IEEE Transactions on Nuclear Science, Vol. NS-27, No. 1, February 1980

POTENTIAL HEALTH EFFECTS OF MAN-MADE ACTINIDES COMPARED WITHNATURAL RADIONUCLIDES

Naomi H. Harley*New York University Medical CenterInstitute of Environmental Medicine

550 First AvenueNew York, NY 10016

Summary

Alpha emitters within the body are known to pro-duce both bronchogenic carcinoma (lung cancer) andosteosarcoma (bone cancer). The alpha dose from natu-rally occurring radionuclides to these critical sitesis compared with that which might be expected fromthe alpha emitters produced in nuclear power reactors(plutonium, americium and curium). A background doserate of about 9500 mrad to cells in bronchial epithe-lium and 100 mrad to cells on bone surfaces for a50 year exposure is estimated for the total of allthe natural radionuclides. About 0.25 and 2 mradrespectively are estimated for a 50 year exposure tothe man-made actinides when soil is contaminated tothe same level as is found for the natural actinidesuranium and thorium (about 1 pCi/g). Since the poten-tial for cancer induction ought to be related to alphadose, contamination of the environment with man-madealpha-emitting actinides can be evaluated on thisbasis.

Introduction

Most of the criticism regarding the nuclear in-dustry apply specifically to the potential healtheffects from radioactivity. The main concern is forthe man-made alpha emitting actinides that are pro-duced in any nuclear reactor. These are the alpha

238, 239, 240 241, 243 242, 244emitters ' ' Pu, ' Am and ' Cm.All of these nuclides have relatively long half-lives

(except 42Cm) (see Figure 1), and return by decayinto their original elements, naturally-occurringuranium or thorium.

If these nuclides are dispersed onto soil in theenvironment, they will undoubtedly be inhaled throughresuspension of particulates. Ingestion is not con-

sidered a significant route of uptake since absorptionin the G.I. tract is low. Through inhalation theydeliver an alpha dose to cells in bronchial epitheliumin the upper respiratory tract that are thought to becritical in initiation of bronchogenic lung cancer.

Most inhaled material is deposited in the lower lungrather than the bronchial tree and actinides depositedand retained here translocate primarily to bone sur-

faces and liver with half times of the order of500 days. Alpha emitters on bone surfaces can irra-diate nearby osteoprogenitor cells thought to be thetarget cells in osteosarcoma production. Only thesetwo sites, the upper respiratory tract and bone havebeen implicated in radiation-related human cancers

from alpha emitters within the body.

*The author gratefully acknowledges support from cen-

ter programs from the National Institute of Environ-mental Health Sciences (Grant ES-00260) and theNational Cancer Institute (Grant CA-13343). Thecomputation for this study was done at CourantInstitute of Mathematical Sciences, New YorkUniversity, supported by DOE contract EY-76-C02-3077.

This type of particulate inhalation is not with-out precedent, since naturally-occurring actinidessuch as 234, 238U and 228, 230, 232Th in the earths

crust behave similarly and enter the body throughinhalation of resuspended soil. Other naturally-

226 210occurring alpha activities, Ra and P0o, are knownto be present in the skeleton through dietary intake

of 226Ra, 210Pb (the grandparent of Po) and Po

itself. Both Ra and its daughter products residemainly in the skeleton and deliver an alpha dose tocells on bone surfaces. The short-lived radon daugh-ters normally present in all environmental atmospheres,deposit on bronchial surfaces and deliver their doseto cells in bronchial epithelium. These radon daugh-ters are currently thought to deliver the highest doseto humans of any of the natural radionuclides.

It is the purpose of this work to evaluate thedose that man-made actinides could potentially deliverin an accident, and the populations likely to be ex-posed. These are compared with the alpha dose receivedby the global population from the naturally-occurring

actinides, as well as R ,210po radon and itsshort-lived radon daughters all of which must beconsidered when assessing radiation-related healtheffects from very low levels of alpha activity.

Natural Activities

Radium

Fisenne et al. have analyzed 360 samples of humanvertebrae from 13 countries for naturally-occurring

Ra and calcium. They indicate that the 6Ra con-centration in 130 samples all from New York Cityappears to be distributed log normally. There was nodifference in concentration either with age or sex.

Naturally-occurring 226Ra is derived through dietaryintake and the concentration has been shown to beuniform throughout the skeleton. Combining their datawith other published results, a population distribution

for 26 countries with a total of 1.3 x 109 persons(35% of the world's population) is derived which shows

that the median Ra concentration is 23 fCi/gCawith a geometric standard deviation of 1.6. The doseto cells on bone surfaces from the median measuredconcentration is 0.4 mrad/year. Because of the largenumber of samples and countries represented, thisvalue is probably representative of the dose the globalpopulation receives from this natural alpha emitter.

