This article originally appeared in Environmental Research Volume 130, April 2014, Pages 34–42 at



Post-Vietnam Military Herbicide Exposures in UC-123 Agent Orange Spray Aircraft


Peter A. Lurker1

Fred Berman2

Richard W. Clapp3

Jeanne Mager Stellman4*


1 Germantown Consultants, LLC, Germantown, OH, USA

2 The Center for Research on Occupational and Environmental Toxicology, Oregon Health and Science University, Portland, OR, USA

3 Boston University School of Public Health, Boston MA, USA

4Department of Health Policy & Management Mailman School of Public Health, Columbia University, New York, NY, USA


Corresponding author: Jeanne Mager Stellman

Department of Health Policy & Management

Mailman School of Public Health, Columbia University

600 West 168th Street, 6th floor, New York, NY, 10032

212-342-3123; cell: 718-614-0635; fax: 212-305-3405.



KEYWORDS: Agent Orange, dioxin, exposure modeling, veterans, Vietnam War


Background: During the Vietnam War, approximately 20 million gallons of herbicides, including ~10.5 million gallons of dioxin-contaminated Agent Orange, were sprayed by 34 C-123 aircraft that were subsequently returned to the United States, without decontamination or testing, to three Air Force reserve units for transport operations (~1971-1982). In 1996, observed dioxin contamination led to withdrawal of these C-123s from public auction and to their smelting in 2009. Current Air Force and Department of Veterans Affairs policies stipulate that “dried residues” of chemical herbicides and dioxin had not lead to meaningful exposures to flight crew and maintenance personnel, who are thus ineligible for Agent Orange-related benefits or medical examinations and treatment. Sparse monitoring data are available for analysis.

Methods: We used three complementary approaches for modeling potential exposures to dioxin in the post-Vietnam war use of the aircraft by: (1) using 1994 and 2009 Air Force surface wipe data to model personnel exposures and to estimate dioxin body burden for dermal-oral exposure for dried residues using modified generic US Environmental Protection intake algorithms; (2) comparing 1979 Air Force 2,4- dichlorophenoxyacetic acid and 2,4-5- trichlorophenoxyacetic acid air samples to maximum saturated vapor pressure concentrations to estimate potential dioxin exposure through inhalation, ingestion and skin contact with contaminated air and dust; and (3) applying emission models for semivolatile organic compounds from contaminated surfaces to estimate airborne contamination.

Results: Model (1): Body-burden estimates for dermal-oral exposure were 0.92 and 5.4 pg/kg body-weight-day for flight crew and maintainers. The surface wipe concentrations were nearly two orders of magnitude greater than the US Army guidance level. Model (2): measured airborne concentrations were at least five times greater than saturated maxima, yielding dioxin estimates that ranged from 13.2 – 27.0 pg/m3 and supporting the likelihood of dioxin dust adsorption. Model (3): Theoretical models yielded consistent estimates to Model 2 (11, 23 and 46 pg/m3), where the range reflects differences in experimental value of dioxin vapor pressure used. Model (3) also supports airborne contamination and dioxin dust adsorption.

Conclusions: Inhalation, ingestion and skin absorption in aircrew and maintainers were likely to have occurred during post-Vietnam use of the aircraft based on the use of three complementary models. Measured and modeled values for dioxin exceeded several available guidelines. Deposition-aerosolization-redeposition homeostasis of semivolatile organic compound contaminants, particularly dioxin, is likely to have continually existed within the aircraft. Current Air Force and Department of Veterans Affairs policies are not consistent with the available industrial hygiene measurements or with the widely accepted models for semivolatile organic compounds.


KEYWORDS: Agent Orange; phenoxyherbicides; dioxin; exposure modeling; veterans; Vietnam War


  • Thirty-four US Air Force UC-123 aircraft that were used to spray approximately 10 million gallons of dioxin-contaminated herbicides and 10 million gallons of other herbicides during the Vietnam War were returned to the United States for use by Air Force Reserve units, 1971-1982, without prior decontamination and with no monitoring of personnel.
  • Current Air Force and Department of Veteran Affairs policies stipulate that no meaningful dioxin exposures to flight crews or maintainers occurred because dioxin was bound in “non-available dried residue.”
  • We sought to evaluate this policy and the potential exposures from surface contaminated dried residues using three different, complementary methods for modeling dioxin exposure: (1) application of the US Environmental Protection Agency generic model for dermal-oral absorption from measured surface residues; (2) application of the industrial hygiene saturated vapor pressure model to historical air samples; and (3) thermodynamic modeling for surface emissions of semivolatile organic compounds.
  • Model 1, the dermal-oral exposure model yielded estimates of 0.92 and 5.4 pg/kg-BW day for flight crews and maintainers (1971-1982). Both estimates exceed the US EPA acceptable daily intake value of 0.7 pg/kg BW-day (US Environmental Protection Agency, 2012). Values derived from surface wipes were more than five times greater recommended screening levels.
  • Models (2) and (3) for airborne contamination yielded consistent results that ranged from 11 to 46 pg/m3 dioxin, which exceeds the German Maximum Allowable Worker Concentration of 10 pg/m3, the only available standard.
  • It is likely that post-Vietnam UC-123 flight crews and maintainers were exposed via the dermal-to-oral, inhalation and ingestion routes of exposure to dioxin.
  • Re-examination of current federal policies with regard to these exposures is indicated.


