Fallout From Nuclear Weapons Tests in the 50's and Cancer Risks

8/7/2019 Fallout From Nuclear Weapons Tests in the 50's and Cancer Risks

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Fallout from Nuclear Weapons Tests

and Cancer Risks

Exposures 50 yea rs ag o still h ave h ea lth implicatio nstoday that w ill continue into the future

Steven L . Simon, Andre Bouville and Charles E. Land

P rior to 1950, only limited consider-ation was given to the health im-pacts of worldwide dispersion of ra-dioactivity from nuclear testing. But in

the following decade, humanity beganto significantly change the global ra-diation environment by testing nuclear

StevenL. S imon r ec ei ve d a P hD. i n r ad io lo gi ca l h e al ths c ie n ce s f rom Co lo r ado S ta te Un iv er si ty i n1985. Heh as s er ve d o n th e fa cu lti es o f t he Un iv er sit y o f U ta hand t he Un ive rs it yo fNo rt h C a ro li na a t C h ap el H ill .H e s pe nt f iv e y ea rs d ir ec ti ng th eM a rs ha ll I sl an dsNa ti onw ide Rad io log ic a l S tudy and a ls o pa rt ic ip a tedi n t he r adi ol og ic a lmon it or ing o f t hef o rm e rnucleart es t s ite s a t J oh ns to n I sla nd , i n A lg er ia , a nd i n F re nc hPo lynes ia . S imon joi ned t he Na ti ona l Cance r I ns ti tu te(NCD in 2 00 0 a nd n ow fo cu se s o n r etr os pe ctiv e d os e

e s timat ion i n suppo rt o f ep id em io log ic s tud ie s o f r adi o-a c ti ve f a ll ou t and occupat ional expo sur e i n med ic in e .A nd re B ou ville w as b or n in F ra nc e a nd o bta in ed h isP hD. i n p hy sic s a t t he Un iv er sit e P a ul -S ab ai ie r inTou lo us e in 1 97 0. H e s er ve d a s a c on su lta nt to th eUn it ed Sc ien ti fi c Commit te e on t he E f fe ct s o fA tom icRadiat ion for30 year s and t he I nt er n at ional Commis -s ion on Rad io log ic a l P r o te ct ion ( ICRP)fo r17 years.B o uv ill e j oin ed th e N a tio n al C a nc er I ns tit ute i n 1 98 4a nd h as s in ce b ee n h ea vi ly in vo lv ed i n t he e stima tio no frad ia t ion doses resul tingf r omradioactive falloutf r oma tmo sp he ric n uc le ar w e ap on s t es ts a nd fr om t heChornobyl acc iden t. Char lesE. Lan d r ec ei ve d a P hD .i n s ta ti st ic s fr om t he Un iv er sit y o f Ch ic ag o. H e s tu d-i ed t he r is k o f r adi at ion -r e la te d c ancer a t t he A tomicBomb Ca su alt y Comm is sio n a nd th e R a dia tio n E ff ec tsRe se a rch Founda ti on i n H i ro shima, J apan , b e fo r e jo in -i ng t heNCI in 1975. L an d s er ve d o n th e I CRPfo r20y ea rs a nd w as in str um en ta l in p ro du cin g th e 1 98 5Na ti ona l I ns ti tu te s o f Hea lt h r adi o- ep id em io log ic a l t a-b le s a n d t he 2 00 3 NC I-CDC i nte ra cti ve c omput er p ro -g r am deve loped t o a s s is t a d jud ic a ti on o f compen sa ti onclaimsfo rc ancer s f ol low ing occupat ional expo su r es t orad ia t ion . His resea rch in te res t s include quan ti fy ingl ife time rad ia t ion- re lated cancer r i sk wi th emphasi son impl ic a ti on s o f unce r ta in ty f or publ ic po li cy. Ad -d re ss f or S imon: D iv is io n o f C a nc er E p id em io lo gy a ndGenet ic s, Na ti ona l Cance r I n st it ut e, Na ti ona l I n st i-tuteso fHea lt h, 6120 Execu ti ve B lvd ., Be th e sda ,MD

20892 . Ema il :[email protected].