Many areas with elevated 26Ra are known to exist butthese are not considered in this work. The averagedietary exposure is given in Table 1.

Radon and Its Short-Lived Daughters

Radon-222 is a naturally-occurring alpha emitting

noble gas that is the daughter product of 6Ra. Radonis liberated from radium near the earths surface. Its

0018-9499/80/0200-0682$00.75© 1980 IEEE682

Figure 1. Actinide Decay Scheme

1a/6

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TABLE 1. NATURALLY OCCURRING ALPHA EMITTERS AND THEIR AVERAGE DOSE RATE TO CRITICAL CELLS

Bronchial Dosetmrad mradYear 50 Year

Bone Cell Doset±mrad mradYear 50 Year

0.4 20

DietAirCigarettes

210Po

222

2RnRnDaughters (RaA, RaB, RaC)

DietAirCigarettes

Air

Air

1.25 pCi/day0.01 pCi/m30.0004 pCi/mg tar

1.63 pCi/day0.003 pCi/m3J0.001 pCi/mg tar

X 200 pCi/m3

X 180 pCi/m3

0. 05 2.5 1.7 84

1.0 50 4.7 230

10 0.05 2.5

190 9500

Naturally-Occurring Actinides

228Th

230Th232Th

234u238u

Air.,

it

it

it

tAlpha dose rate to shallow basal cells in bronchial epithelium 22 pm belou

0.05

- 0.02 0.5

- 0.002 0.3

0.02 **

- 0.002 **

w the surface in segmental bronchi.

ttAlpha dose rate to cells 10 pm from bone surface.

228Th bone dose rate not estimated since the majority in bone is formed from 8Ra in Bone.

Uranium bone surface dose rate not estimated since its behavior in bone is not well documented.

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ExposureRoute

Diet

Exposure

1.6 pCi/day

half-life is 3.8 days and since there are no majorsinks in the environment, it becomes widely dispersedin the atmosphere. The global average outdoor radonconcentration over the continents and near ground

level is reported to be about 200 pCi/m3 2. Indoorconcentrations can be many times higher since thebuilding structure acts to contain the radon releasedfrom underlying soil. Ventilation with outdoor airof 4 air changes per hour will reduce indoor concen-trations to outdoor levels. However, this high arate is rarely attained and so indoor levels addsubstantially to population dose.

Radon gas is soluble in body fluids and fat.

This solubility allows a dose rate of 2.5 x 107 radyear

per pCi/m to be delivered to cells on bone surfacesfrom radon (and short-lived daughters) dissolved in

fatty marrow .

Short-lived 22Rn daughters 218Po(RaA), 214Pb(RaB)and Bi(RaC) are present in all environmental atmo-spheres. Their concentrations can be expressed as

ratios to that of their parent Rn activity(Rn/RaA/RaB/RaC) and the ratio is normally near

1/0.9/0.7/0.7 . These short-lived daughters are foundnormally attached to the ambient aerosol (except inthe case of RaA which is partially unattached(unattached RaA/Rn % 7%). A few percent of theseaerosol particles deposit in the bronchial tree andthus deliver an alpha dose to cells in bronchialepithelium. All of the unattached RaA deposits inthe bronchial tree and so delivers a disproportion-ately large fraction of the dose. Measurements madein 23 homes in the New York New Jersey area by George

and Breslin showed that the average indoor Rn

concentration was 830 pCi/mi and the average outdoor3

concentration was 180 pCi/m . The annual bronchialdose from an exposure to these concentrations isestimated to be 190 mrad per year.

Pb and 20Po

20Pb and l Po are inhaled, since these nuclidesare produced through decay of airborne Rn, as well

5as ingested through dietary intake . The major routeof entry to the body for nonsmokers is ordinarily

through the diet (1.2 pCi Pb/day, 1.6 pCi210 6

Po/day) . However, for smokers it has been shownthat the skeletal burden of 210Pb and 210Po is aboutthree times higher then for nonsmokers (680 pCi 210Pofor smokers versus 240 pCi Po for nonsmokers).Mainstream cigarette smoke contains both Pb and

l pO and measurements indicate the concentration to

be about 1 fCi Po/mg tar and 40% of this value210 8 210for Pb . It is the Po which delivers the alpha

dose to the skeleton since Pb is a beta emitter.Most of the Po present in the skeleton, however,is thought to be supported by its longer-lived 20Pb

grandparent since the biological half-life of 210Poin body tissues is short. An estimate of the dose to

cells on bone surfaces from these levels of Po inthe skeleton is about 5 mrad per year for smokersand about 2 mrad per year for nonsmokers.

