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The extent and patterns of usageof Agent Orange and otherherbicides in Vietnam
Jeanne Mager Stellman*, Steven D. Stellman†‡, Richard Christian§, Tracy Weber* & Carrie Tomasallo** Departments of Health Policy and Management and † Epidemiology, Mailman School of Public Health, Columbia University, 600 West 168th Street, New York,New York 10032, USA‡ Institute for Cancer Prevention, One Dana Road, Valhalla, New York 10595, USA§ 2102 Old Stage Road, Alexandria, Virginia 22308, USA...........................................................................................................................................................................................................................

Herbicides including Agent Orange were sprayed by United States forces for military purposes during the Vietnam War (1961–1971) at a rate more than an order of magnitude greater than for similar domestic weed control. In 1974, the US National Academyof Sciences published estimates of the extent and distribution of herbicides sprayed. Here we present revised estimates,
developed using more-complete data. The spray inventory is expanded by more than seven million litres, in particular withheavily dioxin-contaminated herbicides. Estimates for the amount of dioxin sprayed are almost doubled. Hamlet census data
reveal that millions of Vietnamese were likely to have been sprayed upon directly. Our identification of specific military herbicide
targets has led to a more coherent understanding of spraying. Common errors in earlier interpretations of the spray data are also
discussed.
Between 1961 and 1971 herbicide mixtures, nicknamed by thecoloured identification band painted on their 208-litre storage
barrels, were used by United States and Republic of Vietnam forces
to defoliate forests and mangroves, to clear perimeters of military
installations and to destroy ‘unfriendly’ crops as a tactic for
decreasing enemy food supplies1. The best-known mixture wasAgent Orange. About 65% of the herbicides contained 2,4,5trichlorophenoxyacetic acid (2,4,5-T), which was contaminatedwith varying levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin(TCDD). Herbicide mixtures are listed in Table 1.
In 1970, the US Congress directed the US Department of Defense(DoD) to engage the National Academy of Sciences to conduct acomprehensive study (NAS-1974) of the ecological and physiological effects of defoliation in Vietnam2. NAS-1974 relied on achronological record, the HERBS file3, which contained flightpath coordinates of Air Force spraying missions carried out between
August 1965 and December 1971 and from 1968 on US Armyhelicopter spraying missions. In 1985, the DoD supplemented thisfile with the Services-HERBS file, derived from additional record
searches. The HERBS file error rate was about 10%, attributablelargely to transcription, data entry and pilot recording errors4.
Under contract to the NAS, using data more complete than werethen available, we undertook, in close collaboration with the US
Armed Services Center for Research of Unit Records (CRUR), tocorrect both files (see Methods) and during this process discoveredmuch additional archived data.
Military herbicide operations in Vietnam became a matter ofscientific controversy from their inception5,6. In April 1970, 2,4,5-T
was banned from most US domestic uses, on the basis of evidence of
its teratogenicity7. The Agent Orange Act of 1991 requested theInstitute of Medicine (IOM) to assess the strength of the evidence
for association between exposure to military herbicides and diseasein veterans and the feasibility of conducting further epidemiologicalstudies. The IOM recommended that the Department of Veterans
Affairs develop historical reconstruction methods for characterizing
exposure to herbicides in Vietnam8,9. The present report is the resultof that recommendation.
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Background to military use of chemical defoliants
The DoD’s Advanced Research Project Agency’s (ARPA) Project
Agile was instrumental in the US development of herbicides as amilitary weapon, an undertaking inspired by the successful British
use of 2,4,5-T to destroy jungle-grown crops during the insurgency in
Malaya. ARPA supported tests on combinations and concentrationsof herbicides; calibration studies of the spray delivery system toachieve the desired 28 l ha21 (3 gallons/acre) rate; and experiments on
optimal conditions to minimize spray drift10. ARPA also developed
the Hamlet Evaluation System11 which collected the political census
data that we use here for estimating population exposures.
The first large-scale US military defoliation took place in Camp
Drum, New York, in 1959, using Agent Purple (a 50:50 mixture of
2,4-D and 2,4,5-T) and a spray system which was the model for thoseused in Vietnam. Herbicide tests were run from August to December
1961 in the Republic of Vietnam (RVN), using dinoxol and trinoxol12,13. An insecticide test series was also undertaken. The firstmajor herbicide shipment arrived in RVN in January 1962; defoliation targets were sprayed during September and October 1962
(Agent Purple); crop destruction targets were sprayed in November
1962 (Agent Blue)14. Systematic testing of herbicides and calibrationof herbicide delivery systems continued for several years15.
A 1962 pact assigned ownership of the herbicides to RVN when
they entered its territory. Vietnamese physically handled the herbicides during off-loading, transport, and transfer to storage tanks.
RVN ownership complicated United States Air Force (USAF)
logistics and record-keeping, and disposal when Agent Orange use
was abandoned in mid-1970 (ref. 16). US policy emphasized that its
forces were assisting the RVN in the herbicide programme. C-123
aircraft carried out the missions camouflaged and equipped withremovable identification insignias. Crop destruction aircraft
bore South Vietnamese markings and were accompanied by aVietnamese crew member under a State Department/DoD concept
known as Farmgate17. Flight crew wore civilian clothing.