2,4-D              2,4-Dichlorophenoxyacetic acid

2,4,5-T           2,4,5-Trichlorophenoxyacetic acid

A                      Aircraft interior surface

AT                   Averaging time

BW                  Body weight

CFa                  Area conversion factor

CF wt               Weight conversion factor

Cs                      Contaminant surface concentration

ED                    Exposure duration

EF                     Exposure frequency

Fom_part       Volume fraction organic matter in airborne particles

FTga                 Decimal fraction absorbed from gastrointestinal tract

FTre                  Decimal fraction contaminant removed from skin-to-mouth

FTsm                Decimal fraction of contaminated skin touched to mouth

FTss                  Decimal fraction contaminant transferred surface to skin

FTwe                 Decimal fraction of contaminant collected onto wipe

h                        Convective mass-transfer coefficient

I                         Systematic Intake

IARC                 International Agency for Research on Cancer

Koa                    Octanol/air partition coefficient

Kp                      Airborne particle/air partition coefficient

NIOSH              National Institute of Occupational Safety and Health

OSHA               Occupational Safety and Health Administration

Q                       Ventilation rate

RH                    Probability of Ranch Hand aircraft

SA                     Exposed skin surface area

STP                   Standard temperature and pressure

SVP                   Saturated vapor pressure

SWSL               Surface wipe screening level

UC-123             Ranch Hand aircraft, known as the “Provider”

UV                     Ultraviolet light

WD                    Type of worker

yo                       Gas-phase concentration in contact with the emission surface

ρparticle            Density airborne particles



PAL expresses gratitude for the opportunity to have been assigned to work with a team of medical professionals at the United States Air Force School of Aerospace Medicine to identify any post Vietnam C-123 Ranch Hand exposure data and conduct an exposure assessment (November 2011-April 2012) and also the hard work and long hours the US Air Force Reserve Military Airlift personnel, including the C-123 staff did to get their passengers, patients and cargo delivered, who facilitated his work as an active duty bioenvironmental engineer conducting in-flight industrial hygiene surveys (1980-1984) on US Air Force Reserve missions.


1.1 Historical context.

Between 1962 and 1971, the United States Air Force carried out Operation Ranch Hand in which approximately 20 million gallons of herbicides were sprayed by Fairchild UC-123 aircraft over a relatively small area (~16%) of the Republic of South Vietnam in order to defoliate vegetation used for concealment and to destroy crops used by enemy combatants. Approximately 10.5 million gallons were a 50:50 mixture of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), popularly known as Agent Orange. The 2,4,5-T was contaminated with 2,3,7,8‑tetrachlorodibenzodioxin, which will be referred to here as dioxin. The herbicides were shipped in color-coded drums, which accounts for their nicknames. Table I summarizes the known quantities of herbicides sprayed and number of aircraft (sorties) associated with each mission and Table II shows the distribution of missions by agent used and number of aircraft in the mission. (Stellman, Stellman, Christian, Weber, & Tomasallo, 2003). Some Operation Ranch Hand aircraft also sprayed the insecticide malathion. Table II provides data on the number of sorties (individual airplanes flown per mission) that were required to carry out this vast operation. The last Agent Orange Ranch Hand mission was on April 16, 1970 and missions using other herbicides ended January 7, 1971 (U.S. Department of Defense, 1970).

After service in Vietnam, the UC-123 spray planes were reassigned, from 1971 to 1982, to the Air Force Reserve for aero-medical evacuation missions. They were not decontaminated or tested for herbicides or dioxin contamination levels before their return to stateside service. No personal air samples or biological monitoring for herbicide exposure are known ever to have been collected from flight crew or aircraft maintenance personnel during post-war aircraft use. A complete list of all the Operation Ranch Hand aircraft and their fate has not been made public by the Air Force. Using unofficial lists, we estimate that about 34 aircraft carried out all the Ranch Hand operations shown in Tables I and II.

Operation Ranch Hand aircraft were equipped with a 1000-gallon tank and pump to force liquid herbicide under pressure into lines connected to spray booms, one under each wing and a third beneath the centerline of the aircraft (Young, 2009). On average, each aircraft flew about 6000 herbicide missions and became heavily contaminated with chemical residues during loading, maintenance, fueling and while on missions. Few precautions were taken inasmuch as the herbicides were not thought to be harmful to humans (Military Assistance Command-Vietnam, 1966). Planes were usually flown with pilot and co-pilot cockpit windows and aft rear cargo door open (Meek, 1981). A typical Ranch Hand mission employed more than one aircraft flying in formation, but, as shown in Table II, missions could include from one to twelve aircraft. Spray legs were often repeated in a single mission such that planes would fly through previously sprayed airspace. Herbicide mist would enter the aircraft and deposit throughout their interiors. If pressurized spray lines were broken through malfunction, battle damage or maintenance mishap, they would release significant amounts of liquid herbicide into the aircraft interior.

1.2 Contamination arises as an issue.

In 1979, air samples for 2,4,5-T, 2,4-D and malathion, but not dioxin, were taken from the interior of the aircraft known as “Patches” at Westover Air Force Base following complaints of persistent chemical odors (Conway, 1979). Patches had flown herbicide missions in Vietnam from 1961-1965. It is uncertain whether Patches was used for herbicide missions 1965-1967; however, in 1967 it was assigned to insecticide missions only. The bulk of herbicide spraying took place after Patches ceased to spray these chemicals. In 1980, Patches was retired to the National Aviation Museum of the United States Air Force (“Fairchild C-123k Provider,” n.d.), then to the USAF Museum at Wright-Patterson Air Force Base, OH. At the museum, staff concerns about dioxin exposure led to another round of testing. Based on a three-sample surface wipe survey of Patches, Weisman recommended restorers use Tyvek® coveralls and full-face respirators with high efficiency particulate filters and public entry and interior storage of materials or spare parts be prohibited (Weisman & Porter, 1994).

