 50 50 15 50 70 50

gonads 0.20 0.92 0.79 1.39 0.92 0.88 1.29 breast 0.05 0.29 0.20 0.27 0.19 0.18 0.22 red marrow 0.12 Ö.72 0.48 0.69 0.48 0.46 0.59 lungs 0.12 8.00 4.30 125 2.24 3.18 1.28 LLI wall 0.12 1.00 0.97 1.43 1.21 1.26 1.65 liver 0.05 0.70 0.52 0.79 0.49 0.46 0.54 remainder 0.05 0.66 0.40 0.30 0.28 0.31 0.26

TOTAL 12.3 7.70 6.10 5.81 6.70 5.80

28

ы

о i.

Froction per micron 9 P r О СП О n 1 T

II

I • I 4 I I I I • i l l

Froction per micron P r - !* N V (Л О СЯ О О» О

т СИ

ш с.

Fraction per micron о -(Я О ~r

Fraction per micron О — - • (O M Crf ся о ся о in b

MASS DISTRIBUTION COUNT DISTRIBUTION 3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

1.5

! i i i —

m -S i : ! -MMAD=1 um

"U-. ' 1 2 3 4

Pcrtide Diameter (/xm)

MASS DISrRIBUTlON

1.0 -

0.5

0.0

<гш\.г ff»1.5 »»2

— <J»4.5

MWAD=3 jum

1 2 3 Particle Diameter Cum)

2 3 3articie Diameter ( д т )

COUNT DISTRIBUTION !.S

С о

1.0

S.

и

0.0

I " • « " ' • г 1 Í ff-1.2 : - - a - 1 . 5 1 - -ff-2

i Í ,' \ MMAD-3 Mm -

j ' * - ^ » ; \ / \ 1 1 N • - - / \

A \ -

1 2 3 Particle Diameter (yum)

Figure Bl.a Representative particle sizes and mass distributions characterized by MMAD = \цт and Zpm,- respectively (in linear scale)

UOJOILU J9d UOIjwüJJ

31

Pb

Figwn BS. Distributions of uirboru Pb maaa with reaped to particle aizt (ref. David and Oaborn, 1985)

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35

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KFKM992-01/E A. Czitrovszky et al.: Laser analysis of aerosol panicles to calculate distribution of aerosol electric charges and size

KFKM992-02/A F. Glück et al.: Order-a radiative corrections for semleptonic decays oi polarized baryons

KFKM992-03/A L. Diosi. Coarse graining and decoherence translated into von Neumann Language

KFKI-1992-04/E L. Diósi: On the Calrieira-Leggett Master Equation

KFKM992-05/B+M J. Kadlecsik: Tensor Manipulation Package for General Relativity Calculations

KFKM992-06/B GS. Hall: Symmetries in General Relativity - a brief survey С Hoenseiaers: Nonlinear Partial Differential Equations and Solitons

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Ш.*Щ- «992-08/A K. Szlachanyi, P. Vecsernyés: Quantum Symmetry and Braid Group Statistics in G-spIn Models

MKM992-09/G A. RHCZ: Estimation of Quasi-Critical Reactivity

KFKI-1992-10/A+В A. Frenkei and F. Károlyházy: Progress in the Kmodel of Quantum mechanics

KFKI-1992-11/A J. Révai, Y. Nogami: On the theory of atomic ionization following alpha decay

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KFKH 992-13/A S. Krasznovszky: Analysis of the KW Distribution Using a Generalization of the Meijer's G-function

KFKM992-14/M Mária Törő: Performance Analysis of the IIASA Network

KFKI-1992-15/A F. Glück: Measurable distributions of unpolarized neutron decay

KFKI-1992-16/J Sz. Tőrök, Sz. Sándor et. all: Source apportionment of individual aerosol partides at Hungarian background stations

KFKI-1992-17/G G. Pór, L. A. Sokolov "KAZMER" A Complex noise diagnostic system for 1000 MWePWRWWER Type Nuclear Power Units

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KFKM992-19/G.I P. Pellionisz, S.K. Jha and 6 1 . Goswami: Acoustic Emission Experiments for PHWR Technology Development

KFKI-1992-20/K P. Zombori, A. Andrási, I. Németh: A Nrw Method for the Determination of Radionuclide Distribution in the Soil by In Situ Gamma-Ray Spectrometry

KFKI-1992-21/A L. Díósí: On Digital Randomness of Quantum Coordinates

KFKM992-22/B С Hoenselaers, Z. Perjés: Remarks on the Robinson-Trautman solutions

KFKM992-23/G B. Lukács and A. Rácz: Objective Judgement by Kaiman Filtering in the Generalized Landsbergian Scheme

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KFKM992-26/G J. Végh, J. Huszár, J. Láz: Development of an X Window based operator's interface for a core monitoring system

KFKM992-27/G Vértes P.: Code lor Calculation of Spreading of Radioac'Vity in Reactor Containment Systems (TIBSO code)

KFKM992-28/B,G B. Lukács, A. Rácz: Killing Symmetries in Neutron Transport

KFKM992-29/D Z. Donkó and M. Janossy: Modeling the cathode region of noble gas mixture discharges using Monte Carlo simulation

KFKI-1992-30/A P. Varga: To the Problem of the Coherence Length of Neutrons

KFKM992-31/A P. Lévai and J. Zimányi: Multi-heavy baryons in ultrarelativistte heavy ion collisions

KFKM992-32/C B.Lukács, Sz. Bérezi. E. Lábos. I. Molnár: Mutual Dynamics of Organisation Leveis in Evolution

KFKM992-33/G E. Végh. J.S. Jánosy, P. Ivancsics: Extension of the Paks fuü-scalesSImulator to severe accidents

KFKM992-34/M,N Sz. Ferenczi: Proceedings of the 1st Austrian-Hungarian Workshop on Transputer Application

KFKM992-35/A LP. Csernal. J.I. Kapusta, Gy. Kluge and E.E. Zabrodin: Phase Transition Dynamics In Ultrarelativistic Heavy Ion Collision

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Kiadja a Központi Fizikai Kutató Intézet Felelős kiadó: Gadó János Szakmai lektor: Koblinger László Nyelvi lektor: Kobllnger László Példányszám: 191 Készült a PROSPERITÁS Kit nyomdaüzemében Budapest, 1993. március hó