The herbicide targets and US Air Force project folders
US Air Force (USAF) operations, codenamed Operation Ranch
Hand, dispersed more than 95% of all herbicides used in Operation
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Trail Dust, the overall herbicide programme. Other branches of the
US armed services and RVN forces, generally using hand sprayers,
spray trucks (Buffalo turbines), helicopters and boats, sprayedmuch smaller quantities of herbicide. Operation Ranch Hand wasorganized into projects that underwent a complex combined South
Vietnamese and US approval system which could sometimes last as
long as one year. Each project consisted of specific targets that were
often amended or deleted during the approval process. Crop
destruction also required White House approval until 1963, after
which final approval was delegated to the US Ambassador to the
RVN.
We reconstructed the project number to which each mission in
the HERBS file belonged by concatenating two data fields. Aggregating missions by project number transforms the HERBS file froma chronological listing of criss-crossing flight paths into targetrelated groups of flights flown at different points in time (Figs 1 and
2). The importance of projects and targets has not been sufficiently
appreciated. NAS-1974, and even the USAF itself18, inverted thehierarchy thus: “All missions within a target formed a project”.
With US National Archives staff assistance we located a collectionof USAF operational project folders for about 50% of the projects
and nearly 60% of the volume of herbicide inventoried in the
HERBS file19. Many folders contained ‘after action reports’ or otherdocumentation on completed missions which had not been
included in any HERBS file and permitted us to identify some
200 new missions that pre-date August 1965, the date of the earliest
missions on the NAS-1974 HERBS file. The early years of theVietnam War have incorrectly been regarded as of minor importance with regard to herbicide spraying. In total, about 1.9 million
litres of Agent Purple were sprayed between 1962 and 1965, which isparticularly significant because herbicides manufactured in theearly 1960s were almost certainly more heavily TCDD-contaminated than those produced later20. Further, pre-1965 spraying was
limited to a relatively small area (Fig. 1c), which thus may be atparticular risk for TCDD contamination. Recent data on TCDD
residues in soil sampled near US Army Special Forces camps where
Agent Purple was sprayed21, and results of soil assays at the testing
grid in Eglin Air Force Base, Florida22, support this interpretation.

