Agent Orange Investigative Report Series, No. 2
Contract: VA-101-12-C-0006

INVESTIGATIONS INTO THE
ALLEGATIONS OF AGENT
ORANGE/DIOXIN EXPOSURE
FROM FORMER
RANCH HAND AIRCRAFT

Compensation Service
Department of Veterans Affairs
810 Vermont Ave., NW
Washington, DC 20420
A. L. Young Consulting, Inc.
Alvin L. Young, PhD
Kristian L. Young, MA
November 2012

 A. L. Young Consulting, Inc.
1810 Tranquility Road
Cheyenne, WY, 82009-2903

 

30 November 2012

Mr. Michael D. Pharr

Contract Officer?s Representative
Compensation Service
Department of Veterans Affairs
810 Vermont Ave, NW
Washington, DC 20420

Dear Mr. Pharr,

Please ?nd attached to this letter the Final Report on Investigations into the Allegations of Agent Orange/Dioxin
Exposure from Former RANCH HAND Aircraft. This report is the second of many reports that will be prepared in
fulfillment of Contract 01-i2?C-0006, Development of on Archival Directory of A gent Orange Documents. The
goal of developing this directory is to Search and identify the thousands of documents, reports, and correspondence
located within our National Archives and Records Administration and other document repositories that relate to the use
of ?Tactical Herbicides? including Agent Orange, outside of Vietnam.

As in the case of the UC-123K controversy, the Compensation Service did not have the records related to history of the
aircraft, nor the detailed scienti?c studies conducted at Hill Air Force Base Utah on the quarantined aircraft
that were stored by the 309th Aerospace Maintenance and Regeneration Group at Davis?Monthan Air Force Base Ari-
zona. Thus, the Department of Veterans Affairs has been dependent on limited documentation related to events involv-
ing possible exposure to Agent Orange.

This current report documents three factors critical for exposure assessment that were identi?ed by former United
States Air Force Reserve personnel who ?ew or maintained the and the C-123K Post?Vietnam from 1972 to
1982. The 1st factor included a detailed history of the more than 200 aircraft that were modi?ed to meet the
needs of various military activities during the Vietnam War. The 2nd factor included a detailed examination of the vari?
ous alleged ?dry? Agent Orange residues that remained within the UC-123K aircraft assigned to Reserve units. The 3rd
factor examined whether the residues were actually available through dermal or inhalation routes to the aircrews ?ying
the Ks, and if the exposure was signi?cant to cause disease. The results could not prove that the Air Force
Reserve personnel were not exposed to Agent Orange or its associated dioxin contaminant, but all the analytical
and scienti?c studies suggested that if they were exposed, that exposure was negligible.

Sincerely, 
04/1444, 3Q Ego um 

Alvin L. Young, 

Professor

DISCLAIMER FOR VA REPORTS
The conclusions reached in this report are based upon a comprehensive review of
the historical records maintained in the publicly available files of the National
Archives and Record Administration, and other archival repositories. However, the
conclusions reached do not necessarily represent those of the Department of
Veterans Affairs or any other Department or Agency of the United States
Government.
This report is part of the Agent Orange Investigative Report Series, and should be
considered as an amendable or living document. If additional authenticated
documents or records are found that address the topic of this report, a re-evaluation
of the conclusions may be necessary.

 INVESTIGATIONS INTO THE ALLEGATIONS OF AGENT
ORANGE/DIOXIN EXPOSURE FROM FORMER
RANCH HAND AIRCRAFT
EXECUTIVE SUMMARY
Allegations made by former Air Force Reserve aircrews and maintenance
personnel have raised health concerns about residual amounts of Agent Orange
remaining in Post-Vietnam C-123K aircraft, that had been deployed by the
Reserves between 1972 and 1982. Despite a recent exposure assessment by the
Air Force Research Laboratory, Wright-Patterson AFB Ohio and a recent
Advisory Opinion by the Department of Veteran Affairs, that “it was unlikely that
any dioxins from such residues would lead to adverse health effects”, the
controversy has continued. The present report on “Investigations into the
Allegations of Agent Orange/Dioxin Exposure from Former RANCH HAND
Aircraft” is intended to present factors not previously evaluated by the earlier
assessment and Advisory Opinion.
There were three factors identified that were critical to the investigation. The first
factor was a critical need to search the historical records on the history of the
aircraft series C-123K and UC-123K. The “U” designation was for those aircraft
used in defoliation and crop destruction missions in Operation RANCH HAND
during the Vietnam War. The second factor was to understand the nature of the
“dry” Agent Orange residues that were found in some of the aircraft, but especially
those found in “Patches”, an aircraft “retired” to the Air Force Museum in 1980
and that underwent decontamination in 1994 prior to its display to the public. The
third factor was to determine how the exposures could have occurred and their
significance to the health of the Air Force Reserve personnel who comprised the
aircrews and who serviced the aircraft after they were returned to the United States
and used as cargo aircraft during the period of 1972-1982.
Results of the investigation into Factor Number 1. Twenty-three of 34 UC123Ks were returned to the United States in 1970-1971, and after reconditioning
were assigned to Air Force Reserve units. These aircraft had been assigned to
RANCH HAND beginning in May 1968, but most of the UC-123Ks arrived in
Vietnam between December 1968 and November 1969, a time when defoliation
1

 operations were significantly reduced. Moreover, after November 1969, the
Department of Defense directed that the use of Agent Orange be restricted and the
tactical herbicides Agents Blue and White be substituted. Four of the UC-123K
were reassigned to the Aerial Spray Flight at Rickenbacker AFB Ohio. Thus, Air
Force Reserve crews were more likely to have flown in the 47 C-123Ks that were
returned from Vietnam, rather than assigned to one of the 19 remaining aircraft
that had been flown in Operation RANCH HAND, or if formerly assigned to
RANCH HAND, an aircraft that very likely did not spray Agent Orange.
Verification of the tail numbers provided in a veteran-prepared report confirmed
that only 6 of 26 aircraft assigned to the 731st Tactical Airlift Squadron, Westover
AFB Massachusetts were former RANCH HAND aircraft.
Results of the Investigations in Factor Number 2. The allegations put forth by
former Air Force Reserve crew and maintenance personnel were that the residues
within the 19 UC-123Ks reassigned post-Vietnam to their Reserve units were from
Agent Orange, and that the magnitudes of these residues were exemplified by
analytical studies conducted during the decontamination of “Patches”, a former
RANCH HAND aircraft donated to the National Museum of the US Air Force,
Wright-Patterson AFB Ohio. A search of the historical records provided a detailed
history of “Patches” to include its assignments in international locust control
programs, its use in tests and evaluations of spray equipment at Eglin AFB,
Florida, to its use in Vietnam not only in Operation RANCH HAND, but
frequently reconfigured for its use in hauling cargo or for insecticide missions in
Operation FLYSWATTER. The extensive activities of “Patches” put in doubt the
analytical results of the 4 (and only 4) samples analyzed for dioxin and furans in
1994. Indeed, the fingerprint of the analytical results suggested the potential
contamination or cross contamination by PCBs, the insecticide Lindane, and other
aromatic materials. Clearly “Patches” was not a representative aircraft for
determining Agent Orange residues. Certainly the odors reported by veterans could
be attributed to such pesticides as DDT or Carbanyl (Sevin®) rather than Agent
Orange.
In 1996 and 2009, UC-123K aircraft in quarantine storage at the 309th Aerospace
Maintenance and Regeneration Group (AMARG) at Davis-Monthan AFB were
sampled for the residual Agent Orange constituents 2,4-D, 2,4,5-T and the
associated dioxin, TCDD. One hundred forty samples (140) were collected from 4
2