weapons in the atmosphere. By the ear-ly 1960s, there was no place on Earthwhere the signature of atmosphericnuclear testing could not be found in

soil, water and even polar ice.Cancer investigators who specialize inradiation effects have, over the interven-ing decades, looked for another signatureof nuclear testing-an increase in cancerrates. And although it is difficult to de-tect such a signal amid the large num-ber of cancers arising from "natural" or"unknown" causes, we and others havefound both direct and indirect evidencethat radioactive debris dispersed in theatmosphere from testing has adverselyaffected public health. Frequently, how-ever, there is misunderstanding aboutthe type and magnitude of those effects.Thus today, with heightened fears aboutthe possibilities of nuclear terrorism, itis worthwhile to review what we knowabout exposure to fallout and its associ-ated cancer risks.

Historical BackgroundThe first test explosion of a nuclearweapon, Trinity, was on a steel tower insouth-central New Mexico on July 16,1945. Following that test, nuclear bombswere dropped on Hiroshima and Naga-

saki, Japan, in August of 1945. In 1949,the Soviet Union conducted its first testat a site near Semipalatinsk, Kazakh-stan. The U'.S; the Soviet Union andthe United Kingdom continued testingnuclear weapons in the atmosphere un-til 1963, when a limited test ban treatywas signed. France and China, coun-tries that were not signatories to the1963 treaty, undertook atmospheric test-ing from 1960 through 1974 and 1964through 1980, respectively. Altogether,504 devices were exploded at 13 prima-

ry testing sites, yielding the equivalent

explosive power of 440 megatons ofTNT ( se e F ig ure 2 ).

The earliest concern about health ef-fects from exposure to fallout focused on

possible genetic alterat ions among off-spring of the exposed. However, herita-ble effects of radiation exposure have notbeen observed from decades offollow-upstudies of populations exposed either tomedical x rays or to the direct gamma ra-diation received by survivors of the Hiro-shima and Nagasaki bombs. Rather, suchstudies have demonstrated radiation-re-lated risks ofleukemia and thyroid cancerwithin a decade after exposure, followedby increased risks of other solid tumorsin later years. Studies of populations ex-posed to radioactive fallout also point toincreased cancer risk as the primary latehealth effect of exposure. As studies ofbiological samples (including bone, thy-roid glands and other tissues) have beenundertaken, it has become increasinglyclear that specific radionuclides in falloutare implicated in fallout-related cancersand other late effects.

Nuclear Explosions: The BasicsNuclear explosions involve the suddenconversion of a small portion of atomicnuclear mass into an enormous amount

of energy by the processes of nuclearfission or fusion. Fission releases energyby splitting uranium or plutonium at-oms, each fission creating on averagetwo radioactive elements (products), onerelatively light and the other relat ivelyheavy. Fusion, triggered by a fission ex-plosion that forces tritium or deuteriumatoms to combine into larger atoms, pro-duces more powerful explosive yieldsthan fission. Both processes create threetypes of radioactive debris: fission prod-ucts, activation products (elements that

become radioactive by absorbing an ad-

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ditional neutron) and leftover fissionablematerial used in bomb construction thatdoes not fission during the explosion.

A nuclear explosion creates a largefireball within which everything isvaporized. The fireball rises rapidly,incorporating soil or water, then ex-pands as it cools and loses buoyancy.The radioactive debris and soil that are

initially swept upwards by the explo-sion are then dispersed in the direc-tions of the prevailing winds. Falloutconsists of microscopic particles thatare deposited on the ground.

How People Are Exposed to FalloutThe radioactive cloud usually takes theform of a mushroom, that familiar iconof the nuclear age. As the cloud reachesits stabilizat ion height, it moves down-wind, and dispersion causes vertical andlateral cloud movement. Because wind

speeds and directions vary with altitude(Figure 3), radioactive materials spreadover large areas. Large particles settle lo-cal ly, whereas small particles and gasesmay travel around the world. Rainfallcan cause localized concentrations farfrom the test site. On the other hand,large atmospheric explosions injected ra-dioactive material into the stratosphere,10 kilometers or more above the ground,where it could remain for years and sub-

Figure 1. Between 1945 and 1980, the U.S. , the

U.S.S.R, the U.K., France and China carried

out more than 500 atmospheric test s of nuc learweapons totaling the explosive equivalent of