Revised spray inventory
We have re-estimated the volume and type of herbicides sprayed
between 1961 and 1971 to have 7,131,907 more litres than the‘uncorrected’ NAS-1974 inventory and 9,440,028 l more than NAS1974’s ‘corrected’ inventory, in which about 10% of all missions had
been discarded because of obvious recording errors. Figure 3 shows
the areas sprayed; Fig. 4 shows the yearly distribution of spray; Fig. 5
shows the numbers of sorties flown, 1961–1971. Table 2 gives
estimates of the frequency with which land areas were repeatedly
sprayed.
Contamination of 2,4,5-T with TCDD varied widely by production run, manufacturer, and the percentage 2,4,5-T in theformulation. In early 1966, Agent White, which did not contain
2,4,5-T and hence was not TCDD-contaminated, began to replace
Agent Orange. Chemical market forces had led to a shortage ofAgent Orange. From a tactical perspective, Agent White was lesssatisfactory than Agent Orange because several weeks were requiredfor defoliation to begin. It was accepted by the DoD, however,
because Agent Orange would apparently no longer be available insufficient quantities23. Agent Blue was the agent of choice for crop
destruction by desiccation throughout the entire War, but morethan four million litres of the other agents, primarily containing
2,4,5-T, were also used on crops.
Procurement records show that at least 464,164 l of Agent Pink
and 31,026 l of Agent Green, with comparatively higher TCDD
levels, were purchased, but we have been able to document little
more than 50,000 l as having been sprayed in RVN and about
15,000 l that were used in tests. We have identified missions which
dispersed about 1,900,000 of Agent Purple, but available procurement data show purchase of only 548,100 l (ref. 24), indicating thatthe procurement records are incomplete.

Extent of dioxin contamination
Estimates of how much TCDD was deposited in Vietnam are basedon the volume of 2,4,5-T-containing herbicide sprayed (which has
been revised upward on the basis of the new inventory), and on
TCDD contamination levels. Estimates of the mean contaminationlevel used by the USAF18 and by NAS-1974 are probably too low.

Table 1 Use of military herbicides in Vietnam (1961–1971)13,15,29
Name

Chemicalconstituents

Concentration active
ingredient

Estimated quantities
sprayed (litres)*Years used...................................................................................................................................................................................................................................................................................................................................................................

Agent Pink†

60%–40% n-Butyl: isobutyl esterof 2,4,5-T§

961–1,081 g l21
acid equivalent‡

1961;1965

50,312 sprayed; 413.852 additionalon procurement records

Agent Green†

n-Butyl ester 2,4,5-T(Should have same acidequivalent as Agent Pink)(Unclear but within
timeframe for Agent Pink)

31,026 shown on procurement
records

Agent Purple†

50% n-Butyl ester 2,4,-D;
30% n-butyl ester 2,4,5-T;
20% isobutyl ester 2,4,5-T

1,033 g l21 acid equivalent

1962–1965

1,892,773

Agent Orange†

50% n-Butyl ester 2,4,-D;
50% n-butyl ester 2,4,5-T

1,033 g l21 acid equivalent

1965–1970

45,677,937 (may include Agent
Orange II)

Agent Orange II †50% n-Butyl ester 2,4-D;
50% isooctyl ester 2,4,5T

910 g l21 acid equivalent

After 1968 (?)

Unknown but at least 3,591,000
shippedAgent Whitek

Acid weight basis: 21.2%
tri-isopropanolamine salts of 2,4-D
and 5.7% picloram

By acid weight: 240.2 g l21
2,4,-D and 64.9 g l21 picloram

1966–1971

20,556,525

Agent Blue
(powder){

Cacodylic acid (dimethylarsinicacid) and sodium cacodylate

Acid: 65% active ingredient;
salt: 70% active ingredient

1962–1964

25,650Agent Blue
(H2O solution)

21% sodium cacodylate þcacodylic acid to yield at least 26%
total acid equivalent by weight