An estimate of the bronchial alpha dose from

Po arising from both inhaled Pb and Po insmokers and nonsmokers is 0.3 mrad per year and 0.05mrad per year. These calculated values have recentlybeen given support by detailed measurements withtrack etch film in the bronchial tree of deceased

9smokers and nonsmokers

Natural Actinides

The nominal concentration of the natural acti-

nides 228, 230, 232Th in soil is 1.0 pCi/g for eachisotope. The nominal concentration of the natural

actinides 234, 238U is 0.6 pCi/g for each isotope.To establish a reference atmosphere for these nuclidesin the lower atmosphere it is assumed that the mass

loading of air due to resuspended soil is 100 pg/m3

Measured values of this quantity do not vary by morethan about a factor of two. The nominal atmosphereis thus 0.1 fCi/m3 each for 228, 230, 232Th and

0.06 fCi/m3 for 234, 238U. This leads to dailyinhalation intakes of 2 fCi and 1.2 fCi for eachthorium or uranium nuclide, assuming a breathing rate

of 20 m per day.

Lucas has shown that the skeletal burden ofnatural thorium increases linearly from zero at birth

to about 1.3 pCi at age 90. Harley and Pasternack11have shown that this skeletal burden is consistent

with the inhalation intake of 0.1 fCi/m using the

ICRP Task Group Lung Model parameters for transloca-tion of material from lung to bone. They estimatedthe doses from these nuclides to cells in bronchialepithelium and to cells on bone surfaces. The doseto cells on bone surfaces was estimated for a 50 yearexposure interval so that a long-term comparisonmight be made with that obtained from man-made acti-nides if an accidental release produced persistentair concentrations. The dose to cells on bone sur-faces from any of the actinides is complicated by thefact that the actinide translocated from the lung isdeposited uniformly on bone surfaces and thereafter,resorption and accretion mixes this surface deposituniformly at a rate of about 8 pm per year untilthe entire bone volume is homogeneously contaminated.This burial process requires about 10 years beforeuniform distribution is observed and the dose rateto the sensitive surface cells from the originaldeposition is decreasing during this time, becomingconstant after uniform distribution is retained.This is thought to be a reasonably faithful represen-tation of the way actinides behave in human bone.For calculation, each individual inhalation intake(of one month's duration) and the dose to target cellslocated 10 pm from the bone surface is followed for50 years as material deposited in the lung is removedto bone surfaces and then redistributed with burial.The total dose is accumulated by summing that fromeach individual exposure over the 50 years.

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TABLE 2. MAN-MADE, ALPHA EMITTING ACTINIDES AND THE DOSE RATES EXPECTED FORA 50 YEAR EXPOSURE TO THE REFERENCE ATMOSPHERE

Bronchial Dosemrad50 Year

0.06

0.06

0.06

0.08

Bone SurfaceDosemrad50 Year

0.50

0.48

0.48

0.38

238Pu

239Pu

241A

244cm

ExposureRoute

Air

Air

Air

Air

Exposure

0.1 fCi/m3it

It

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The details of the dose calculation are given in refer-11

ence . The dose estimates are given in Table 1.

Man-Made Actinides

In the event of a major contaminating accident,the man-made actinides 238 239, 240Pu, 241Am nd242 244Cm would be dispersed in the environment. Itis unlikely that this dispersal could be global exceptin the event of extensive nuclear war or weaponstesting. Weapons testing has already distributed

350000 Curies of ' Pu over the earths surfaceresulting in an average surface soil concentration in

2the U.S. of 0.002 Ci/km . Several accidents in theU.S. have been reported which resulted in soil contam-ination with the long-lived, man-made actinides. Oneexample was a partial filter breakthrough at a weapons

reprocessing plant that released 4 Ci of airborneplutonium to an unplanted field. Surface soil contam-ination averaged 3.2 pCi/g. As a reference contami-nation in this work a soil level of 1.0 pCi/g foreach of the man-made actinides is assumed to maintain

3an air concentration of 0.1 fCi/m . The 50-year doseestimates to bone surface cells and bronchial epithe-lium are given in Table 2. The dose from an actualcontaminating event could be determined using thesoil ratio as the multiple of the dose estimatesreported here.