440 megatons of TNT. These tests injected ra-

dioactive material into the atmosphere, much

of which became widely dispersed before be-

ing depos ited as fa llout . Cancer invest igators

have been s tudying the heal th effec ts of radio-

active fallout for decades, making radiation

one of the bes t-unders tood agents of envi ron-

mental injury. The legacy of open-air nuclear

weapons t esting incl udes a small but si gni fi-

cant increase in thyroid cancer, leukemia and

certain solid tumors. Mushroom clouds, such

as the one from the 74-kiloton test HOOD on

July 5, 1957 (detonated from a ball oon at 1,500

feet a lt itude), a re a universal ly recognized icon

of nuclear explosions. The charact eristic cap

forms when the fireball from the explosion

cools sufficiently to lose buoyancy. HOOD

was the largest atmospheric test conduct ed at

the Nevada Test Site (and in the continental

U.S .). Fortuna tely, the U.S., the Soviet Union

and the U.K. stopped atmospheric testing in

1963,when the nat ions s igned the Limi ted Tes t

Ban Treaty. (France ceased atmospheric testing

in 1974 and China in 1980.) President John F.

Kennedy signed the treaty on October 5, 1963

(bottom). (Top phot ograph from the Nevada

Tes t Si te , U.S. Depar tment of Energy. Bot tom

photograph from the National Archives.)

www.americanscientist.org 2006 Sigma Xi, The Scientific Research Society. Reproduction

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United States

United Kingdom

6 5, 1 0 9Marshall Islands

41,10Mururoa

12,21Johnston

Atoll

6,7 /3 ,1Christmas Island/

Malden Island

J 4 . P91,239

Novaya Zemlya

Figure 2. Primary atmospheric nuclear weapons test sites were widely distributed around the globe, with South America and Antarctica the

only continent s to be spared. No spot on Eart h escaped the fallout, however, as larger tests injected radioacti ve material into the stratosphere,

where it could remain for several years and disperse globally. The numbers shown at each test site indicate the number of tests and (following

the comma) the total yield in the equivalent of megatons of TNT. (Data here and in Figures 3 and 7-10 from Ncr 1997.)

sequently be deposited fairly homoge-neously ("global" fallout). Nuclear testsusually took place at remote locations atleast 100 kilometers from human pop-ulations. In terms of distance from thedetonation site, "local fallout" is within50 to 500 kilometers from ground zero,"regional fallout" 500-3,000 kilometersand global fallout more than 3,000 ki-lometers. Because the fallout cloud dis-perses with time and distance from theexplosion, and radioactivity decays overtime, the highest radiation exposures aregenerally in areas of local fallout.

Following the deposition of fallouton the ground, local human popula-

tions are exposed to external and in-ternal irradiation. External irradiationexposure is mainly from penetratinggamma rays emitted by particles onthe ground. Shielding by buildingsreduces exposure, and thus doses topeople are influenced by how muchtime one spends outdoors.

Internal irradiation exposures canarise from inhaling fallout and absorb-ing it through intact or injured skin,but the main exposure route is fromconsumption of contaminated food.

Vegetation can be contaminated when

fallout is directly deposited on exter-nal surfaces of plants and when it isabsorbed through the roots of plants.Also, people can be exposed when theyeat meat and milk from animals graz-ing on contaminated vegetation. In theMarshall Islands, foodstuffs were alsocontaminated by fallout directly depos-ited on food and cooking utensils.

The activity offallout deposited on theground or other surfaces is measured inbecquerels (Bq),defined as the numberof radioactive disintegrations per sec-ond. The activity of each radionuclideper square meter of ground is importantfor calculating both external and internal

doses. Following a nuclear explosion,the activity of short-lived radionuclidesis much greater than that of long-livedradionuclides. However, the short-livedradionuclides decay substantially dur-ing the time it takes the fallout cloud toreach distant locations, where the long-lived radionuclides are more important.

Iodine-131, which for metabolic rea-sons concentrates in the thyroid gland,has a half-life (the time to decay byhalf) of about eight days. This is longenough for considerable amounts to

be deposited onto pasture and to be

transferred to people in dairy foods(Figure 4). In general, only those chil-dren in the U.S. with lactose intol-erance or allergies to milk productsconsumed no milk products, partic-ularly in the 1950s and 1960s whenthere were fewer choices of preparedfoods. Radioiodine ingested or inhaledby breast-feeding mothers can also betransferred to nursing infants via themother's breast milk.