Acid weight: 360.3 g l21

1964–1971

4,715,731...................................................................................................................................................................................................................................................................................................................................................................
Other chemicals used in testing programme but not in Vietnam operations include Modified Orange (4-amino-3,5,6-trichloropicolinic acid (picloram) added to Orange), Dalapon, Bromacil, Tandex,
Monuron, Diuron and maleic hydrazide. Dinoxol (1890 l) (butoxyethanol esters of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-T), Trinoxol (1455 l) (40% ethanol ester of 2,4,5-T) and 378 l Conc D (30%ethyl ester of 2,4,-D in H2O) were also used in tests during 1961.
* Nominal application rate: 4.78 kg ha21.
† Contaminated with varying levels of TCDD.
‡ Acid equivalent is the mass of pure acid that results from complete de-esterification or deamination of salts and esters. Total ester masses are approximately 20% greater.
§ 80–20% mixture when mixed with Agent Green. Agent Green was never sprayed alone but was immediately mixed with Agent Pink for spraying.
kProprietary product of Dow Chemical Company (Tordon 101).
{Ansul Chemical Co. product Phytar 560 was only arsenical before July 1969.

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After Agent Orange spraying ended in May 1970, the USAF was
required to dispose of very large stockpiles of surplus herbicide thatwere ultimately incinerated aboard the M/T Vulcanus in 1977. (TheUS government had also assumed liability for large unblended
inventories of 2,4,5-T and 2,4-D from contracted suppliers, either
paying for storage or stockpiling the chemicals at Kelly Air Force
Base, Texas, in December 1970 (ref. 25). We do not know the fate of
the stockpiles.)
TCDD concentrations varied widely in the mixtures. The USAF
was required to provide the US Environmental Protection Agency
with an Environmental Impact Statement (EIS) prior to incineration26. The EIS sheds light on TCDD levels in the over three million
litres of herbicide stockpiled at the Naval Construction Battalion
Center (NCBC), Gulfport, Mississippi, and the approximately
26,000 208-litre barrels in the Johnston Island stockpile.
TCDD concentrations ranged from 6.2 to 14.3 p.p.m., andaveraged 13.25 p.p.m. in samples drawn for incineration-effluent
modelling studies from 28 different NCBC barrels chosen by the
USAF as representative of the seven manufacturers of the NCBC
stockpile26. However, in other samples drawn from the NCBC
stockpile, the TCDD range was about 0.05 to 13.3 p.p.m. (weighted
average 1.77 p.p.m.). (NAS-1974, however, calculated a range of,0.05 to 17.0 p.p.m., and an arithmetic mean of 2.99 p.p.m. fromthe same analytical data2.) The large discrepancies between theeffluent modelling study and the other samples analysed by the

USAF and NAS-74 can be explained by information in the background analytical documentation in which runs analysed byNAS-1974 are consistent with the ‘low dioxin’ analytical series27,28.
The documentation also reports dioxin levels to be heterogeneous,even within production runs. USAF chemists had concluded thatgeneralization from tested barrels to untested ones in the same
production batch was not reliable. NAS-1974 appears not to havebeen provided information either on the heterogeneity of production runs or that they were generalizing from data consistent
with a ‘low dioxin’ analytical series.
Two hundred random samples from the Johnston Island stockpile were also analysed for TCDD content. The Johnston Island
stockpile is likely to have been primarily contracted for in 1967 orlater and to consist almost exclusively of Agent Orange, because
Agents Purple, Pink and Green were not manufactured after the useof Agent Orange began29 and in mid-1966 Ranch Hand missions
were curtailed because of a severe herbicide shortage17. NAS-1974
calculated a mean TCDD level of 1.91 p.p.m. ^ 20% for the stockpile2. USAF documentation that is widely viewed as authoritative18,
however, disputes this mean and contends that the four highest
values (17, 22, 33 and 47 p.p.m.) must have been Agent Purple, and
not Agent Orange, because these values exceeded the mean reported
for the NCBC inventory, citing a personal communication from amilitary officer who recalled that as many as 20 drums of Agent
Purple may have been present in the stockpile and redrummed into

Figure 1 Herbicide projects, targets and spraying19. a, Spraying operations were directed
at specific targets, 487 of which are shown. Labels in a refer to areas blown up in thecorresponding parts of Figs 1 and 2. b, Some areas were targeted in multiple projects at
different times: black box, 1964; red boxes, 1965; green boxes, 1966. c, We identified
targets active before 1965; at least 4.7 million litres of herbicides were used to destroy
33,339 ha of crops and defoliate 101,300 ha of land. Dioxin contamination may beparticularly relevant for this time period. d, Correspondence between HERBS file Agents
Orange and White mission paths (orange and white lines) and target boxes, three of which
enclose a railroad right-of-way. In Figs 1 and 2, waterways such as canals and rivers areshown in blue.