 aircraft with known histories of defoliation missions in Vietnam. The results
indicated that no Agent Orange residues were found on the exterior of any aircraft
or in air samples taken inside the tightly-closed aircraft. Two of the aircraft had
trace levels of residues, near the lowest limit of analytical detection, on the
fuselage floor, and were essentially considered “clean”. The other two aircraft had
levels of Agent Orange residues on all interior fuselage surfaces that were tested.
The average concentrations found in these two aircraft were statistically near the
risk-based screening level for dioxins, based on a one-year industrial exposure
scenario. The question remained, were these residues actually capable of providing
a measureable exposure or dose to aircrew or maintenance personnel?
Results of the Investigation into Factor Number 3. The contaminant TCDD
found in the dry residues within Post-Vietnam UC-123Ks was not water soluble.
The only method for extracting and measuring TCDD within the aircraft interior
surfaces was through the use of wipe samples “wetted” with the organic solvent
hexane. Although there were measurable levels of TCDD within these dried
residues, studies of dermal contact with TCDD have found that any exposures that
occurred were “negligible” because the skin is a major barrier to TCDD uptake,
contributing less than 1% over the long term to the body burden. Vapor exposures
to TCDD at near ambient temperatures were extremely unlikely to result in any
significant dose because TCDD is not volatile below 420° C (~ 780 °F).
Four epidemiological or analytical studies of Vietnam veterans or professional
sprayers of 2,4,5-T herbicide provided supporting evidence that “primary” or
“secondary” exposure to TCDD associated with the spraying of Agent Orange
would not have resulted in diseases caused by the herbicides or its associated
TCDD. However, it is important to note that all the analytical and scientific studies
cannot prove that the Air Force Reserve aircrews and maintenance personnel
assigned to the UC-123K were not exposed to Agent Orange and its associated
dioxin contaminant. However, all the analytical and scientific studies suggested
that if they were exposed, that exposure was negligible.

3

 INTRODUCTION
In the March/April 2008 issue of Orion Magazine, an article was published titled:
“Agent Orange: A Chapter from History That Just Won’t End- The author’s
article focused on his visit to the Aerospace Maintenance and Regeneration Center
(AMARC) on Davis-Monthan Air Force Base (AFB), Arizona where the
remaining UC-123K RANCH HAND aircraft were stored for more than 22 years
[1]. Why the visit? “Because I’ve come to bear witness to American folly, to rest
my eyes on the flying machines that flattened the forest of Southeast Asia, poisoned
its people, and changed my life.” The author, Ben Quick, described how his father
had served in Vietnam and was the victim of a “chemical rain (i.e., Agent Orange)
that falls on American troops as they slink through the hinterlands of Vietnam in
search of Viet Cong.” Mr. Quick cited stories of Agent Orange and its associated
dioxin, and thus concluded that the reason the UC-123K aircraft remained in an
isolated location at AMARC was because the aircraft were contaminated by Agent
Orange and its associated dioxin. This very sincere and emotional article triggered
a cascade of concern by various Air Force Reserve aircrews that had flown some
of those aircraft from 1972 – 1982, some 2 to 10 years after cessation of the
RANCH HAND defoliation program during the Vietnam War [2].
In 2011, a retired Air Force officer who had served with an Air Force Reserve Unit
filed a complaint with the Air Force Inspector General alleging that the Air Force
knew that UC-123Ks were used for spraying Agent Orange in Vietnam and that the
Air Force had failed to properly inform post-Vietnam aircrews of the risks [3]. The
Air Force issued a “Consultative Letter” released on 27 April 2012 and prepared
by the Air Force Research Laboratory, Wright-Patterson AFB, Ohio titled: UC-123
Agent Orange Exposure Assessment [4]. Subsequently, on 25 September 2012,
the Veterans Benefits Administration, Department of Veterans Affairs issued an
“Advisory Opinion” on “Service-connection based on exposure to Agent Orange
due to flying C-123 aircraft” [5]. Both the Consultative Letter and the Advisory
Opinion essentially agreed that it was unlikely that “any dioxins from residual
Agent Orange on aircraft surfaces, or that any exposure would lead to adverse
health effects.” The Compensation Service recommended that such claims
associated with Agent Orange exposure be denied service-connection. Never-theless, the Department of Veterans Affairs released a Public Health Notice on Agent
Orange Residue on Post-Vietnam War Airplanes that concluded: “Although the
4

 risk of long-term health problems from exposure to Agent Orange residue on postVietnam C-123 airplanes is minimal, Veterans who believe they have exposurerelated health problems may file a claim for disability compensation. These claims
will be decided on a “case-by-case basis” [6].
Despite the actions and conclusions by the Department of the Air Force and the
Department of Veterans Affairs, the controversy has continued. Although the
allegations were primarily related to the potential of remaining residues to provide
a source for exposure and dose, there are other factors that need to be evaluated.
FACTORS CRITICAL TO THE EVALUATION OF THE ALLEGATIONS
In October 2012, a veteran-prepared report “Request for Congressional Assistance
with C-123 Veterans’ Claims: Establishing Agent Orange Exposure to Veterans”
was distributed to various Congressional delegations [7]. The essence of the report
focused on the following three issues or factors critical to establishing that the
United States Air Force Reserve crews and maintenance personnel were exposed to
toxic chemical residues from post-Vietnam aircraft, and were thus entitled to
service-connected compensation:
• That the aircraft alleged to be the sources of the exposure to residues of
Agent Orange and its associated dioxin (2,3,7,8-tetrachlorodibenzo-pdioxin, TCDD) were the former RANCH HAND UC-123Ks, and that these
aircraft were generally the only aircraft that Air Force Reserve personnel at
selected Air Reserve units flew or maintained;
• That the residues within these remaining UC-123Ks were from Agent
Orange, and that the magnitudes of these residues were exemplified by
analytical studies conducted during the decontamination of “Patches”, a
RANCH HAND aircraft donated to the National Museum of the US Air
Force, Wright-Patterson AFB Ohio; and,
• That the Agent Orange/ dioxin residue within the aircraft was a primary
route of exposure and posed a far greater risk than those experienced by the
RANCH HAND crews that flew the UC-123Ks in Vietnam because those
crews were exposed for generally just one year, not multiple years as were
the Air Force Reserve crews.