The two nuclear weapons droppedon Hiroshima and Nagasaki weredetonated at relatively high altitudesabove the ground and produced mini-mal fallout. Most of the injuries to the

populations within 5 kilometers of theexplosions were from heat and shockwaves; direct radiation was a majorfactor only within 3 kilometers. Mostof what we know about late health ef-fects of radiation in general, includingincreased cancer risk, is derived fromcontinuing observations of survivorsexposed within 3 kilometers.

Understanding Radiation DoseRadiation absorbed dose is the energyper unit mass imparted to a medium

(such as tissue). Almost all radionu-


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elides in fallout emit beta (electron)and gamma (photon) radiation. A cas-cade of events follows once tissue isexposed to radiation: The initial radia-tion scatters, and atoms in the body areionized by removal of weakly boundelectrons. Radiation can damage DNAby direct interaction or by creatinghighly reactive chemical species that

interact with DNA.The basic unit of the system used

internationally to characterize radia-tion dose is the gray (Gy), defined asthe absorption of 1 joule of energy perkilogram of tissue. (The internationalsystem of units is gradually supplant-ing the previous system based on doseunits of rad, but conversion is easy: 1Gy = 100 rad.) For perspective, it ishelpful to remember that the externaldose received from natural sources ofradiation-from primordial radionu-

elides in the earth's crust and from cos-mic radiation-is of the order of 1 mil-ligray (mGy, one-thousandth of a gray)per year; the dose from a whole-bodycomputer-assisted tomographic (CT)examination is about 15-20 mGy, andthat due to cosmic rays received duringa transatlantic flight is about 0.02mGy.

Examples of Fallout ExposuresDoses from fallout received in the 1950sand 1960s have been estimated in re-cent years using mathematical exposureassessment models and historical fall-

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1954), was responsible for most-al-though not all-of the radiation ex-posure of local populations from allof the tests. The fallout-related dosesreceived as a result of that one test atBikini Atoll are the highest in the his-tory of worldwide nuclear testing.

Wind shear (changes in directionand speed with altitude) and an unex-pectedly high yield resulted in heavyfallout over populated atolls to the eastof Bikini rather than over open seas to

1-131releasedin bomb

test fallout

traveledaway

on wind

o26

o

people(often children)

drank themilk

some 1-131in milk

col lected inthyroid gland

Figure 3. Wind shear (variations in wind speed and direction with altitude) causes fallout to

spread over large areas. The 43-kiloton test SIMON was detonated at 4:30 a.m. local time on

Apr il 25 ,1953, a t the Nevada Tes t Si te. Tra jec tories of the fal lout debri s c louds across the U.S . a re

shown for four alt itudes. Each dot i ndicat es six hours. The numbered dots are the date i n April .



o

10,000 . ...18,00030,000 ')40,000_ '


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o

fallout arrival time (hours)

7.5 223.5 5

grO;""d zero external d o ' :(9"Y)30 10___ Rongelap

Atoll

Bikar Atoll

o

BikiniAtoll

Utrik Atoll

Taka Atoll a 4l~Alinginae Atoll

Rongerik Atoll

- ...........Rongelap Island

o 50 100 200 300 400 500

distance from detonation site (kilometers)

Figure 5. BRAVO, detonated on March 1, 1954, was a IS-megaton thermonuclear device that

resulted in the highest radiation exposures to people of any nuclear test. Unanticipated wind

direction and an explosive yield higher than expected sent a fallout cloud from the Bikini test

site towards the inhabited atolls of Rongelap, Ailinginae, Rongerik and Utrik in the Marshall

Islands. Doses from external exposure were about 1-2 Gy on Rongelap and Ai linginae.

the north and west. About 31j2 hoursafter the detonation, the radioactivecloud began to deposit particulate,ash-like material on 18 Rongelap resi-dents who were fishing and gather-ing copra on Ailinginae Atoll about135 kilometers east of the detonationsite, followed 2 hours later by deposi-tion on Rongelap Island 65 kilometersfarther to the east, affecting 64 resi-dents. The fallout arrived 21j2 hourslater at Rongerik Atoll another 40 kilo-meters to the east, exposing 28 Ameri-can weathermen; about 22 hours after

detonation, it reached the 167residentsof Utrik Atoll.