Figure 2 We could not locate target documentation for half the Ranch Hand missions on
the HERBS file. a, Missions clearly directed at specific targets. Our current work is to
identify probable targets through such mapping. Colours represent the flight paths ofspray missions delivering Agents Orange and White b, HERBS file problem. Some
purposes defined in the HERBS file documentation3, such as waterways andcommunication lines, are often labelled defoliation in the spray tapes, as shown.
NAS-1974 and others have analysed spraying by HERBS file purpose but this is
inaccurate.

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Figure 3 Volumes of herbicide sprayed. a, b and c represent known volumes of Agents
Orange, White and Blue, respectively, sprayed by US military forces in RVN, 1961–1971.
Volumes are calculated for individual grids spaced at 0.01 degrees (,1.2 km2), that
divide up Vietnam in a geographic information system developed by us45. Colours in a, b
and c correspond to volumes as shown in key. Arrows in a and c point to missions in Laosand in the Parrot’s Beak region of Cambodia, respectively. d, Grids sprayed with volumes
greater than 4,800 l (about 10% of total), with marker size increasing in proportion tovolume and colours corresponding to herbicide codenames. All herbicides containing
2,4,5-T are represented by orange markers.

Agent Orange containers. (The hypergeometric probability of
selecting four of the 20 Agent Purple drums from the stockpile is1.32 £ 1025.) In fact, the range observed is completely consistent
with the USAF’s own analysis of the range and heterogeneity of
TCDD levels27,28. By 1988, Young, the senior author of the USAF
documentation, dropped the word “may” and simply reported thefour high values to have been Agent Purple30. This latter reference
has been relied upon as authoritative by the IOM8, and many others.
A 1971 NAS-1974 analysis of six core soil samples collected fromthe central calibration grid at Pran Buri, Thailand, over which all
ARPA test flights had flown, found TCDD levels ranging from nondetectable (,0.0012 p.p.m.) to 0.0233 p.p.m. and 2,4,5-T residue
from non-detectable (,0.02 p.p.m.) to 0.61 p.p.m.. NAS-1974 estimated the original herbicide to have contained ,3 to 50 p.p.m.
TCDD, consistent with the range observed in the Johnston Island
stockpile2. (The USAF documentation18 incorrectly asserts that
NAS-1974 had erred in attributing the TCDD to Agent Orange
rather than to Agents Purple and Pink. These misstated findings areused as further rationale for assuming the four high TCDD values tohave been Agent Purple.)
Although Agent Purple is, indeed, likely to have been more highly
contaminated with TCDD (an archived sample of Agent Purple at
Eglin Air Force Base contained 45 p.p.m. TCDD18 and historical
TCDD contamination data show early 1960s contamination levels
to have been much higher)20, it is also likely that mean TCDD levels
in Agent Orange were far higher than 3 p.p.m. for much of theherbicide used. An average value closer to 13 p.p.m. may be more
realistic.
If 3 p.p.m., the mean associated with the ‘low dioxin’ series isconservatively applied to the new inventory we have presented here,
the estimate for TCDD present in the spray grows to 221 kg from
NAS-1974 estimates of 106–163 kg. Applying 32.8 p.p.m. and65.5 p.p.m. as the average TCDD in Agents Purple and Pink, we
obtain an additional 165 kg, or 366 kg in total (which still does nottake into account the herbicides sprayed by RVN forces, andpossibly by US Army and Navy forces by trucks, boats, handsprayers and helicopters, nor the more than 400,000 l of Agent
Pink shown in procurement records but not found in any recorded
missions). If, indeed, dioxin contamination of Agent Orange could
be fourfold or more higher, then this increased dioxin load grows

proportionally. It is also possible that some missions recorded ashaving dispersed Agent Orange did, in fact, spray the much more
highly contaminated but unaccounted-for Agent Pink, but we knowof no way to determine this. It is more likely that the unaccountedfor herbicides were used by Vietnamese troops, although about
50,000 l of Agent Pink do appear on the 1965 inventory.