Other planes from the spray fleet were stored at the 309th Aerospace Maintenance and Regeneration Group facilities at Davis-Monthan Air Force Base, Arizona, and subsequently offered for public sale; however, surface contamination tests revealed 2,4-D and 2,4,5-T above an unstated detection level (Porter, 1997). Extensive and costly follow-up tests for dioxin were recommended, but to our knowledge no further testing was undertaken. Instead, given public health concerns over dioxin, the Air Force Materiel Command Law Office withdrew the aircraft from sale in December 1996 (U.S. Dept. of Air Force, 1996). This withdrawal led to unsuccessful litigation by purchasers for damages from investments made based on sales contracts. The Court denied claims for damages because “the C-123s evidenced the presence of hazardous chemical contamination and under applicable regulations, the aircraft could not be sold until they were decontaminated” (Board of Contract Appeals, General Services Administration, 2000).

In 2009, some of the aircraft stored by the Aerospace Maintenance and Regeneration Group were tested for dioxin residues. Of 138 samples, only 16 samples were taken from interior surfaces in two Ranch Hand aircraft. Each interior sample was positive for dioxins (US Dept. of Air Force, 2009). As expected, all exterior samples were below detection limits given that dioxins rapidly decompose in sunlight (Choudhry & Webster, 1989). The available dioxin surface wipe data from both testing rounds are summarized in Table III. All but two aircraft were smelted at an off-base contractor-operated smelting unit for conversion to aluminum ingots. The aircraft remain on display, but, unlike many other displayed aircraft, the public is not permitted entry into any of these aircraft.

1.3 Health and Policy Considerations

Dioxin exposure is a major health consideration for herbicide-exposed veterans, and 2,4,7,8-tetrachlorodibenzodioxin is the most potent dioxin congener. Dioxin is an impurity created during the manufacture of 2,4,5-T. Limited post-war testing of unused military herbicides revealed dioxin contamination levels as high as 45 ppm in Agent Purple and 13 ppm in Agent Orange (Stellman, Stellman, Christian, Weber, & Tomasallo, 2003). Dioxins are highly persistent in the environment. Their high lipophilicity leads them to be stored for long periods in body fat. The biological half-life in humans has been estimated at between 5 and 10 years (Milbrath, Wenger, Chang, et al., 2009). Acute adverse health effects from dioxin exposure include chloracne, a severe acne-like condition (Suskind, 1985). Epidemiological studies have shown an association between dioxin and non-Hodgkin lymphoma (Bertazzi, Consonni, Bachetti, et al., 2001), soft tissue sarcoma (Zambon, Ricci , Bovo, et al., 2007), chronic lymphocytic leukemia (Blair and White, 1985), and cancers of the larynx, lung, and prostate (IOM, 2006). The International Agency for Research on Cancer has classified dioxin as a human carcinogen (Group 1) (IARC Working Group on the Evaluation of Carcinogenic Risks to Humans, 1997). In animals, dioxin is a developmental toxicant causing skeletal deformities, kidney defects, and weakened immune responses in the offspring of animals exposed to dioxin during pregnancy (Abbott, Harris, & Birnbaum, 1992; Holladay et al., 1991). Indeed, it was data on possible birth defects in laboratory animals associated with 2,4,5-T that set off a string of administrative actions to restrict both domestic use and military use of the chemical in Vietnam (Hay, 1982). Long simmering controversies over the health effects of Agent Orange led Congress to pass the Agent Orange Act of 1991 (Martini, 2012). A provision of the Act instructs the Department of Veteran Affairs to contract with the Institute of Medicine to conduct scientific reviews of military herbicides used in Vietnam and of Vietnam-veteran health. The Institute of Medicine publishes biennial reviews of all available scientific evidence on health effects of the herbicides (Institute of Medicine (U.S.) Committee to Review the Health Effects in Vietnam Veterans of Exposure to Herbicides., 2009). The Secretary of the Department of Veteran Affairs takes Institute of Medicine recommendations on the likely relationship between military herbicide exposure and specific diseases into consideration in developing benefit policies for Vietnam veterans. Sixteen diseases in veterans or their offspring were eligible for compensation in 2013.

Current Department of Veterans Affairs policy limits automatic awarding of military herbicide benefits to veterans with service in Vietnam or its interior waterways. Other veterans, those who did not have “boots on the ground” but may have come into contact with the same military herbicides, specifically produced for use in Vietnam, such as during disposal and testing operations, are not granted presumption of exposure but must establish, individually, the fact of his or her exposure. However, crew and maintenance personnel who operated the spray planes 1971-1982 in the United States are specifically denied benefits because the risk for exposure is “extremely low and therefore, the risk of long-term health effects is minimal” (emphasis in original) ( U.S. Dept. of Veterans Affairs, 2012). Similarly, the Air Force has concluded that potential Agent Orange exposures to post-Vietnam C-123 flight crews and passengers were unlikely to have exceeded acceptable regulatory standards or to have predisposed persons in either group to experience future adverse outcomes (Smallwood, 2012).

1.4 Approach

Here we apply three different and complementary accepted modeling methodologies to the previously described historical Air Force sampling data in order to estimate potential exposure in people who may have worked on or in proximity to the contaminated spray aircraft during post-Vietnam War assignments. We compare our estimates to available guidelines and standards and discuss implications of our findings with respect to current Veterans Administration and Air Force policies.




2.1 Dioxin Dermal-Oral Exposure from Direct Contact

We used the surface wipe data obtained by two Air Force studies (US Department of the Air Force (USAF), 2009; Weisman & Porter, 1994) shown in Table III to estimate potential intake from dermal-to-oral ingestion associated with hand-to-mouth transmission. May, et al., (2002) and later the US Army Center for Health Promotion and Preventive Medicine (2009) adapted the generic intake model (Equation 1), developed by the US Environmental Protection Agency (1989), to derive risk-based wipe surface screening levels for industrial scenarios.