Doses received by the Rongelapgroup were assessed by ground andaerial exposure rate measurementsand radioactivity analysis of a commu-nity-pooled urine sample. The dosesreceived before evacuation were essen-tially due to external irradiation fromshort-lived radionuclides and internalirradiation from ingestion of short-livedradioiodines deposited on foodstuffsand cooking utensils. Thyroid doses, inparticular, were very high: At Rongelap

Figure 6. Subsequent to the explosion of BRAVO at the Bikini test site, teams of medical doc-

tors and health physicists made annual trips to the Marshall Islands to check on the health of

islanders accidentally exposed to radioactive fallout. In this photograph, Dr. Robert Conard

is examining a Marshall Islander for any thyroid abnormalities. (Photograph courtesy of

Brookhaven National Laboratory.)

550

they were estimated to be several tensof Gy for an adult and more than 100Gy for a one-year old. Estimated thy-roid doses atAilinginae were about halfthose at Rongelap, and doses at Utrikwere about 15 percent of those at Ron-gelap. The external whole-body dosesestimated were about 2 Gy at Rongelap,1.4 Gy at Ailinginae, 2.9 Gy at Rongerik

and 0.2 Gy at Utrik. Much lower expo-sures have been estimated for most ofthe other Marshall Islands atolls.

Twenty-three Japanese fishermen onthe fishing vessel Lucky Dragon werealso exposed to heavy fallout. Theirdoses from external irradiation wereestimated to range from 1.7 to 6 Gy.Those doses were received during the14 days it took to return to harbor;about half were received during thefirst day after the onset of fallout.

Semipalatinsk, Kazakhstan. The Semi-

palatinsk Test Site, in northeastern Ka-zakhstan near the geographical centerofthe Eurasian continent, was the Sovietequivalent of the U.S. Nevada Test Site;88 atmospheric tests and 30 surface testswere conducted there from 1949through1962. The main contributions to localand regional environmental radioactivecontamination are attributed to particu-lar atmospheric nuclear tests conductedin 1949, 1951and 1953.

Doses from local fallout originatingat the STS depended on the locationof villages relative to the path of thefallout cloud, the weather conditionsat the time of the tests, the lifestyles ofresidents, which differed by ethnicity(Kazakh or European), and whetherthey were evacuated before the fall-out arrived at the village. Some uniquecircumstances included strong windsthat resulted in short fallout transittimes and little radioactive decay be-fore deposition for at least one test.Also, the residents of the area wereheavily dependent on meat and milkfrom grazing animals, including cattle,

horses, goats, sheep and camels.Dose-assessment models predict a de-

creasing gradient in the ratio of externalradiation doses to internal doses frominhalation and ingestion with increas-ing time from detonation to fallout ar-rival. The relatively large particles thattend to fall out first are not efficientlytransferred to the human body. At moredistant locations in the region of localfallout, internal dose is relatively moreimportant because smaller particles thatpredominate there are biologically more

available. For example, in rural villages


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along the trajectory of the first test (Au-gust 1949)at the Semipalatinsk Test Site,average estimated radiation dose fromfallout to the thyroid glands of juvenileresidents decreased with increasing dis-tance from the detonation, but the pro-portion ofthat total due to internal radi-ation sources increased with distance. At110 kilometers from the detonation site,

the average dose was 2.2 Gy, of which73 percent was from internal sources,whereas at 230 kilometers, 86 percentof the average dose of 0.35Gy was frominternal sources

Nevada Test Site (NTS). The NTS wasused for surface and above-ground nu-clear testing from early 1951 throughmid-1962. Eighty-six tests were con-ducted at or above ground level, and14 other tests that were undergroundinvolved significant releases of radio-active material into the atmosphere.