5

 A BRIEF HISTORY OF THE C-123, UC-123B, AND THE UC-123K
In order to understand how many UC-123K aircraft were available to Air Force
Reserve units after the termination of the RANCH HAND program in Vietnam, it
was necessary to review the history of the C-123 aircraft. That history began in
July 1955 when the Tactical Air Command’s 309th Troop Carrier Group, Ardmore
AFB Oklahoma took delivery of the first Fairchild C-123B “Provider”, a twinengine transport designed for assault operations into landing zones that had been
only rudimentarily prepared. Fairchild Corporation of Hagerstown, Maryland
produced 300 C-123B aircraft between the years 1954-1958 [8].
In November 1961, six Providers were sent to South Vietnam to start Operation
RANCH HAND, the defoliation program. In December 1961, an additional
squadron of C-123Bs (16 aircraft/squadron) were deployed to Vietnam from the
464th Troop Carrier Wing, Pope AFB, North Carolina. By the fall of 1964 there
were four USAF C-123B squadrons flying airlift and airdrop missions. All of these
squadrons, including RANCH HAND aircraft, were assigned to the 315th Air
Commando Wing (later renamed the 315th Special Operations Wing) and which
would remain the principal organization for all C-123B squadrons until 1970 [8].
By March 1965, RANCH HAND (now designated as the 12th Air Commando
Squadron) was deploying seven UC-123Bs (the “U” designating spray aircraft) for
defoliation and crop destruction missions [9]. In April 1968, the first UC-123Ks
arrived at Bien Hoa Air Base, Vietnam. The 12th Air Commando Squadron was the
last of the five units in the 315th Wing to get the improved aircraft [8].The UC-123
“K” models were reworked “B” models with a powerful J-85-17 jet engine on each
wing outboard of the conventional engines, improved engine armor plating, a
strengthened windshield, a larger spray pump, and a flowmeter to assure a constant
chemical flow rate of 3 gallons per acre [9].

Additional UC-123Ks continued to arrive in May 1968. By the end of June 1969,
29 UC-123Ks were assigned to the RANCH HAND squadron, and by November
1969, shortly after it reached a peak of 34 assigned aircraft, the squadron was
suddenly reduced to 14 aircraft with the released aircraft reassigned to airlift units
or returned to the Continental United States (CONUS) [8, 9]. On 1 January 1970,
6

 the 315th Special Operations Wing was re-designated as the 315th Tactical Airlift
Wing, while the RANCH HAND unit retained its Special Operations Squadron
title. On 31 March 1970, USAF Headquarters again directed reduction of the spray
squadron to eight aircraft – six for herbicide and two for insecticide – by the end of
June 1970 [8]. In July 1970, the remaining aircraft were moved to Phan Rang, the
Headquarters of the 315th Tactical Airlift Wing. On 28 January 1971, the Joint
Chief of Staff officially cancelled all further USAF herbicide missions. RANCH
HAND crewmen continued flying the two insecticide missions (Operation
FLYSWATTER) until December 1971 [9].
A total of 46 aircraft were modified for spray operations in the ten years of
herbicide and insecticide operations in Vietnam. This included 12 UC-123Bs that
were never modified as “K” models, and 34 UC-123Ks. The last “B” model
aircraft left Vietnam in January 1969. Nine RANCH HAND aircraft were lost to
crashes, including 1 UC-123K in February 1971 [9]. As previously noted,
beginning in November 1969 many of the UC-123Ks were either transferred to
USAF airlift squadrons, or transferred to the South Vietnam Air Force, or assigned
to Air America operations. These transfers required the removal of the spray
systems including the 1,000-gallon tank, console, and spray boom. Never-the-less,
the aircraft retained their designation as “UC-123Ks”. The tanks and spray booms
were not returned to CONUS, but left at Da Nang, Bien Hoa, or Phan Rang.
A search of the historic records concluded that 183 C-123Bs were modified to “K”
models, to include the 34 UC-123Ks assigned to RANCH HAND. Between 2
February 1962 and 27 December 1971, 62 of the various modified C-123s were
lost to crashes/accidents in Vietnam. Like the UC-123Ks, the remaining C-123K
aircraft were also widely distributed as the gradual withdrawal of US Forces from
the Republic of Vietnam occurred. A large number of C-123Ks were transferred to
the South Vietnam Air Force, with the first squadron transferred in 1970, and
three additional USAF C-123K Squadrons by September 1971. Air America
received 35 various C-123Bs, C-123Ks, or UC-123Ks; these were primarily
assigned to airlift missions in Laos [10].
The records indicated that a total of 47 USAF C-123Ks and 23 UC-123Ks returned
to CONUS throughout 1970 – 1971 with most of these aircraft being initially
assigned to 309th Aerospace Maintenance and Regeneration Group (AMARG),
7

 Davis-Monthan AFB Arizona. All of these aircraft underwent a reconditioning
prior to reassignments to selected USAF Air Reserve units in 1971-1972. It should
be noted that 4 of the UC-123Ks were assigned directly to the Air Force Reserves
907th Tactical Airlift Group, Rickenbacker Air National Guard Base Ohio. These 4
aircraft retained their special configuration for aerial spraying as part of USAF’s
aerial insecticide operations [11]. Eighteen of the 23 UC-123Ks returned to
AMARG between 1980 and 1986.
CONCLUSIONS
Contrary to the allegation, both C-123K and UC-123Ks aircraft were assigned to
USAF Reserve units during the period 1972 -1982. A few of the UC-123Ks had
been assigned to RANCH HAND beginning in May 1968, but most of the UC123Ks arrived in Vietnam between December 1968 and November 1969, a time
when defoliation operations were significantly reduced. Moreover, after November
1969, the Department of Defense directed that the use of Agent Orange be
restricted and the tactical herbicides Agents Blue and White be substituted. Thus,
Air Force Reserve crews were more likely to have flown in C-123Ks, rather than
assigned to an aircraft that had been formerly assigned to RANCH HAND, or if
formerly assigned, an aircraft that very likely did not spray Agent Orange.
Verification of the tail numbers provided in the veteran-prepared report confirmed
that only 6 of 26 aircraft assigned to the 731st Tactical Airlift Squadron, Westover
AFB Massachusetts were former RANCH HAND aircraft [7].
THE ALLEGATIONS ON THE RESIDUE REMAINING IN THE UC-123Ks
The October 2012 veteran-prepared report repeatedly emphasized that the residues
remaining within the UC-123K aircraft assigned to Air Force Airlift Reserve units
were those related to Agent Orange [7]. The basis for this conclusion came
primarily from the analyses conducted in 1994 on one aircraft, namely “Patches”
(tail number 56-4362) that had been donated to the National Museum of the United
States Air Force, Wright-Patterson AFB Ohio in 1980 [12]. During its service in
RANCH HAND, the aircraft had taken more than 600 hits from enemy ground fire,
hence, the name “Patches” [13].
The history of “Patches” is informative as to the types of potential residues that
could be present within its air frame. “Patches” was one of the original six C8