Equation 1


I = intake (milligram/kilogram (mg/kg) body weight-day)

C = chemical concentration

CR = contact rate (inhalation rate, ingestion rate, absorption rate)

EFD = exposure frequency and duration

BW = body weight

AT = averaging time

In the surface-screening level model, the contact rate (CR in Equation 1) is the product of estimates for the following factors:

  • exposed skin surface area (SA)
  • decimal fraction of contaminant transferred from surface to skin (FT)
  • decimal fraction of contaminated skin touched to mouth (FTsm)
  • decimal fraction of contamination removed from skin to mouth (FTre)
  • weight conversion factor (CFwt)
  • decimal fraction absorbed from gastrointestinal tract (FTga)

Exposure frequency and duration (EFD in Equation 1) are estimated by four factors:

  • exposure frequency, hand to mouth events per day (EF)
  • work days per year (WD)
  • exposure duration (ED)
  • probability of being on a Ranch Hand aircraft (RH)

Exposure frequency factors were derived as follows. Pilot and crew flight time is based on interview data obtained by one of us (PAL) from a Westover Air Force Base, Air Force Reserve pilot assigned to a C-123 between 1973 and 1981 (Lurker, 2013). We also used that author’s (PAL) experience (1984-1986) as an industrial hygienist for Aerospace Maintenance and Regeneration Group and his personal observations of the four museum volunteers to modify parameters. Because the Air Force has not made public the identifying numbers of the aircraft used in Operation Ranch Hand, we relied on experienced personnel involved in the Westover Air Force Base operations who reported that eleven of the 24 C-123 aircraft assigned at Westover Air Force Base were previously Ranch Hand aircraft (Lurker, 2013). For purposes of our model we assumed that the remaining twenty-two Ranch Hand aircraft were evenly divided between the two other twenty-four plane squadrons (Pittsburgh International Airport Air Reserve Station and Rickenbacker Air Force Base). Therefore, we hypothesized there to be an 11/24 or 0.46 probability that any single mission in the post-Vietnam period for these three Air Force Reserve squadrons would have been on a Ranch Hand aircraft (RH = 0.46).

To be conservative in our estimate for the concentration C, we used the upper confidence limit of both the 1994 and 2009 aircraft. While we believe the 1994 measures on Patches are much more likely to replicate 1971-1982 exposure levels because they are closer in time to the events and the aircraft sampled in 2009 had been stored outdoors in the Tucson AZ desert where ultraviolet radiation and intense internal cabin heat would have degraded most of the dioxin present, we decided to err on side of caution.

Substitution of the parameters shown in Table IV leads to Equation 2 for estimating systemic intake (I):

Equation 2

The values we used for these factors, their units and sources are given in Table IV.

2.2. TCDD Airborne Contamination Estimates Using Maximum Saturation Vapor Pressure

In the second model, we applied the maximum saturation vapor pressure method to determine whether the airborne concentrations of herbicides measured by Conway (1979) exceed predicted levels expected to arise from vapor pressure alone. This method is widely used in industrial hygiene and inhalation toxicology, where Henry’s Law is used to estimate the maximum concentration of a solid or liquid substance that will become a gas in a closed space (Reinke, 2009). At standard temperature and pressure, the maximum saturation vapor pressure is simply the product of the vapor pressure and the molecular weight of the substance in question. If the measured concentration exceeds the maximum saturation vapor pressure, then an additional source of contamination, such as adsorption onto dust particles, must also be present.

We used the following vapor pressures: 1.4 x 10-7 mm Hg (Chemical Buyers, 2012) and 2‑ x 10-6 mm Hg (Walters, 2013) to calculate the saturation vapor pressures for 2,4-D and 2,4,5‑T, respectively, shown in Table V. Conversion factors are given in the footnote.

We then compared the saturated vapor pressure for 2,4-D and 2,4,5-T to the airborne concentrations in the air samples drawn by Conway (1979). Each measured value exceeded the maximum saturation vapor pressure. The ratios between the measured air concentrations and the maximum saturated vapor pressures are also shown in Table V. Because each substance in a mixture of substances will exert its own independent vapor pressure, we can assume that dioxin will also be present at a concentration that exceeds its maximum vapor pressure, just as the measured chemicals. In order to be conservative, we chose the lowest ratio of observed to maximum vapor pressure, which is five, and used this value to extrapolate the likely range of airborne concentrations that would have been found had Conway’s analysis included dioxin. Because the vapor pressure of dioxin is difficult to measure, a range of values has been reported in the literature. We used three different published saturated vapor pressures of dioxin, converted to mm Hg, in our model: 1.5 x 10-9 mm Hg (National Toxicology Program 2011) and 7.4 x 10-10 mm Hg (Podoll, Jaber, & Mill, 1986), 3 x 10-9 mm Hg (Weschler and Nazaroff, 2008). We used a published range of likely contamination levels of dioxin in 2,4,5-T: 45 parts per million and 13 ppm (Stellman et al., 2003).

2.3 TCDD Airborne Concentration Using Thermodynamic Emission Models

Finally, we employed a third model, based on theoretical emissions of semivolatile organic chemicals, like dioxin, using first principles of thermodynamics (Weschler and Nazaroff 2008), to estimate dioxin contamination levels in the interior of the spray aircraft, as illustrated schematically in Figure 1. We adapted the Little et al. (2012) generalized approach to calculate the extent to which dioxin will either be in the air above the dried residue or will have been adsorbed onto dust in the aircraft, a phenomenon that has been widely observed and for which Little et al. provide essential dioxin-specific parameters.