In 1979 the U.S. Department of En-ergy described a methodology for esti-mating radiation doses to populationsdownwind of the NTS. Doses from in-ternal irradiation within this local fall-out area were ascribed mainly to inha-lation of radionuclides in the air and toingestion of foodstuffs contaminatedwith radioactive materials. Doses frominternal irradiation were, for most or-gans and tissues, substantially smallerthan those from external irradiation,with the notable exception of the thy-roid, for which estimated internal doseswere substantially higher. Estimatedthyroid doses were ascribed mainly toconsumption of foodstuffs contami-nated with iodine-131 (I-131) and, toa lesser extent, iodine-133 (I-133), andto inhalation of air contaminated withboth I-131 and I-133.Thyroid doses var-ied according to local dairy practicesand the extent to which milk was im-

more than 3,000

1,000-3,000 0-m

o.nc

300-1,000 mme n

"0

100-300 m(f)

.nc

30-100 nJm3

10-30m

< D

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Figure 7. Cesium-137 deposition density resulting from the cumulative effect of the Nevada

tests generally decreases with distance from the test site in the direction of the prevailing

wind across North America, although isolated locations received significant deposition as a

result of rainfall.

ported from less contaminated areas.Bone-marrow doses less than 50 mGy

were estimated for communities in alocal fallout area within 300 kilometersof the NTS, where ground-monitoringdata were available, and an order ofmagnitude less for other communitiesin Arizona, New Mexico, Nevada, Utahand portions of adjoining states.

Investigators at the University ofUtah estimated radiation doses to thebone marrow for 6,507 leukemia casesand matched controls who were res-idents of Utah. Average doses wereabout 0.003 Gy with a maximum ofabout 0.03 Gy. Subsequently, thyroiddoses were estimated to members ofa cohort exposed as school children insouthwestern Utah and who are partof a long-term epidemiology study.The mean thyroid dose was estimat-ed to be 0.12 Gy, with a maximum of1.4 Gy. Among children who did notdrink milk, the mean thyroid dose wason the order of 0.01 Gy.

In response to Public Law 97-414(en-acted in 1993),the U.S. National Cancer

Institute (NCI) estimated the absorbeddose to the thyroid from I-131 in NTSfallout for representative individuals inevery county of the contiguous UnitedStates. Calculations emphasized thepasture-cow-milk-man food chain, butalso included inhalation of fallout andingestion of other foods. Deposition ofI-131 across the United States was re-constructed for every significant eventat the NTS using historical measure-ments of fallout from a nationwide net-work ofmonitoring stations operationalbetween 1951 and 1958. Thyroid doseswere estimated as a function of age atexposure, region of the country and di-etary habits. For example, for a femaleborn in St. George, Utah, in 1951 andresiding there until 1971, the thyroiddoses are estimated to have been about0.3Gy if she had consumed commercialcow's milk, 2 Gy if she had consumedgoat's milk, and 0.04 Gy if she had not

1-3

more than 100

30-100

10-30

3-10

0-1

Figure 8. Total external and internal dose to the red bone marrow of persons born on January 1,1951, from all Nevada tests is shown at left. To-

tal external and internal dose to the thyroid of adults in 1951 from all Nevada tests is shown at right. Note that the dose is roughly proportional

to the deposition density shown in Figure 7.




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m1,000-3,000 e n

""0m

300-1,000 ~nJm

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'E 0.1-0.3


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Nevada Test Site fallout global fallout

thyroid or red red bone thyroid or red red bone

bone marrow thyroid marrow bone marrow thyroid marrow

external dose internal dose internal dose external dose internal dose internal doseradionuclide half-life (mGy) (mGy) (mGy) (mGy) (mGy) (mGy)

carbon-14 5730 y 0.1 0.1

cesium-137 30 y 0.01 0.009 0.009 0.3 0.1 0.1

strontium-90 28.5y 0.02 0.0009 0.2[0.002]a [0.5]a

tritium 12.3 y 0.07 0.07

antimony-125 2.7y 0.03

ruthenium-106 368 d 0.001 0.002 0.04

manganese-54 313 d 0.04

cerium-144 284 d 0.02

zirconium/niobium-95 64 d 0.08 0.2

strontium-89 52 d 0.001 0.03

ruthenium-103 39 d 0.03 0.02

cesium-136 13 d 0.002 0.002

barium/lanthanum-140 13 d 0.2 0.006 0.05

iodine-131 8 d 0.02 5 0.001 0.4 0.00009

[30]a [2]a [0.0002]atellurium/iodine-132 3.3 d 0.1 0.06 0.001

neptunium-239 2.4 d 0.02

iodine-133 0.9 d 0.02 0.04

zirconium/niobium-97 17 h 0.02

rounded totals: 0.5 5 0.1 0.7 0.7 0.6

[30]a [2]a [0.9]a

Figure 11. Average doses i n mill igray (rrrGy) for adul ts (unless accompanied by a superscripted "a," which denotes a chi ld born January 1, 1951)