 123Bs located at Pope AFB North Carolina that was modified for aerial spraying
and was sent to South Vietnam, arriving in January 1962. However, before it was
involved in defoliation missions, it was diverted at the request of the Department
of State to the Middle East for locust control. It departed 2 May 1962 from Saigon
to Tehran, Iran where it sprayed over 17,000 acres in Iran and Afghanistan with the
insecticide Lindane [9, 14], returning to Langley AFB Virginia on 10 June 1962.
On 14 June 1962, it was redeployed to Eglin AFB Florida to participate in a 30-day
test of aerial spray equipment on Test Range C-52A of the Eglin Military
Reservation [9]. While at Eglin, it sprayed the tactical herbicide “Purple” in the
first tests of the modified aerial spray equipment [15]. Following its return to
Vietnam, it was immediately dispatched to treat locust infestations with 57%
Malathion [16]. From January through May 1963, it was temporarily converted to
supporting logistical operations delivering ammunition, general cargo including
maintenance supplies, and personnel [9].
From June 1963 through most of 1966, “Patches” supported RANCH HAND
operations in both defoliation and crop destruction missions. However, on 14
October, “Patches” was reconfigured and dispatched again to treat locust
infestations in Thailand with 95% Malathion [9, 16]. In April 1967, Patches was
ordered permanently assigned to Malathion duty under the direction of the MACV
(Military Assistance Command, Vietnam) Surgeon General’s Office and in support
of Operation FLYSWATTER [17]. In June 1968, “Patches” left the insecticide
flight to return to the United States for modification as a K model. In October
1968, “Patches”, now a UC-123K, returned to Vietnam and temporarily returned to
flying defoliation missions, primarily involving- White (from a review of the Daily
Air Activity Reports, 1 October – 1 December 1968). However, in late November
1968, it was returned to mosquito control duty. After termination of Operation
FLYSWATTER in December 1971, “Patches” returned to CONUS in 1972 and
served in the Air Force Reserves in an airlift capacity until it was retired to the
USAF Museum in 1980 [13].
A 1994 – 1997 decontamination of “Patches” at the USAF Museum focused
exclusively on the presence of dioxins and furans with the data converted to
2,3,7,8-TCDD equivalents (TEQs) and reported in nanograms per wipe sample.
The assumption was that the 17 congeners identified of polychlorinated dibenzo-pdioxins (PCDD) and polychlorinated dibenzofurans (PCDF) were congeners that
9

 confirmed the residue was from Agent Orange [12]. There were only four wipe
samples reported in the 1994 analyses, and the TEQ equivalents ranged from 4.1
ng/m2 to 1,400 ng/m2. An analytical study conducted by Dow Chemical Company
of 82 samples of 2,4,5-T confirmed that the only quantifiable dioxin in 2,4,5-T was
the 2,3,7,8-TCDD, although some samples showed traces of the penta (PnCDD),
hexa (HxCDD), and hepta (HpCDD) PCDDs [18]. The wipe sample having the
largest concentration of TCDD may have been a result of the aircraft spraying
Agent Purple while at Eglin AFB in 1962, since Purple had much higher levels of
TCDD than Agent Orange [15]. The other PCDDs and the PCDFs may have been
present as a result of PCB-leaking electric transformers, and pentachlorophenoltreated ammunition boxes, both frequently transported as cargo in Vietnam [19].
The potential presence of PCBs and pentachlorophenol contributing to the residue
also presented the possibility that the actual analytical methods may have had
interferences from polychlorinated aromatics, and the values were not accurately
determined [20]. This may have been especially true of the potential presence of
2,3,4,6-tetrachlorobenzene and 2,3,4,6-tetrachloro-phenylmethyl ether, both
persistent products of the breakdown of Lindane, the insecticide sprayed by
“Patches” in 1962 [14, 20]. Two subsequent wipe samples taken after
decontamination in 1995 showed an average interior 2,3,7,8-TCDD concentration
of 45 ng/m2 [12].
The above assessment suggested that residues in “Patches” may not have been
“representative” of the residues that may have persisted in other UC-123K aircraft
deployed by Air Force Reserve units. From 1986 through 2010, there were 18 UC123K aircraft being stored with AMARG at Davis-Monthan AFB. These aircraft
were owned and managed by the 505th Aircraft Sustainment Squadron (ACSS),
Hill AFB Utah. Four of the 18 aircraft were sampled for residual Agent Orange
components, namely the herbicides 2,4-D and 2,4,5-T and the associated dioxin
contaminant 2,3,7,8-TCDD [21]. The history of all 4 aircraft indicated they had
been deployed in RANCH HAND missions in Vietnam (tail numbers 54-086, 544571, 55-4532, and 55-4544) [9, 10]. A total of 140 samples were collected from
the 4 aircraft. The wipe samples consisted of gauze pads wetted with hexane for
dioxin samples and with water for herbicide samples. Importantly, a
comprehensive sampling protocol ensured that all key internal and external

10

 surfaces were sampled in replicate for all four aircraft. Additionally, air samples
were taken from within each aircraft [21].
The results of the sampling and analyses for the four aircraft are shown in Figures
1 and 2. The dioxin data were expressed in TEQs to be consistent with the data
from “Patches”, although the primary dioxin was 2,3,7,8-TCDD with traces of the
HpCDD and Octadibenzo-p-dioxin (OCDD) being detected in a few samples [21].
The fingerprint pattern of the 17 congeners identified in samples from “Patches”
was not present in samples from the 4 aircraft sampled at AMARG. The analytical
results for the 4 former RANCH HAND aircraft sampled at AMARG indicated:
There were no detectable levels of the phenoxy herbicides or associated TCDD on
the exterior of the 4 aircraft that were sampled;
There were no detectable levels of the phenoxy herbicides or TCDD found in any
of the air samples collected inside the 4 aircraft sampled;
Two of the 4 aircraft had trace levels of 2,4-D and 2,4,5-T at the detection level of
230 µg/m2 and 150 µg/m2, respectively (these were considered very low values);
and
Two of the 4 aircraft had low levels of dioxin and phenoxy herbicides on all
interior surfaces that were sampled (average concentrations of 14.6 and 18.2 ng/m2
TEQ, 518 and 502 µg/m2 2,4,5-T, and 587 and 453 µg/m2 2,4-D for aircraft 554571 and 55-4532, respectively) [21].
Some additional observations: 1. Interior floor areas were not found to be more
heterogeneously contaminated than interior wall surfaces. In fact, interior floor
concentrations were uniform in the two aircraft with measureable residual
contamination; 2. The results were consistent with previous sampling for phenoxy
herbicides that was conducted in 1996. Both aircraft that were found to have trace
concentrations (55-4544 and 54-0585) had non-detectable levels of herbicides on
the fuselage floor in the 1996 samples. The two aircraft that had low levels of
dioxins (TEQs) and herbicide concentrations in all interior surface samples (554532 and 55-4571) also had detectable herbicide levels in samples taken from the
floor in 1996; and, 3. Concentrations of dioxins found during the 2009 sampling

11

 event were significantly lower than concentrations found in “Patches” in 1994, or
composite samples collected in 1995 after decontamination, i.e., 45ng/m2 [21].

Concentration (ng/m2)

30
25

23 ng/m2 screening level value
20
15
10
5
0
55-4571

55-4532

55-4544

54-0585

Aircraft Tail Number

Figure 1. Average Interior Concentrations of Dioxins Reported as ng/m2 TEQ, Compared to
the Risk-Based Screening Level Value of 23 ng/m2. Error bars indicate 95% upper confidence
limits for average values approaching the risk-based standard [21].

12

 100000

100,000 µg/m2 screening level value
Concentration (ug/m2)

10000

1000

100

10

1
55-4571

55-4532

55-4544

54-0585

Aircraft Tail Number

Figure 2. Average Interior Concentrations of 2,4-D (blue, diagonal fill) and 2,4,5-T (red, solid
fill), Compared to the Risk-Based Screening Level Value of 100,000 µg/m2. Note log scale of
concentration axis [21].