The concentration of dioxin in the atmosphere above the surface, y, will be a function of yo, its vapor pressure and the area A of residue in the aircraft capable of emitting the dioxin, as well as the ventilation rate and mass-transfer coefficient, Q and h, respectively (Equation 3a). While the model does take the ventilation rate Q is taken into account, it is not a critical factor because the surface contamination is a continual sink for emitting gases to be adsorbed onto dust. However, the driving force for potential occupational exposure is not such emission, which will be very low, but rather the adsorption of dioxin onto the dust particles (National Institute for Occupational Safety and Health, 1984). Dioxin is preferentially and strongly attracted to the dust and will partition onto the solid dust phase from the air phase above the surface. The degree of dust loading will be a function of the total mass of suspended particles, TSP, and Kp, the airborne particle/air partition coefficient (Equation 3b). A partition coefficient measures the comparative tendency for a substance to reside in one of two neighboring immiscible phases. In our model, the phases are the dust and the air above contaminated surfaces in the aircraft. Kp is the product of how much organic material is present in the dust (Fom_part, divided by the density of the dust particles, ρ particle,) and the ease with which dioxin preferentially transfers to the dust particles, measured by the octanol/air partition coefficient Koa, which Weschler and Nazaroff (2008) have shown to be the appropriate constant for describing the expected partitioning of a chemical between the gas phase and dust (Equation 3c).

Equation 3


Table VI gives the specific parameters we used for estimating the predicted concentration of dioxin for the UC-123 situation. Because the area of exposure could vary for crew and pilots, we calculated y twice, once with an area of 280 m2 and a second time with a doubled area of 560m2. Also, the Little et al method is strongly dependent on the value used to estimate the gas-phase concentration at the emission surface, yo. We thus used three published values for dioxin vapor pressure in our model.



3.1 TCDD Dermal-Oral Exposure from Direct Contact

Based on Equation 1, the estimated intake factor for the dermal-oral route was 0.92 pg/kg BW-day for flight crews and 5.4 pg/kg BW-day for maintainers at an assumed 95% upper confidence limit surface wipe concentration of 285 ng/m2. Both estimates exceed the US EPA acceptable daily intake value of 0.7 pg/kg BW-day (US Environmental Protection Agency, 2012). Figure 2 summarizes the estimated dermal-oral intake by exposure group (flight crew, maintainers, aero-medical evacuation patients, passengers, airborne or paratroopers, Aerospace Maintenance and Regeneration Group, and museum restoration workers). One set of body burden curves is shown at three different body weights, 60, 70 and 80 kilograms. Three exposure guidelines (2.3, 1.0 and 0.7 pg/kg-day, World Health Organization (2002), the Netherlands (Larsen, 2006) and US EPA (2012) respectively, are plotted for comparison. The worst-case maintainer (250 days per year) is also shown.

3.2 TCDD Estimates Using Maximum Saturation Vapor Pressure

Table V compares the Conway (1979) air samples to the calculated saturated vapor pressures. The ranges of ratios of observed-to-expected levels were substantially greater than unity: 63-138 and 5-7for 2,4-D and 2,4,5-T, respectively. The lowest ratio for 2,4,5-T, 5, yielded an estimate of 13 to 27 pg/m3 for dioxin, based on observed contamination levels of 13-45 ppm in historic samples.

3.3 TCDD Estimates Using Thermodynamic Models

Using the emission models developed by Little et al. (2012), with the parameters shown in Table VI and three input values for yo, the vapor pressure, or gas-phase concentration in contact with the emission surface, we calculated y, the airborne dioxin concentration, to be 11, 23 and 46 pg/m3, for an area of 280m2 and to be 12 pg/m3, 24 pg/m3, and 49 pg/m3 for an area of 560 m2. These theoretical values are in the same range as the estimates obtained from the saturated vapor pressure model based on the Conway (1979) air samples. Both the theoretical and the experimental models lead to values that exceed the only available standard for comparison, the German maximum allowable worker concentration of 10 pg/m3.



In this paper we have used three different complementary models to estimate potential occupational exposure to dioxins and military herbicides arising from dried surface residues within contaminated UC-123 Operation Ranch Hand spray planes that had been returned from Vietnam to service in the United States without prior decontamination. Sparse monitoring data (surface wipes and a small number of air samples) were available to us for this modeling. We used the surface wipe data to estimate dermal-oral absorption and the air sample data to estimate the possible concentration of airborne dioxin. As we discuss below, the two models yield levels that exceed recognized guidelines. Similarly, the third method, derived from thermodynamic principles, and not industrial hygiene measurements, also yielded levels that exceed guidelines.

The surface wipe data were used to develop a dermal-oral risk assessment using modification of the U.S. Environmental Protect Agency generic approach for intake, together with parameters defined by May and coworkers (2002) and the US Army (U.S. Army Center for Health Promotion and Preventive Medicine, 2009) in its analyses of dermal exposure to dried dioxin residues in office workers. The Army’s Technical Guidance and its algorithms are, to our knowledge, the only ones available for setting screen levels based on wipe samples. Though developed for occupational exposure to office workers, they are modifiable to other scenarios, using the methods we have applied here. Our calculations yielded occupational exposure estimates of 0.92 pg/kg BW-day for flight crews and 5.4 pg/kg BW-day for maintainers, at an assumed 95% upper confidence limit surface wipes concentration of 285 ng/m2. Other occupational groups were not substantially exposed according to the model. The US Army’s surface wipe screening level for dioxin surface wipe contamination is 3.54 x 10-5 µg/100 cm2 (which is equivalent to 3.54 ng/m2), based on a 10-6 cancer risk assessment (U.S. Army Center for Health Promotion and Preventive Medicine, 2009) and 10-year working lifetime. Our model uses a 12-year working lifetime. The levels measured in the samples were nearly two orders of magnitude greater than this guidance level.