living in the contiguous United States during the era of atmospheric testing are shown for the most important radionuclides. Note that the

radionuclides are organized by half-life, from longest to shortest (in years, y, or days, d), descending, rather than by atomic weight.

estimate fallout-related risks from ex-ternal radiation sources. Estimates ofradiation-related lifetime cancer riskper unit dose from external radiationsources to the organs and tissues ofinterest are shown in Figure 10 for leu-kemia, thyroid cancer and all cancerscombined. Estimated risks, in percent,are given separately by sex, as func-tions of age at exposure.

Thyroid cancer is a rare disease over-all-with U'S, lifetime rates estimatedto be 0.97 percent in females and 0.36

percent in males-and it is extreme-ly rare at ages younger than 25. Fur-thermore, the malignancy is usuallyindolent, may go long unobserved inthe absence of special screening effortsand has a fatality rate of less than 10percent. These factors make it difficultto study fallout-related thyroid cancerrisk in all but the most heavily exposedpopulations. Thyroid cancer risks fromexternal radiation are related to genderand to age at exposure, with by far thehighest risks occurring among women

exposed as young children.

The applicability of risk estimatesbased on studies of external radiationexposure to a populat ion exposed main-ly to internal sources, and to 1-131in par-ticular, has been debated for many years.This uncertainty relates to the unevendistribution of 1-131radiation dose with-in the thyroid gland and its protractionover time. Until recently, the scientificconsensus had been that 1-131is proba-bly somewhat less effective than externalradiation as a cause of thyroid cancer.However, observations of thyroid cancer

risk among children exposed to falloutfrom the Chornobyl reactor accident in1986 have led to a reassessment. An In-stitute of Medicine report concluded thatthe Chornobyl observations support theconclusion that 1-131has an equal effect,or at least two-thirds the effect of internalradiation. More recent data on thyroidcancer risk among persons in Belarusand Russia exposed as young children toChornobyl fallout offer further supportof this inference.

In 1997, NC1 conducted a detailed

evaluation of dose to the thyroid glands

of U'S. residents from 1-131 in falloutfrom tests in Nevada. In a related activ-ity, we evaluated the risks of thyroidcancer from that exposure and esti-mated that about 49,000 fallout-relatedcases might occur in the United States,almost all of them among persons whowere under age 20 at some time dur-ing the period 1951-57, with 95-percentuncertainty limits of 11,300 and 212,000.The estimated risk may be comparedwith some 400,000 lifetime thyroid can-cers expected in the same population

in the absence of any fallout exposure.Accounting for thyroid exposure fromglobal fallout, which was distributedfairly uniformly over the entire UnitedStates, might increase the estimatedexcess by 10 percent, from 49,000 to54,000. Fallout-related risks for thyroidcancer are likely to exceed those for anyother cancer simply because those risksare predominantly ascribable to the thy-roid dose from internal radiation, whichis unmatched in other organs.

External gamma radiation from fall-

out, unlike beta radiation from 1-131,




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E stim ating Your Thyroid C ancer R isk

A Web-based calculatordeveloped by the Na-tional Cancer Instituteis available to anyonewishing to estimate in-

dividual thyroid cancerrisks associated withexposure to I-131 radia-tion in fallout from theNevada Test Site, forpersons who lived in theU.S. during the 1950s.The calculator can beaccessed through the In-ternet at its stand-aloneweb page (http://ntsiI31.nci.nih.gov /) or throughthe main NCI website

(http: //www.cancer.gov/iI31), which provides more general informationabout the NTS, I-131 and radioactive fallout. Information required for thecalculation includes gender, age at exposure, places of residence during theyears 1951-71, and sources and approximate amounts of milk consumed.

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Fallout From Nuclear Weapons Tests in the 50's and Cancer Risks

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