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Estimates of population exposure to herbicides
A Hamlet Evaluation System (HES) in which US district advisors
and Vietnamese district chiefs filled out monthly political survey
and census forms was established in June 1967 and a gazetteer ofplace names and precise geographical locations was also created.
The HES data provide a comprehensive rural census that permits us
to estimate the numbers of hamlets and size of the population
directly sprayed upon31. HES files were not made available to NAS1974 early enough to permit analyses2.
More than 20,585 unique hamlets are represented in the corrected version of the database used here. Population data are not

Figure 4 Litres of herbicides sprayed over 1962–1971. The data are taken from thecorrected HERBS file and do not include Dinoxol and Trinoxol used in 1961.
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Table 2 Estimated area and frequency of spraying
Hectares
Times sprayed
All herbicides

2,4,5-T herbicides.............................................................................................................................................................................

1
2
3
4
5
6
7
8
9
10 þ
Total ha368,556
369,844361,862
341,037
272,709
216,724
153,391
138,610
115,103
293,461
2,631,297

343,426
332,249
275,770
236,232
153,192
119,12775,062
51,371
32,98860,316
1,679,734.............................................................................................................................................................................
Frequency of spraying is the number of times spray mission flight paths overflew grids in Vietnam
GIS developed for herbicide exposure assessment45.

Figure 5 Time course of herbicide sorties. At least 19,905 sorties were run between
1961–1971 (1–34 daily, with a daily average of 10.7 sorties). These data disagree with
USAF estimates18. The number of daily sorties mirrors the course of the Vietnam War
itself: a slow build-up, maximum activity 1968–1969, then a slow but steady decline. The
abrupt spraying drop-off at the end of January 1968 corresponds to the Tet Offensive
wherein North Vietnamese forces carried out massive, coordinated attacks throughout
RVN for nearly 2.5 months. Spray equipment was removed from Ranch Hand aircraft andcrew and aircraft participated in airlift operations.

available for 18% of these hamlets and population data are notsystematically reported each month for all years. Among thehamlets with some population data, 3,181 were sprayed directly
and at least 2.1 million but perhaps as many as 4.8 million people
would have been present during the spraying. Another 1,430
hamlets were also sprayed, but we cannot estimate the population
involved. In all, at least 3,851 out of 5,958 known fixed-wing
missions had flight paths directly over the hamlet coordinates
given in the HES and gazetteer data and about 35% of the total
herbicide sprayed was flown by these missions, although, in general,
flight paths extended beyond hamlet borders.