Our results can also be compared to another set of dioxin exposure guidelines based on an EPA risk assessment paradigm from toxicity studies completed by the National Toxicology Program and validated by the Subcommittee on Dioxin, Committee on Toxicology in their 1988 report “Acceptable Levels of Dioxin Contamination in an Office Building Following a Transformer Fire” (Doull, 1988). The values for re-entry are 25 ng/m2 and 10 pg/m3 on surfaces and in air, respectively. At these levels of contamination, it is calculated that a 50 kg office worker working 250 days per year for 30 years would ingest 2 pg/kg dioxin per day for a cumulative lifetime maximum ingestion of 750 ng. The air and surface contamination re-entry values are exclusive; exposure is to either air exclusively or surface contact. If both air contamination and surface contamination exist, then the safe re-entry level for each must be reduced (e.g. if air contamination is 5 ng/m3, then surface contamination can be no higher than 12.5 ng/ m2 in order to satisfy re-entry guidelines). Based on our 95% upper confidence limit surface wipes concentration of 285 ng/m2 and calculated airborne concentrations of 11 to 46 pg/m2, we estimate that the lifetime exposure limit of 750 ng would have been reached in less than 3 years for an airman working full-time and this concentration is conservative, as discussed in the methods section.

The estimated daily intake of 0.92 pg/kg BW-day for flight crews and 5.4 pg/kg BW-day for maintainers exceeds the EPA 0.7 pg/kg BW-day acceptable daily intake (US EPA, 2012). The EPA estimate is based on lifetime exposure and our calculations are for a likely occupational exposure period, so the two values are not directly comparable. Our estimates suggest that post-Vietnam flight crew and maintainers will have exceeded their lifetime doses, particularly since expected background exposures are not included. Also, while our dermal-oral model used the worst-case scenario for years of exposure, it is likely to have underestimated the actual time spent in the aircraft, which was based on flight hours logged. Actual residence time was likely to be 25% to 50% higher (Lurker, 2013).

It is important to emphasize that, because surface wipe and air monitoring samples were collected some thirty and nine years, respectively, after the last spraying of herbicides in Vietnam, our analyses likely underestimate the degree to which aircraft personnel were exposed to dioxin. In the intervening years, surface dioxin contamination would have been substantially reduced through degradation, vaporization and adhesion to dust, mechanical removal from normal wear-and-tear, and cleanup efforts to remove chemical odors. The data showing higher internal dioxin surface contamination in Patches, from samples collected ~24 years after Viet Nam, as compared to data from the aircraft stored under Sonoran desert conditions, from samples taken ~39 years post Viet Nam, supports this notion of time and environmental effects to reduce surface dioxin contamination. Similarly, it is likely that herbicide and insecticide air concentrations were also reduced during the intervening nine years prior to air sampling. Nevertheless, we have used the values from all interior aircraft samples in our dermal to oral route of exposure model. Given the intervening time prior to sampling and sparse available data, it is remarkable that the three models used to estimate dioxin contamination yielded such consistent results.

We used two other models to estimate inhalation exposure of flight crews and maintainers and found that they were likely to have been exposed to airborne concentrations of TCDD that exceed the only available standard for comparison, the German maximum allowable worker concentration limit of 10 pg/m3.

The first inhalation model, based on a standard industrial hygiene and inhalation toxicology method of saturated vapor pressures, showed that the measured airborne levels of 2,4‑D and 2,4,5‑T were two orders of magnitude greater than predicted by the saturated vapor pressure, providing strong empirical evidence that the contaminants were adsorbed onto dust particles, which were continually deposited and re-suspended within the aircraft. The US National Institute for Occupational Safety and Health (1984) has noted that dust-adsorbed dioxin is a likely route of exposure, far exceeding exposure from gases arising from vapor pressure alone.

Our extrapolation for the concentration of dioxin present in the atmosphere is also likely to be an underestimate because we used standard temperature and pressure, while the conditions on the aircraft were often not standard. Extremes of temperature, changes in atmospheric pressure, vibration and other factors would have likely increased the vaporization rate, and hence led to higher levels of available dioxin, particularly since the interior of the aircraft was shielded from ultraviolet light, thereby minimizing ultraviolet degradation. This contention is supported by the positive interior wipe samples taken nearly four decades after the last herbicide exposures occurred. Further, the saturated vapor pressure model provides a conservative estimate of maximum exposure based on a closed environment model and based on a liquid. The aircraft had many air exchanges per hour and the residue was dried, yet the levels measured by Conway (1979) were orders of magnitude greater than the conservative saturated vapor pressures. Finally, Conway did not use pre-filters to trap particulates and, therefore, underestimated airborne concentration.

Model 3, based on theoretical emissions from contamination measured in the aircraft yielded results that were consistent with the levels of dioxin present estimated by the saturated vapor pressure method. Air samples with levels substantially above saturation, more than a decade after the last herbicide missions, strongly indicate that the aircraft must have been thoroughly coated with a film of herbicides and dioxins during Operation Ranch Hand and that there had never been an opportunity for the chemicals to be cleared by ventilation, either during the War or afterwards in the Air Force Reserves. The herbicides/dioxins had, in effect, become a permanent persistent presence on surfaces, as well as in the dust particles in the air, until the aircraft were destroyed. Given, in essence, an infinite sink for emissions from the legacy surface residue, there would have been a continuous reservoir for adsorption onto dust, even if regular ventilation were present. This is entirely consistent with the behavior of semivolatile organic compounds, as noted by Little et al. (2012). There is no reason to expect dioxin present in the surface residue to behave differently from 2,4,5-T and 2,4-D. In fact, there is good reason to believe that the relative proportion of dioxin present on dust would be greater than the phenoxyherbicides, because the Koa for dioxin is substantially larger than those for the herbicides (Weschler & Nazaroff, 2008) and Koa is the best predictor of the compound’s adsorption onto dust.

