1. HERBICIDAL DISSIPATION AND DEGRADATION IN RELATION TO THE ASSESSMENT OF INDIRECT EXPOSURE OF GROUND TROOPS
Alvin L. Young, Ph.D.
Institute for Science and Public Policy
The University of Oklahoma

2. Defoliated Mangrove

3. Defoliated Upland Forest

4. Background and Context
Recent report of the US Institute of Medicine
(National Academy of Sciences)
– Contractor from Columbia University
– Use of historical records for model to estimate
“exposure opportunity”
– Wide range of assumptions and “imputations”
The records they relied upon have important
information for assessing exposure in troops and
in the population of South Vietnam.
Also, information on bioavailability is important
in assessing exposures to Agent Orange.

5. Herbicide Application Systems I
Systematic study and testing of equipment also limited
potential exposures by making spray applications
efficient and precise; thereby, greatly limiting spray
swath.
Testing and evaluation were conducted at Eglin AFB,
Fl, and to a lesser degree, at the Pran Buri Calibration
Grid, Thailand, and were refined during the 1960’s.
The fixed wing spray system, used in more than 95%
of the defoliation and crop destruction missions, had
16 nozzles on each wing boom and 8 nozzles on the
tail boom designed to produce a median spray droplet
size of 320 to 350 µm.

6. Herbicide Application Systems II
98% of the particles were 100 µm or greater =>
rapid settling velocity.
A full tank of herbicide contained 3,600 liters with
about 5% remaining post mission.
The release of herbicide took approximately 3.5 – 4
minutes with a spray swath 80 m wide (±6 m) and
14 km in length.
Deposition was extremely precise with little “drift.”

7. Spray Particle Distribution

8. Clear demarcation of spray swath edge; if spray drifted one would expect a “ragged” edge.
Photo courtesy of Paul Cecil, Historian, RANCH HAND Assoc.

9. Close-up of Defoliated Forest

10. Characteristics of Herbicide Spray Missions I
43% of the RANCH HAND spray missions
consisted of three aircraft.
Over 70% consisted of three aircraft or less.
The aircraft flew in close formation so as to
produce a single swath of defoliation.

11. Typical three-airplane spray mission

12. Characteristics of Herbicide Spray Missions II
Air speeds were tested, and optimum for
defoliation was at knots, 35-50
meters above ground level, depending on
the terrain.
Spray planes flew only in favorable
weather conditions – in calm wind (i.e.,
less than 10 knots), in the absence of
precipitation, and in the early morning.
Weather requirements were critical, and
missions often were cancelled or
retargeted due to unacceptable weather.

13. Characteristics of Target Area I
More than half of the Vietnamese jungle
subjected to spraying operations was
double and triple canopy jungle.
Foliage density can be quantified as a leaf
area index (LAI), defined as the total area
in proportion to the ground surface below.
– For example, a value for the LAI of 2.0 means
that there are 2m² of leaves/1m² of ground surface.
– The LAI is used for calculations involving photo-
synthesis and to define the amount of canopy
penetration by light, rain, or herbicides.

14. Dense Forest in Vietnam

15. Characteristics of Target Area II
Agent Orange was used for mature or
secondary forests with LAI values from 2
(open secondary) to 5 (mature forest).
In relatively undisturbed dense forests, the
target canopy had an LAI of 3-5 and 87-97%
of the herbicide sprayed was intercepted.
– If forest was sparse, the higher percentage was
likely.
Herbicide evaporated rapidly and residual
dissipated rapidly.
Experiments under similar conditions
indicated no more than 6% of herbicide ever
reached the ground (usually 1-6%).

16. Dioxin in the Environment I
Agent Orange contained on average about two
parts per million of dioxin.
The dioxin contaminant degrades rapidly.
– In sunlight, the half-life of dioxin from Agent
Orange on leaves is less than 6 hours.
– Loss of dioxin continues even in reduced light (cloudy
days and shade).
– A half-life of 7 to 10 hrs. was observed even when
ultraviolet light intensities were low.
=> 99% of dioxin on foliage will be gone in 2-3
days.

17. Dioxin in the Environment II
Early morning missions were typically
followed by hours of sunlight.
Defoliation of upper canopy layers let in
additional sunlight, increasing the degradation
of dioxin in vegetation below the canopy.
In sunlight, dioxin continues to break down
at a significant rate in the waxy cuticle of plants.

18. Dioxin in the Environment III
Less than 6% of herbicide would have
penetrated canopy.
Dioxin is immobile in plant tissue and
in soil.
Dioxin has very low water solubility and
rapidly binds to sediment.

19. Human Skin Absorption Studies
Skin absorption of 2,4,5-T and dioxin
are low.
– After 2 hours of saturated contact of
2,4,5-T over a large area of bare skin
only 0.15—0.46% entered the
body and was eliminated in urine.

20. Anecdotal Reports of Spraying
Yet, some 25% of troops report being
sprayed directly with Agent Orange.
– Most likely troops recall spraying of
insecticide to control mosquitoes from
the same type of aircraft as herbicide
aircraft.
– Insecticide was sprayed on troops and
hamlets regularly from 1967 to 1972.

21. Operation “Flyswatter”
From March 1967–February 1972, three
RANCH HAND “Silver Spray Birds”
routinely sprayed malathion on 14 bases
and their adjacent Vietnamese cities,
every 9-14 days.
More than 1.5 million liters were
disseminated.

22. Dioxin Body Burdens Serum dioxin in troops is consistent with
little or no exposure.
– In 1988, CDC compared serum dioxin in 646
US Army veterans who served as ground troops
in the most heavily sprayed regions of Vietnam
with those of 97 Vietnam-era veterans who did
not serve in Vietnam.
– The distributions of dioxin levels were ‘nearly
identical’ in the two groups, both with means
medians of about 4 ppt dioxin.
Only veterans who had elevated serum
levels of dioxin were RANCH HAND and
Chemical Corps; both had direct contact
with the liquid herbicide.

23. Summary and Conclusions
Troops and civilians were unlikely to be
sprayed directly.
Procedures and equipment reduced
likelihood of spray drift.
Entering spray areas soon after spraying
was unlikely because of unexploded
ordnance from “fire suppression” activities.
Degradation of Agent Orange by sunlight
was extensive.
Contact with residual was unlikely to
involve significant exposure to dioxin
because of rapid degradation and low
bioavailability.