In Figure 2, note that aircraft 55-4544 had no detectable levels of 2,4,5-T,
suggesting that this UC-123K was a late arrival in 1969 to RANCH HAND and
was very likely used primarily for spraying of Agent White, a formulation of
picloram (Tordon) and 2,4-D.
Another issue related to the UC-123Ks controversy, was the issue of
“smells/odors” in the aircraft. All three of the tactical herbicides had distinct odors.
Although TCDD does not have an odor, Agent Orange had a “butanol-like” odor
that was very persistent, i.e., years. Malathion and Lindane also had persistent
odors, but there was another source not identified in the C-123Ks that returned
from Vietnam, and that had to do with the odor associated with the quarantine
procedures used for all returning aircraft and equipment from Vietnam [22].
During the gradual withdrawal of US Forces from the Republic of Vietnam,
equipment and material which were not designated for turnover to the Vietnamese
Air Force (VNAF) were returned to CONUS for further utilization. This
“retrograde cargo” was required to undergo international quarantine procedures
designed to eradicate disease vectors, insects and other pests, thus preventing their
13

 introduction into the United States [22]. If a UC-123K or any of the C-123K
models transported cargo in Vietnam and were reassigned to CONUS, they were
frequently tasked to carry retrograde cargo, and hence were required to undergo
quarantine procedures. The Military Quarantine Inspector was responsible for the
inspection and certification of aircraft and retrograde cargo. The processing and
quarantine procedures were conducted at major military installations in Vietnam.
When the cargo was palletized and loaded onto the aircraft and ready for treatment,
it was covered with plastic and tied down with cargo nets. The treatment consisted
of a micronized DDT and Carbaryl (Sevin®) forcefully injected under the plastic
covers. Even when the cargo was removed at destination, the odor of these
insecticides persisted in the aircraft for many years [22]. It should be noted that
EVERY C-123 aircraft, including the C-123K models, returning from Vietnam
was subjected to quarantine procedures. The odors from these persistent pesticides
were present in these aircraft for many years, and were likely those odors mistaken
for Agent Orange, i.e., noting that 20 of the 26 aircraft identified in the October
2012 report “Establishing Agent Orange Exposures to Veterans” were not
RANCH HAND aircraft but alleged to have been, based on odors presumably
associated with residues [7].
CONCLUSIONS
The allegations put forth by former Air Force Reserve crew and maintenance
personnel were that the residues within the 19 UC-123Ks reassigned post-Vietnam
to their Reserve units were from Agent Orange, and that the magnitudes of these
residues were exemplified by analytical studies conducted during the
decontamination of “Patches”, a former RANCH HAND aircraft donated to the
National Museum of the US Air Force, Wright-Patterson AFB Ohio. A search of
the historical records provided a detailed history of “Patches” to include its
assignments in international locust control programs, its use in tests and
evaluations of spray equipment at Eglin AFB, Florida, to its use in Vietnam not
only in Operation RANCH HAND, but frequently reconfigured for its use in
hauling cargo or for insecticide missions in Operation FLYSWATTER. This
extensive multiple activities of “Patches” put in doubt the analytical results of the 4
(and only 4) samples analyzed for dioxin and furans in 1994. Indeed, the
fingerprint of the analytical results suggested the potential contamination by PCBs,
the insecticides Lindane , and other aromatic materials. Clearly “Patches” was not
14

 a representative aircraft for determining Agent Orange residues. Certainly the
odors reported by veterans could be attributed to pesticides rather than Agent
Orange, namely DDT and Carbanyl (Sevin®).
In 1996 and 2009, UC-123K aircraft in quarantine storage at the 309th Aerospace
Maintenance and Regeneration Group (AMARG) at Davis-Monthan AFB were
sampled for the residual Agent Orange constituents 2,4-D, 2,4,5-T and the
associated dioxin, TCDD. One hundred forty samples (140) were collected from 4
aircraft with known histories of defoliation missions in Vietnam. The results
indicated that no Agent Orange residues were found on the exterior of any aircraft
or in air samples taken inside the tightly-closed aircraft. Two of the aircraft had
trace levels of residues, near the lowest limit of analytical detection, on the
fuselage floor, and were essentially considered “clean”. The other two aircraft had
levels of Agent Orange residues on all interior fuselage surfaces that were tested.
The average concentrations found in these two aircraft were statistically near the
risk-based screening level for dioxins, based on a one-year industrial exposure
scenario. The question remained, were these residues actually capable of providing
a measureable exposure or dose to aircrew or maintenance personnel?
THE ALLEGATIONS THAT SIGNIFICANT EXPOSURE OCCURRED
The most important and relevant allegation was that the herbicide and dioxin
(Agent Orange) “dry residues” within the UC-123K aircraft represented a primary
route of exposure and, thus potentially posed a far greater risk than those
experienced by the RANCH HAND crews that flew those same UC-123Ks in
Vietnam. The supposition was that RANCH HAND aircrews and maintenance
personnel were exposed to Agent Orange for generally just one year, not the
multiple years as were contended by the post-Vietnam Air Force Reserve crews.
The assumption that analytical values of the “dry dioxin residues”, obtained
through the use of “wipe” samples taken from within the interior surfaces of PostVietnam UC-123Ks, are determinants of the degree and level of individual
exposures is simply not valid. The dioxin, 2,3,7,8-TCDD, tenaciously adheres to
surfaces and is essentially inert because it is not susceptible to chlorination or
dechlorination reactions, thus its long persistence time [23]. Extensive studies on
the photodegradation of TCDD were conducted by Crosby et al., [24]. They found
15

 that in sunlight and in the presence of Agent Orange, the TCDD molecule was
readily dechlorinated (destroyed) because the n-butyl formulation of Agent Orange
provided a hydrogen donor essential for the dechlorination of TCDD [24]. The acid
forms of 2,4-D and 2,4,5-T (the forms founds in the 2009 studies of residues in 4
UC-123Ks at AMARG, Figure 2) do not contribute the necessary organic
hydrogen donor, and hence the continued persistence of TCDD [21]. The studies
of TCDD persistence at Eglin AFB Florida confirmed that in the absence of the
herbicide and sunlight, TCDD residues were still detected 25 years after massive
levels of Agents Purple and Orange had been aerially sprayed on Test Area C-52A
in the early and mid-1960s [15].
In their assessments of exposure to the TCDD within the UC-123K aircraft, Air
Force Reserve personnel suggested that there were two major routes of exposure.
The first was the residue that aircrews or maintenance personnel came in dermal
contact with, and the time (duration and frequency) of that contact. The second
route of exposure was through inhalation. It was logical for the aircrews to assume
that if they could smell an odor, then through inhalation they were being exposed
to what was in the odor [7, 25].
Dermal Exposure: Dioxin (TCDD) is essentially water insoluble. In both the
studies conducted with “Patches” (1994) and the 4 aircraft at AMARG (2009), the
TCDD residues on the interior surfaces of the aircraft were removed through the
use of wipes “wetted” with the organic solvent hexane [12, 21]. Although there
were measurable levels of TCDD in the residues, extensive studies have shown
that actual dermal contact with TCDD contributes no more than 1% (and probably
considerably less) over the long term to the body burden, and that 1% was
considered by the Center for Disease Control and Prevention (CDC) to be a
“negligible” exposure [26]. Thus, the skin is a major barrier to exposure from
TCDD [26]. The risk assessments that have used the analytical data from the
hexane wipe samples failed to recognize that those analytical values cannot be
extrapolated to represent a human “dose”. This approach has been labeled the “big
leap” in defining exposure in a population, e.g., aircrews, and the environmental
matrix, e.g., the residues on the interior walls, and the storage of dioxin in the
human body [27].