Finally, in most occupations with potential dioxin-exposure, dermal absorption is the primary route of dioxin exposure (Kerber, Reitz, & Paustenbach, 1995). Our model considered only hand-to-mouth dermal factors and did not include this important source of contamination. Dermal absorption modeling is difficult and only limited hexane surface data are available to us. The VA has questioned the utility of hexane-based surface sampling: “There is a low probability that transfer of TCDD in food or water or from hand-to-mouth could occur among these crew members, especially given that the sampling for TCDD on the aircraft surfaces required use of a solvent (hexane) to displace and dissolve any residue” (U.S. Dept. of Veterans Affairs, 2012). However, hexane-wipes are a standard sampling method and it is likely that at least some dermal exposure occurred for the following reasons. While hexane can reach chemicals lodged in areas inaccessible to skin contact and overestimate exposure for porous surfaces, the surfaces on the aircraft were not porous. Further, hexane wipes do not completely extract all chemicals, as demonstrated by repeat sampling, and thus can underestimate exposures. While it is true that dioxin is extracted more efficiently by hexane than by skin in laboratory experiments, it is important to note that dioxin uptake always occurred in every experiment. Human skin has a high level of lipids, making it attractive to lipophilic compounds like dioxin, although absorption depends on the area of skin in contact with the chemical, as well as on sweat, number of hours of contact, pressure exerted and other factors (Slayton, Valberg, & Dallas Wait, 1998). The likelihood that absorption through clothing could occur is confirmed in at least one experiment where cotton fabric appears to increase absorption (Midwest Research Institute (MRI), 1994). This route of entry would thus add to the exposures we have also shown likely to occur, namely, dermal-to-oral and inhalation of contaminated dusts.

Our findings, the results of three different modeling approaches, contrast with Air Force and VA conclusions and policies (Smallwood, 2012; Murphy, 2013). The VA concept of a “dried residue” that is biologically unavailable (Dick, Irons, Terry, & Peterson, 2012) is not consistent with widely accepted theories of fugacity and basic thermodynamics of the behavior of surface residues. Aircraft occupants would have been exposed to airborne dioxin-contaminated dust as well as come into direct skin contact, and our models show that the level of exposure is likely to have exceeded several available exposure guidelines.


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Number of Ranch Hand Missions, Sorties and Gallons Sprayed by Herbicide Type and Year.a

Orange50% n-Butyl ester 2,4,-D; 50% n-Butyl ester 2,4,5‑T1961-1965210564493525
WhiteAcid weight basis: 21.2% Tri-isopropanolamine salts of 2,4-D and 5.7% Picloram1966-1969136252124,976,885
Blue21% sodium cacodylate + cacodylic acid to yield >= 26% total acid equivalent by weight1966-19693491008897,850
Purple50% n-Butyl ester 2,4,-D; 30% n-Butyl ester 2,4,5-T; 20% Isobutyl ester 2,4,5-T1961-1965267566471,043
Pink60%-40% n-Butyl: Isobutyl ester of 2,4,5-T1961-196561513,291
UnspecifiedSpecific agent not stated in mission records.1961-1965455000

a Adapted from Stellman, et al. (2003).


TABLE II. Distribution of identified Ranch Hand missions by herbicidal agent and numbers of aircraft (sorties) flown, 1961 – 1971.a

Number of Aircraft (Sorties) in Mission


a Adapted from Stellman, et al. (2003)
Table III. Dioxin Interior Ranch Hand Aircraft Surface Wipe Samples in Three Aircraft, 1994 and 2009

Sample LocationConcentration, ng/m2
Patches, 1994a1,400
Patches, 1994250
Patches, 1994200
A/C 4571, 2009b18.42
A/C 4571, 200927.58
A/C 4571, 200921.66
A/C 4571, 20094.65
A/C 4571, 20097.72
A/C 4571, 20091.3
A/C 4571, 20099.28
A/C 4571, 200932.22
A/C 4571, 200910.3
A/C 4532, 200925.72
A/C 4532, 200926.35
A/C 4532, 200929.37
A/C 4532, 200912.96
A/C 4532, 20096.4
A/C 4532, 200911.66
A/C 4532, 200914.96

a US Air Force – Weisman samples on “Patches” (Weisman & Porter, 1994)

b US Air Force samples on aircraft stored outdoors in Tucson, AZ (US Department of the Air Force, 2009)


Table IV. Definitions of the Intake Factor Parameters for Post Vietnam UC-123 Exposurea

Parameter DefinitionValueComments
ISystemic intakecalculated (pg/kg BW-day)Picogram/kilogram body weight–day
CsContaminant surface concentrationµg/100 cm295% upper confidence limit value: 285 ng/m2
RHProbability of being on a Ranch Hand aircraft0.46 (unitless)Based on 11 Ranch Hand aircraft among 24 C-123 aircraft at Westover Air Force Base (Lurker, 2013)
CFaArea conversion factor0.0001 m2/cm2
SAExposed skin surface area326 cm2Surface area of both palm sides of the hand (US Army Center for Health Promotion and Preventive Medicine, 2009)
FTssDecimal fraction contaminant transferred surface-to-skin0.063 (unitless)(US Army Center for Health Promotion and Preventive Medicine, 2009)
FTreDecimal fraction contaminant removed from skin-to-mouth1.0Assumed to be 1 for conservative model
CF wtWeight conversion factor1000 pg/ng
FT gaDecimal fraction absorbed from gastrointestinal tract0.87(ATSDR, 1998)
EFExposure frequency hand-to- mouth events per day3/day(May, et al, 2002)
Work Days for Various Types of Workers
WDNotionally Exposed Maintainer70 days/yearReserve Technician working one weekend/month and one two-week annual tour plus extra person-days for mission requirements
Flight Crew42 days/yearBased on Reserve Pilot Flight Logs
Aero-medical Evacuation Patient1 days/yearPatient with one aero-medical evacuation/year
Passenger3 days/yearThree flights per year
Airborne3 days/yearThree flights per year
Aerospace Maintenance and Regeneration Group Personnel3.5 days/yearEstimation based on author (PAL) observations
Museum Restoration Worker2.5 days/yearEstimation based on author (PAL) Wright-Patterson Air Force Base industrial hygienist experience (2006-2009)
Exposure duration12 years1971-1982
Decimal fraction of contaminant collected onto wipe0.50 (unitless)Organic Compound (US Army Center for Health Promotion and Preventive Medicine, 2009)
EDAveraging time4380 days365 days/years * 12 years
FTweDecimal fraction of contaminant collected onto wipe0.50 (unitless)Organic Compound (US Army Center for Health Promotion and Preventive Medicine, 2009)
ATAveraging time4380 days365 days/years * 12 years