Discussion

All of the alpha emitters that should be consider-ed when assessing human health effects from low levelsof alpha radioactivity are shown in Tables 1 and 2.The approximate exposures and modes of entry to thebody are also shown. The annual dose rate or the50 year dose is also shown for both bronthial epithe-lium and bone surfaces. It can be seen that, in allcases, the dose from natural radionuclides (particu-

210larly from radon daughters in the lung and Po inthe sekeleton) far exceeds that from a contaminatingevent that maintains a soil level of 1 pCi/g and anair concentration of 0.1 fCi/m3 for each of the man-made actinides for 50 years. The dose rate from thenatural emitters is delivered to the global populationwhile that from a contamination from the man-madeactinides would affect a local population.

This does not say that the effects of the man-made alpha emitters in initiating bronchial or bonecancer are negligible. It has been estimated, forexample, that about 1/5 of the spontaneous (not relat-ed to smoking) bronchogenic lung cancer in the U.S.,or about 10 cancers per million persons per year,can be attributed to naturally-accurring radon

daughters 4

The same kind of reasoning is difficult to applyin the case of osteosarcoma or bone cancer. Based onthe experience of the radium dial painters, thereappears to be a threshold for cancer induction ofabout 1000 rads to the cells on bone surfacesHowever, if there is a low-lying linear componentas has been suggested 6, a comparison may be establish-ed using the total dose from the naturally-occurring

226 210actinides, Ra and Po as a basis for comparison.From Table 2, it can be seen that a contaminatingevent which gives a soil level of 1 pCi/g will delivera 50 year dose that is about 1/50 of that deliveredby background radionuclides. The bone dose from

210Po due to smoking is 100 times as great.

On this basis it should be possible to establisha realistic approach to potential carcinogenic effectsof the man-made actinides in the environment and todetermine the level which will insure that there willbe no measurable increase in cancers throughaccidental environmental exposure.

References

1. Fisenne, I. M., H. W. Keller. The Worldwide

Distribution of Ra in Asked Human Bone.USDOE, Environmental Measurement LaboratoryQuarterly EML 356, July 1979.

2. NCRP 45, Natural Background Radiation in theUnited States, National Council on RadiationProtection and Measurements, Washington D.C.1975.

3. Harley, N. H., B. S. Pasternack. The Beta Doseto Critical Human Tumor Sites from Krypton-85,Health Physics 33, 567-575, 1977.

4. George, A. C., A. J. Breslin. The Distributionof Ambient Radon and Radon Daughters in Residen-tial Buildings in the New York-New Jersey AreaIn The Natural Radiation Environment III, Univ.of Texas School of Public Health, April 1978,Houston, Texas.

5. Parfenov, Y. D. 1 Po in the Environment and inHuman Organism, IAEA Atomic Energy Review 12,75-143, 1974.

6. Spencer, H., R. B. Holtzman, L. Kramer,F. H. Ilcewicz. Metabolic balances of Pb and210Po at Natural Levels,Radiation Res. 69, 166,1977.

7. Holtzman, R. B., F. H. Ilcewitz. 210Pb and 210Poin Tissues of Cigarette Smokers, Science, 153,1259-1260, 1966.

8. Cohen, B. S., M. Eisenbud, N. H. Harley. AlphaRadioactivity in Cigarette Smoke, Submitted toRadiation Research, October, 1979.

9. Cohen, B. S., M. Eisenbud, N. H. Harley. Measure-ment of the Alpha Radioactivity on the MucosalSurface of the Human Bronchial Tree. Submittedto Health Physics, October 1979.

10. Lucas, H. F., D. N. Edgington. Natural Thoriumin Human Bone, Health Physics, 19, 739.

11. Harley, N. H., B. S. Pasternack. PotentialCarcinogenic Effects of Actinides in theEnvironment, Health Physics (In Press).

12. Harley, J. H. World Wide Fallout from WeaponsTests, USAEC Report LA-4756, December, 1971.

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13. Milham, R. C., J. F. Schubert, J. R. Watts,A. L. Boni, J. C. Cury. Measured PlutoniumResuspension on an Old Field at the SavannahRiver Plant in the Southeastern United States,in Transuranium Nuclides in the Environmentp. 409 IAEA, Vienna, 1976.

14. Harley, N. H., B. S. Pasternack. A Model forPredicting Lung Cancer Risks Induced by Environ-mental Levels of Radon Daughters. Submittedto Health Physics, July 1979.

15. Evan$,R. D. Radium in Man, Health Physics 27,497-510, 1974.

16. Marshall, J. H., P. G. Groer. A Possible LowLying Linear Component in the Induction of BoneCancer by Alpha Radiation. In Energy and Health,Ed. N.E. Breslow and A. Whittemore, procedingsof a SIMS Conference Alta, Utah, June, 1978.

17. The author would like to acknowledge Gordon Cookfor photographic reproduction of the Figureand Aimee Miranda for preparation of themanuscript.

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