Aborted missions and emergency dumps
Forty-two missions originally intended to spray 120,000 l of herbicide are known to have ended with emergency herbicide dumps
where the chemical was jettisoned in about 30 s, compared to theusual 4 to 5 min. At least five herbicide-loaded aircraft crashed.
Hundreds of other missions were aborted after take-off because ofpoor weather at the target site, heavy anti-aircraft fire, or mechanical problems. Such aircraft returned to base with herbicide load
intact. It has been erroneously reported that aborted missions
automatically dumped herbicide before landing32. Many flights
were ‘aborted’ before take-off because of mechanical problems.
One extensive but incomplete list of aborted missions is containedin a large-format uncorrected version of the HERBS file, known as
the Map Book, which has served as the basis for much analysis ofpotential exposure, particularly by Vietnamese scientists33. Whenthe Map Book list is checked against the USAF Daily Air Activities
Reports (DAARS), it is found to consist primarily of mechanicalaborts, not herbicide dumps. Conversely, the list does not containmany actual dumps identified by CRUR and has other inaccuracies
which have been corrected over the years by the DoD and now by us.
In 1971, NAS-1974 analysed five soil samples from an area inwhich about 3700 l of Agent Orange had been dumped in December
1968. No 2,4,5-T could be detected. TCDD was not analysed34.
Aborted missions may not represent the significant source of
exposure that it has been represented to be by others, and studies
which have relied on the uncorrected data in the Map Book will, ofnecessity, be inaccurate.
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Discarded drums
Approximately two litres of herbicide residue remains in the 208litre barrel after it has been ‘emptied’. Typically about 20% of theresidue remains after three rinses. Residue, on average, contained
5.96 mg of TCDD, arising from heavily contaminated herbicides
and about 1.25 mg per drum from ‘low dioxin’ herbicides26. Barrel
residues had led to inadvertent defoliation of trees and gardens in
Da Nang, Nha Trang, Bien Hoa, Phu Cat and Saigon civilian areas
near USAF airbases that handled the herbicides when the empty
barrels were transported to local merchants for commercial uses35.
Improved handling procedures were adopted in 1969 following the
Da Nang defoliation incident but the ultimate fate of most of theempty barrels is not known and the extent to which people who used
the barrels for other purposes may have been exposed is not known.

Spraying in Laos and Cambodia
Operation Ranch Hand flew its first missions outside RVN in
December 1965 to defoliate the major reinforcement and supply
route through Laos known as the ‘Ho Chi Minh trail’. Known
spraying was above the 17th parallel (the northern border of RVN),
along Routes 92, 922, 96 and 965 below Tchpone and the Sihanouk
Trail that went from Laos to Cambodia. Missions in both countries
below the 17th parallel can clearly be seen in Fig. 3a and c. A small
amount of crop destruction using Agent Blue was also documented.
Laos was also the site of a brief experiment to determine whether F4E Phantom II jet fighters could successfully be used to carry out
spray operations and avoid anti-aircraft fire. At least five F-4E
missions were flown until a fighter was shot down and the strategy
was abandoned. Documentation of spray activities in Laos isincomplete. The Services-HERBS file shows flight paths for 210
missions, which sprayed about 1.8 million litres. NARA-held
documentation shows as much as 14% more herbicides as having
been sprayed but no coordinates are given so that these data cannot
be included in the revised HERBS file36.
Unlike in Laos, it was official US policy to avoid spraying
Cambodia either directly or indirectly by spray drift37 (cited by
Cecil38). Records show several heavily sprayed regions of RVN, near
Cambodia. The HERBS file shows one five-aircraft Ranch Hand
mission dispersing approximately 19,000 l of Agent Orange on 5
April 1969 inside Cambodia. Another nine missions dispersed
about 136,000 l of Agent Orange while partly over Cambodian
territory (Fig. 3c). At the typical rate of 28 l ha21 this would coverabout 5,500 ha. Undocumented spray drift may have also occurred.
In May 1969 a diplomatic crisis arose when Cambodia charged the
US with repeatedly spraying it and defoliating 70,930 ha; evidence ofdefoliation was confirmed by visiting foreign scientists39. Cambodian claims seem to be exaggerated in that to achieve the extent ofalleged defoliation nearly half of the Ranch Hand flights for April–
May 1969 would have to have been directed towards Cambodia.
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Records are not available to resolve the controversy, particularly
since the area was devastated by US B-52 bombing raids in 1970.

gazetteer data files31. Each hamlet location was assigned a grid identifier and proximity tospray was calculated using our GIS system software.
Received 6 December 2002; accepted 4 March 2003; doi:10.1038/nature01537.