16

 Inhalation Exposure: In considering inhalation of TCDD from the air contained
within the UC-123K aircraft, the single most important property of TCDD is its
“volatility”. To understand how the values of volatility for TCDD are derived, see
Appendix 1.The data in Figure 2 of the Appendix clearly shows that the vapor
pressure of 2,3,7,8-TCDD is extremely low, including at elevated temperatures. At
ambient temperature (around 25o C, 77° F) TCDD is essentially in a solid state and
its vapor pressure is about 9 to 11 orders of magnitude lower than that of liquid
water. The 2,3,7,8-TCDD will only melt around 420o C (788° F). At 100o C (212°
F), the boiling temperature of water, the vapor pressure of 2,3,7,8-TCDD is 7 to 8
orders of magnitude lower than that of water. As a consequence vapor exposures
to TCDD vapors at or near ambient temperatures are extremely unlikely to result in
a significant dose. To explain it in more practical terms, to have had TCDD
volatilize within the crew compartment of the UC-123K, the air temperature would
have had to be approximately 420° C or 788° F.
Supporting Epidemiologic Studies: The veteran-prepared report “Request for
Congressional Assistance with C-123 Veterans’ Claims: Establishing Agent
Orange Exposures to Veterans” claimed that that the exposures received by Air
Force Reserve personnel were “primary exposures”, while RANCH HAND
exposures were “secondary exposures”. In reality, a pathway that would have
represented a primary exposure to Agent Orange and its associated TCDD would
have been a direct exposure to the liquid herbicide. A “secondary exposure”
would have been through secondary pathways such as the consumption of
contaminated food, or the drinking of water with contaminated sediments. These
are called “environmental exposures” and represent an indirect exposure [28].
There are two examples of veterans allegedly receiving environmental exposures.
The first study compared the blood serum TCDD levels in 646 ground combat
troops who served in heavily sprayed areas of Vietnam against 97 veterans who did
not serve in Vietnam [29]. The 646 combat veterans had served one tour in III
Corps, a heavily sprayed part of Vietnam near Saigon. Exposure estimates were
based on military records and on self-reporting. For the Vietnam veterans, the fact
that military records appeared to validate that they were exposed, coincided with
their own perception of being exposed. However, the concentration of TCDD
levels in Vietnam and non-Vietnam veterans were nearly identical, ~ 4 parts per
trillion (ppt) [29]. To the Vietnam veterans in this study, the perception of
17

 exposure and the reality of exposure were not the same, and the use of military
records to determine locations of combat veterans in relation to RANCH HAND
missions were also not good indicators for validating exposure to Agent Orange.
The second study was a 30-year postservice mortality study of a cohort of 9,324
male US Army veterans who had served in Vietnam, and whose presumption of
exposure would have been consistent with the Department of Veterans Affairs
policy [30]. The Vietnam veteran cohort was matched with a cohort of 8989 male
non-Vietnam veterans [30]. The conclusion as reported in 2004:
Vietnam veterans continued to experience higher mortality than non-Vietnam
veterans from unintentional poisonings and drug-related causes. Death rates from
disease-related conditions, including cancers and circulatory diseases, did not
differ between Vietnam veterans and their peers, despite the increasing age of the
cohort (mean age, 53) and the longer follow-up (average, 30 years) [30].
There are two examples of long term populations studies where the cohorts were
exposed to either Agent Orange in Vietnam or to the spraying of 2,4,5-T herbicide.
The first study was the Air Force Health Study (AFHS). In 1982, the US Air Force
initiated the Air Force Health Study, a study of the men of Operation RANCH
HAND, the US-Vietnam allied program for the aerial application of herbicides
during the Vietnam War [31]. For the 20-year study there were two cohorts; one
cohort included 1,261 RANCH HAND veterans, and the other cohort represented
the comparison group that consisted of 19,109 veterans who flew C-130s in
Vietnam. The protocol used a matched retrospective cohort design intended to
independently determine mortality, morbidity, and reproductive health [31].
The strength of AFHS was enhanced during the second physical examination in
1987 with the development of TCDD determination in blood serum at the parts per
trillion level (ppt). Of the 995 RANCH HAND who were fully compliant in 1987
for the physical examination, 932 had serum specimens analyzed by CDC. The
serum values for TCDD ranged from less than 10 ppt (considered “background”)
to 618 ppt. The highest values were found in the maintenance personnel who came
into direct contact with the liquid herbicide, and who were responsible for loading
the herbicide into the planes, cleaning the spray equipment and repairing the
aircraft [31]. During the six examinations conducted over the 20 years, the AFHS
18

 investigated over 300 health endpoints on multiple occasions. The results of the
AFHS did not provide evidence of disease in the RANCH HAND veterans
caused by their elevated levels of exposure to Agent Orange and its associated
TCDD contaminant [31].
The second study of a populations exposed to TCDD involved 2,4,5-T herbicide
applicators in New Zealand [32]. Of 548 men employed as professional pesticide
applicators in New Zealand from 1979 through 1982, nine were selected who had
sprayed 2,4,5-T over a range of 7 to 30 years. Their blood serum levels ranged
from 3 to 131 ppt (mean of 53 ppt TCDD), where the variation in TCDD was
related to their duration of work exposure to 2,4,5-T. The authors concluded that
increased risks from brief exposure to phenoxyherbicides are probably not
attributable to the TCDD that contaminates 2,4,5-T herbicide [32].
CONCLUSIONS
The contaminant TCDD found in the dry residues within Post-Vietnam UC-123Ks
was not water soluble. The only method for extracting and measuring TCDD
within the aircraft interior surfaces was through the use of wipe samples “wetted”
with the organic solvent hexane. Although there were measurable levels of TCDD
within these dried residues, studies of dermal contact with TCDD have found that
any exposures that occurred were “negligible” because the skin is a major barrier
to TCDD uptake, contributing less than 1% over the long term to the body burden.
Vapor exposures to TCDD at near ambient temperatures were extremely unlikely
to result in any significant dose because TCDD is not volatile below 420° C (~ 780
°F).
Four epidemiological or analytical studies of Vietnam veterans or professional
sprayers of 2,4,5-T herbicide provided supporting evidence that “primary” or
“secondary” exposure to TCDD associated with the spraying of Agent Orange
would not have resulted in diseases caused by the herbicides or its associated
TCDD. However, it is important to note that all the analytical and scientific studies
cannot prove that the Air Force Reserve aircrews and maintenance personnel
assigned to the UC-123K were not exposed to Agent Orange and its associated
dioxin contaminant. However, all the analytical and scientific studies suggested
that if they were exposed, that exposure was negligible.
19