Table V. Comparison of Maximum Vapor Concentration to Measured Airborne Concentration and to OSHA Permissible Exposure Limit and German Maximum Allowable Worker Concentration

CompoundCalculated Saturation Vapor Pressure Above Liquid ResidueReported ConcentrationaRatio of Measured Air Concentration to Saturation Vapor PressureUnited
2,4-D0.0017 mg/m30.108 to 0.234 mg/m363-13810 mg/m310 mg/m3
2,4,5-T0.0275 mg/m30.135 to 0.194 mg/m35-710 mg/m310 mg/m3

aAir samples reported by Conway (1979) and converted from parts per million to mmHg (mm Hg/760 mmHg * 106 ppm) * molecular weight/24.45 (mg/m3/ppm)

bOccupational Safety and Health Administration Permissible Exposure Limit. (OSHA, 2013a and 2013b)

cGerman Maximum Allowable Worker Concentration

Table VI. Parameters Used to Estimate Airborne Dioxin Concentration in UC-123

hConvective mass-transfer coefficient0.368 m/hrThibodeaux and Lipsky (1985)
AAircraft interior surface: Surface area = πDL + D2 /4280 m2Assumed cylindrical shape: 15 x 53 feet. D= diameter 4.6 m, L= length= 16 m
KpAirborne particle/air partition coefficient0.0045 m3/µgLittle et al. (2012)
Fom_partVol fraction organic matter in airborne particles0.4Little et al. (2012)
KoaOctanol/air partition coefficient1.12 x 1010(Åberg, MacLeod, & Wiberg, 2008)
ρparticleDensity airborne particles1 x 1012 µg/m3Little et al. (2012)
TSPTotal suspended particles20 µg/m3Little et al. (2012)
QVentilation Rate170 m3/hrAdapted from Meek (1981)
yoGas-phase concentration in contact with the emission surface13 pg/m3
26 pg/m3
53 pg/m3
9.74E-13 atm (Podoll, 1986)
1.97E-12 atm (NTP, 2011)
4 x 10-12 atm (Weschler & Nazaroff, 2008)
yCalculated concentration11 pg/m3
23 pg/m3
46 pg/m3`
Derived, respectively, from the yo values listed above for 280 m2
yCalculated concentration12 pg/m324 pg/m349 pg/m3Derived, respectively, from the yo values listed above for a doubled areas (560) m2



Figure 1

Schematic of Semivolatile Organic Compound Emissions Model. We have adapted the approach and illustration used Little et al. (2012) to model potential exposure to dioxin arising from dried residue in the Operation Ranch Hand UC-123 spray aircraft. Any dioxin present in the residue, at a concentration of c0, would be in equilibrium with the atmosphere immediately above the surface with a concentration given by y0 dependent on the mass-transfer coefficient h. The physical phenomenon that is critical for occupational exposure potential is the adsorption of dioxin onto dust particles (National Institute for Occupational Safety and Health, 1984). Dust adsorption magnifies exposure by many orders of magnitude. Dioxin is highly attracted to dust, as evidenced by its octanol/air partition coefficient, a chemical constant used to calculate transport between two phases that is highly predictive of the atmosphere/dust partition for dioxin and other semivolatile organic compounds (Weschler and Nazaroff, 2008). The availability of dust for adsorption will be a function of the total mass of suspended particles, TSP, and to a very small extent, the ventilation rate Q, because the dioxin in the air atmosphere will be continually replenished by the surface emissions. Specific equations used in this model are given in Section 2.3 and parameters in Table VI.





Estimates of Cumulative Dermal-Oral Intake (pg/kg-BW-day) vs. Days-Per-Year Exposed. Using the 95% Upper Confidence Limit mean value of 285 ng/m2 surface concentration of dioxin for various numbers of work-days per year, we derived estimates using an adaptation of the US Environmental Protection Agency general model for estimating generic intakes (US Environmental Protection Agency, 1989b) to represent likely exposure situations of working conditions in the interior of UC-123 former Ranch Hand aircraft (see Equation 1). Diagonal lines represent dose-variation as a function of bodyweight. Vertical dashed lines represent typical number of annual eight-hour work days used to in the exposure scenarios for dioxin-contaminated surfaces: worst-case maintainer (250 days); reserve maintainer (75 days: 1 two-day-weekend per month, two week annual tour plus 37 extra days); flight crew (42 days); passengers, such as aero-medical evacuation patients and airborne troops (2 days). Intersection of the vertical lines with diagonal lines represents estimated intake, which can be compared to existing guidelines (World Health Organization (2002), the Netherlands (Larsen, 2006) and US Environmental Protection Agency 2012), represented by dashed horizontal lines. In this model flight crew have dermal-oral intake exceeding the US Environmental Protection Agency guideline of 0.7 pg/kg-BW day; maintainers exceed both US Environmental Protection Agency and Netherlands guidelines and worst-case maintainer exceeds all three guidelines.