Discussion
The Vietnam War ended in 1975, yet no large-scale epidemiologicalstudy of herbicides and the health of either the Vietnamese population or war veterans has been carried out. Discussions of healthand ecology studies in Vietnam have recently taken place40. During
the course of developing an exposure methodology, we have
unexpectedly come upon primary data which expanded existingherbicide spraying databases and could help guide the design ofhuman health and of environmental studies.
NAS-1974 found the HERBS file to be a powerful tool forstudying exposure to herbicides. The concordance between
HERBS file mission coordinates, operational folders, and the precise
locations of roadways, rail lines, power lines, canals and so on given
in modern mapping software increases our confidence in the
HERBS file. Viewing the HERBS file as a carefully planned targetbased military exercise, rather than chronological unrelated missions criss-crossing RVN in straight-line paths affords a coherent
analytical approach.
Our analyses using original operational records raise questions
about the spraying data and dioxin contamination relied upon byresearchers and policymakers. For example, we find that the ‘202
Tasks Realized’ document used by NAS-1974 assumed missions thatare missing from the HERBS file41 to represent targeted areas andanticipated spraying rather than operations completed. (Further,the ‘202 Tasks Realized’ document contains many errors, such as
misplaced decimal points for subtotals of volume sprayed, which
cumulate and greatly exaggerate the total volume)42. Therefore,
some NAS-1974 estimates of ‘missing’ spray must be revised downward. On the other hand, dioxin contamination estimates should be
revised upward. Comparatively small amounts of Agent Purple andPink sprayed in Vietnam between 1961–1965 may have deposited alarge percentage of the total dioxin.
Large numbers of Vietnamese civilians appear to have been
directly exposed to herbicidal agents, some of which were sprayedat levels at least an order of magnitude greater than for similar US
domestic purposes20. Other analyses being carried out by us show
large numbers of American troops also to have been directly
exposed or to have served in recently sprayed areas. Areas sprayedduring the early years and in the various test sites around the world15
may be of particular interest for follow-up ecological and epidemiological studies.
A

Methods
We constructed a new version of the HERBS file using operational records for the USAF
12th Air Commando Squadron (ACS), which carried out the Ranch Hand missions and bysystematic comparisons of four different versions of the herbicides files found at the US
National Archives43 and the active file at CRUR. Of 18,087 mission records there were1,264 non-matches between HERB2REV and the current CRUR file (that is, 2.5
million litres out of more than 66.6 million litres). About 3.4% of the total volume isrepresented at CRUR but not the other files. We also identified errors by mapping flightpaths and by consistency checking (for example, legs that were very short or very long, overwater, and so on). Discrepancies which we could not thus resolve were reviewed by a small
panel of experts. The same panel reviewed each proposed amendment to the HERBS file. A
small number of missions which remained ambiguous were discarded. All discarded
missions were from the Services-HERBS.
Flight paths for 65 missions of fixed-wing aircraft missions that had been represented
in the original HERBS file only by a single point (usually the calculated centre-of-mass)
were readily deduced by comparison with similar missions or known targets. (Somemissions are correctly represented as single points because they document perimeter
spraying carried out by spraying specific discrete points, such as outside base camps nearguard posts.) The revised file contains 9,141 missions, primarily by air, but also using othermeans of delivery.
We used digitizing software44 to derive the longitudes and latitudes defining the target
perimeters from hand-drawn maps in the USAF Project Folders19. For folders with no map
we extracted the Universal Transverse Mercator (UTM) coordinates from written
descriptions of the targets and entered them into a Geographic Information System (GIS)
which we created45, similar to a databank model created by NAS1974 (we are willing toassist researchers in using our GIS software for estimating exposure using these data).
Population and hamlet exposure estimates used data contained in the HES and Vietnam686

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Acknowledgements This research was supported by the National Academy of Sciences and
USPHS at the National Cancer Institute. We thank D. Hakenson and his staff at the US Armed
Services Center for Research of Unit Records for technical support and access to military records;
the staff of the National Archives and Records Administration, particularly R. Boylan and
M. O. Adams; N. Heim for assistance with illustrations and F. Benjamin for assistance withmilitary records. R.C. is a retired member of the US Army.
Competing interests statement The authors declare that they have no competing financial
interests.
Correspondence and requests for materials should be addressed to J.M.S.
(e-mail: jms13@columbia.edu).

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