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2012
7. Carter, WT (2012): Request for Congressional Assistance with C-123
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14. MacCuaig RD (1973): The Occurrence of Insecticides in the Blood of Staff
of a Locust Control Organization. WHO Pesticide Series, Report 269,
Lindane (Available from the Armed Forces Pest Management Board
Literature Retrieval System, Accession No. 90706)
15. Young AL, Newton M (2004): Long Overlooked Historical Information on
Agent Orange and TCDD Following Massive Applications of 2,4,5-TContaining Herbicides, Eglin Air Force Base, Florida. Env Sci Pollut Res
11 (4):209-221 (Available from the Armed Forces Pest Management Board
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16. Aerial Spray Operations (1963): Annual Report for 1963, Special Aerial
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a War. Env Sci Pollut Res 15 (1) 3-7 (Available from the Armed Forces Pest
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Workshop, Hanoi Viet Nam, 18-20 June 2007. Env Sci Pollut Res 15 (2):
113-118
19. Rappe C, Marklund S, Kjeller L-O, Bergqvist P-A, Hansson M (1985):
Composition of Polychlorinated Dibenzofurans (PCDF) Formed in PCB
Fires. Chapter 29, IN: Keith LH, Rappe C, Choudhary. Chlorinated Dioxins
and Dibenzofurans in the Total Environment. Butterworth Publishers,
Boston
20. Smith LM, Johnson JL (1985): Evaluation of Interferences from Seven
Series of Polychlorinated Aromatic Compounds in an Analytical Method for
Polychlorinated Dibenzofurans and Dibenzo-p-dioxins in Environmental
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Dioxins and Dibenzofurans in the Total Environment. Butterworth
Publishers, Boston
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Characterization of UC-123K Aircraft, Phase 1. Prepared for the 505
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 Select Engineering Services, 1544 North Woodland Park Drive, Layton
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22. Porter KR (1971): Air Force Retrograde Program. Proceedings of the
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Selected Chlorinated Dioxins on Flyash from Municipal Incinerators Using
Laboratory Simulation of Emission Processes. Chapter 34, IN: Keith LH,
Rappe C, Choudhary. Chlorinated Dioxins and Dibenzofurans in the Total
Environment. Butterworth Publishers, Boston
24. Crosby DG, Moilanen KW, Wong AS (1973): Environmental Generation
and Degradation of Dibenzodioxins and Dibenzofurans. Environ Health
Perspect 5(3): 259-266 (Available from the Armed Forces Pest Management
Board Literature Retrieval System, Accession No. 83739)
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of C-123 Veterans. Veteran Veritas: Advocacy for our Veterans, March 12,
2012
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(2010): Human Uptake of Persistent Chemicals from Contaminated Soil:
PCDD/Fs and PCBs. Regulatory Toxicology and Pharmacology 57: 43-54
27. Needham LL, Patterson DG, Turner WE (2002): Comparison of Assessing
Levels of 2,3,7,8-Tetrachlorodibenzo-p-dioxin in Selected Populations by
Biomonitoring and Exposure Indices. Organohalogen Compounds 59:131134
28. Jones KC, Bennett BG (1989): Human Exposure to Environmental
Polychlorinated Dibenzo-p-dioxins and Dibenzofurans: An Exposure
Commitment Assessment for 2,3,7,8-TCDD. The Science of the Total
Environment 78: 99-116 (Available from the Armed Forces Pest
Management Literature Retrieval System, Accession No. 145214)
29. CDC (1988): Serum 2,3,7,8-Tetrachlorodibenzo-p-dioxin Levels in US
Army Vietnam-Era Veterans. Journal of the American Medical Association
260 (9): 1249-1254 (Available from the Armed Forces Pest Management
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30. Catlin Boehmer TK, Flanders WD, McGeehin MA, Boyle C, Barrett DH
(2004): Postservice Mortality in Vietnam Veterans: A 30-Year Follow-up.
Archives of Internal Medicine 164: 1908-1916
31. Buffler PA, Ginevan ME, Mandel, JS, Watkins DK (2011): The Air Force
Health Study: An Epidemiologic Retrospective. Annals of Epidemiology 9:
673-687
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 32. Smith AH, Patterson DG, Jr., Warner ML, MacKenzie R, Needham LL
(1992): Serum 2,3,7,8-Tetrachlorodibenzo-p-dioxin Levels of New Zealand
Pesticide Applicators and Their Implications for Cancer Hypothesis. Journal
of the National Cancer Institute 84(2): 104-108

23

 APPENDIX 1*
Evaluation of the Vapor Pressure of TCDD over a range of temperatures
Vapor pressure is an important physicochemical parameter for predicting the
atmospheric concentrations of given compounds. Practically, it can be used to
determine the transport and fate of contaminants in the environment and to
characterize exposure in the context of a risk assessment. However, the precise
measurement of the vapor pressure of low-volatility substances is an experimental
challenge. This is the case of dioxins and more specifically 2,3,7,8-TCDD, for
which a the range of values of vapor pressure found in the literature spread over
several orders of magnitude.
Below is a summary of various values of vapor pressure reported in peer reviewed
literature for 2,3,7,8-TCDD. In order to give these values a concrete meaning they
were compared to the vapor pressure of water at different temperature. The data
was synthesized in a graphic format and the numerical values are compiled in
Table 1.
Vapor Pressure of TCDD at Different Temperatures
Generally, the reported vapor pressure of 2,3,7,8-TCDD ranges between 7.4x10

-10

-5

to 3.4x10 mm Hg (9.9x10-8 and 4.5x10-3 Pa) (ATSDR, 1998).
In 1984, Schroy and co-workers identified some data gaps in the physical and
chemical properties of 2,3,7,8-TCDD. At the time, the vapor pressure of solids
was seldom studied and no data was available for TCDD. Therefore, they
undertook a research program to define the physical properties of 2, 3, 7, 8tetrachlorodibenzo-p-dioxin (TCDD), including its vapor pressure. They provided
estimates for a wide range of temperatures between 25 and 421oC. The vapor
pressure spanned over about 11 orders of magnitude, between 1.5x10-9 and 7.6x102
Pa (Schroy et al., 1984). One should take caution with the reliability of these
results since they differ from values published in later years by several orders of
magnitude.
In a subsequent study, the same team reported all the physical and chemical data
available for TCDD at the time, including the vapor pressure between 30 and 71oC

24

 (Schroy et al., 1985). For some undetermined reasons, these results were about
two orders of magnitude higher than those published in 1984.
In 1986, Podoll et al. studied the rates of volatilization and photolysis of TCDD.
They measured the average vapor pressure in air at 25oC to be 7.4 +/- 0.4 x 10-10
Torr which corresponds to about 0.987 x 10-7 Pa (Podoll et al. 1986). This value is
comparable to those published by Schroy et al., Rordorf et al. as well as Delle Site 1
in the same temperature range (Delle Site; 1996, Rordorf 1989; Schroy et al.,
1985). Shroy et al. also reported the boiling point of dioxin to be 421.2oC (Schroy
et al., 1985).
More recently, Li et al. predicted the vapor pressure of 59 PCDDs and 131 PCDFs.
Overall their results were higher than those published by Rordorf et al. even
though the calculation methods were the same. In particular, the vapor pressure of
2,3,7,8-tetrachlorinated dibenzo-p-dioxin, at 25oC was estimated to be 6.2x10-6 Pa,
which was 31-fold higher than the values provided by Rordorf (2.0x10-7 Pa). The
results of these studies are plotted in Figure 1.

*From: Investigations into the Allegations of Agent Orange/Dioxin Exposure from Former RANCH HAND
Aircraft. Agent Orange Investigative Report Series, No. 2, November 2012
1

Delle Site reported vapor pressure values measured for different temperatures using 6 different methods.

25

 1.E+03
1.E+02
1.E+01

Vapor Pressure (Pa)

1.E+00
1.E-01

-40

60

160

260

360

460

1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
1.E-08
1.E-09
Li et al. 2005

Temperature (oC)
Shroy et al. 1985

Podoll et al. 1986

[Shroy et al. 1984]

Rordorf, 1989

Delle Site 1996

Figure 1. Reported values of vapor pressure for 2,3,7,8-TCDD at different
temperatures.

26

 Table 1. Published values of TCDD vapor
pressure as a function of temperature.
Temperature
Pressure
Reference
(oC)
(Pa)
[Shroy et al.
25
1.49E-09
1984]
30
3.40E-09
50
7.15E-08
305
4.95E+01
421.2
7.60E+02
Shroy et al. 1985
30.1
4.53E-07
54.6
1.83E-05
62
4.97E-05
71
1.59E-04
Podoll et al. 1986
25
9.87E-08
Rordorf 1989
25
2.00E-07
50
9.50E-06
75
2.60E-04
100
4.60E-03
125
5.70E-02
Delle Site 1996
24.85
9.90E-08
29.85
2.02E-07a
70.85
2.02E-07a
24.85
1.30E-07
24.85
3.50E-06b
24.85
6.30E-06b
24.85
2.00E-07
24.85
6.20E-07
Li et al. 2005
25
6.20E-06
50
1.90E-04
75
3.50E-03
100
4.50E-02
125
4.10E-01
a
-7
Reported as a pressure of 2.02x10 measured for
a temperature ranging from 303 K to 344 K
b
Reported that a temperature of 298 K
corresponding to a pressure measurement between
3.5x10-6 and 6.3x10-6 Pa
27

 Comparison with the vapor pressure of water
The values of vapor pressure reported above were compared to the vapor pressure
of water at different temperature. These values were calculated using the Antoine
equation expressed as follows:
log 10 (P) = A − (B / (T + C))
where P is the vapor pressure (bar), T is the temperature (K) and A, B and C are
parameters depending to the temperature and determined in various studies.. The
values of these parameters were found on the website of the National Institute of
Standards and Technology (NIST) 2 and are summarized in Table 2.
Table 2a. Parameters used in the Antoine equation to determine the vapor
pressure of water as a function of temperature.
Temperature
(K)

A

B

C

Reference

Comment

379. - 573.

3.55959

643.748

198.043

Liu and
Lindsay, 1970

Coefficients
calculated by
NIST from
author's data.

273. - 303.

5.40221

1838.675

-31.737

Bridgeman and
Aldrich, 1964

Coefficients
calculated by
NIST from
author's data.

304. - 333.

5.20389

1733.926

-39.485

Bridgeman and
Aldrich, 1964

Coefficients
calculated by
NIST from
author's data.

334. - 363.

5.0768

1659.793

-45.854

Bridgeman and
Aldrich, 1964

Coefficients
calculated by
NIST from
author's data.

344. - 373.

5.08354

1663.125

-45.622

Bridgeman and

Coefficients

2

NIST website. Physical properties of water available at:
http://webbook.nist.gov/cgi/cbook.cgi?Name=water&Units=SI&cTG=on&cTP=on

 Aldrich, 1964

calculated by
NIST from
author's data.

293. - 343.

6.20963

2354.731

7.559

Gubkov,
Fermor, et al.,
1964

Coefficients
calculated by
NIST from
author's data.

255.9 - 373.

4.6543

1435.264

-64.848

Stull, 1947

Coefficients
calculated by
NIST from
author's data.

a

Table available on the NIST website:
http://webbook.nist.gov/cgi/cbook.cgi?Name=water&Units=SI&cTG=on&cT
P=on

29

 Table 3. Vapor pressure of
water as a function of
temperature
Temperature
Pressure
o
( C)
(Pa)
-17.25
1,386.354
-0.15
6,041.849
19.85
23,720.56
29.85
42,073.58
30.85
44,542.64
59.85
197,896.1
60.85
207,276.5
69.85
310,853.1
70.85
323,335.8
89.85
697,060.3
99.85
1,007,867
99.85
992,317.2
105.85
1,004,907
299.85
69,619,644
The calculated values of water vapor pressure are reported in Table 3. They are
also compared with the values reported for 2,3,7,8-TCDD in Figure 2.

30

 Vapor Pressure (Pa)

1.E+07
1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
1.E+01
1.E+00
1.E-01 -40
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
1.E-08
1.E-09

Water freezing
About 1 Atmosphere

Water boiling point

60

Li et al. 2005
Delle Site 1996
[Shroy et al. 1984]

160

260

TCDD boiling point
o
(421.2 C)

360

460

Temperature (oC)
Shroy et al. 1985
Rordorf, 1989

Podoll et al. 1986
Water

Figure 2. Comparison of the vapor pressure for 2,3,7,8-TCDD
with that of water at different temperature

 Conclusion
Figure 2 clearly shows that the vapor pressure of 2,3,7,8-TCDD is extremely low,
including at elevated temperatures. At ambient temperature (around 25oC) TCDD
is essentially in a solid state and its vapor pressure is about 9 to 11 orders of
magnitude lower than that of liquid water. 2,3,7,8-TCDD will only melt around
420oC. At 100oC, the boiling temperature of water, the vapor pressure of 2,3,7,8TCDD is 7 to 8 orders of magnitude lower than that of water. As a consequence,
vapor exposures to TCDD vapors at or near ambient temperatures is extremely
unlikely to result in a significant dose.

References
ATSDR. 1998. Chlorinated Dibenzo-p-Dioxin. In: Services USDoHaH, editor.
Toxicological profile. U.S. Department of Health and Human Services, Atlanta,
GA, pp. 721.
Delle Site A. The Vapor pressure of environmentally significant organic
chemicals: A review of methods and data at ambient temperature J. Phys. Chem.
Ref. Data, Vol. 26, No.1, 1997
Li XW, Shibata, E and Nakamura, T. 2005. Thermodynamic prediction of vapor
pressures for polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans,
and polybrominated dibenzo-p-dioxins, Environmental Toxicology and Chemistry,
Vol. 24, No. 9, pp. 2167–2177
Podoll RT, Jaber HM, Mill T. 1986. Tetrachlorodibenzodioxin: rates of
volatilization and photolysis in the environment. Environ Sci Technol; 20: 490492.
Rordorf BF. 1989. Prediction of vapor pressures, boiling points and enthalpies of
fusion for twenty-nine halogenated dibenzo-p-dioxins and fifty-five dibenzofurans
by a vapor pressure correlation method. Chemosphere, Vol.18, Nos.l-6, pp 783788
Schroy JM, Hileman FD, Cheng SC. 1985. Physical/chemical properties of
2,3,7,8-TCDD. Chemosphere 1985; 14: 877-880.
32

 Schroy JM, Hileman FE, Cheng SC. 1984. The uniqueness of dioxins? physical /
chemical characteristics. For Presentation at the 8th ASTM Aquatic Toxicology
Symposium to be held on April 15, 16, and 17, 19S4 at the Draw Bridge Inn, Fort
Mitchell, Kentucky.

33