Health Information Summary for CF Members
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Fate in the Environment
What happens to Agent Orange after it is sprayed?
The environmental fate of Agent Orange has been described in a recent review article (Young et al., 2004a). For Agent Orange to be as effective a herbicide as possible, the maximum amount of spray had to reach the vegetation as quickly as possible. To achieve this, aircraft flew very close to the treetops in calm weather conditions to minimize the amount of spray drifting outside of the target area. Rapid settling of the spray droplets was also important. The spray system used during the Vietnam War used spray nozzles designed to produce a droplet size of 320 to 350 m m (over 98% of the droplets produced were greater than 100 m m). Droplets of this size generally fall rapidly. Tests at Eglin Air Force Base in Florida showed that 87% of sprayed herbicides hit the vegetation within one minute (the remaining 13% took longer to settle due to vortices at the wing tips, drift, or evaporation). US studies showed that even the droplets smaller than 100 m m would have hit the vegetation less than 3 minutes after spraying (Young et al., 2004a).
For spray drift beyond the target area to occur, herbicide would have to remain in the air for extended periods of time, where it would be rapidly degraded by sunlight. Aerial photographs of herbicide-sprayed areas in Vietnam show very distinct and sharp lines between treated (dead) and untreated (healthy) trees. Had there been significant drift either way from the swath of aerial spraying, traces of damage would have been visible as streaks of discoloured foliage (Young et al., 2004a).
The studies showed that little aerially-sprayed Agent Orange reached the forest floor as liquid droplets. In relatively undisturbed dense forests, the forest canopy intercepted 87-97% of the sprayed herbicide. Vegetation below the forest canopy also took up some of the spray, such that the underbrush or forest floor received about 1-6% of the total aerial spray. The Agent Orange that lands on plant surfaces is absorbed into the wax layer of the plant cuticle within minutes and cannot be physically dislodged (Young et al., 2004a) . Agent Orange left on the plant surface breaks down in sunlight within hours (Crosby et al., 1977). It is estimated that very little, if any, Agent Orange can be dislodged from the plant surface 24 hours after spraying (Young et al., 2004a). It is likely that due to the degradation by sunlight of TCDD on leaf surfaces that little material would be left by the time the leaves fall to the ground.
The main contaminant of concern in Agent Orange was TCDD. TCDD is not very soluble in water and binds tightly to soil particles. Estimates of the half-life of TCDD (the amount of time it will take for the concentration of TCDD to be reduced by half) on the soil surface range from 9 to 15 years, whereas the half-life in subsurface soil may range from 25 to 100 years (ATSDR, 1998). Because of tight binding to soil particles, TCDD is unlikely to leech into underlying groundwater (ATSDR, 1998). However, TCDD may enter surface water secondary to soil erosion and runoff. Although some of the TCDD that makes its way to surface waters will be broken down by sunlight or evaporate, most will remain strongly attached to small particles of soil or organic matter and eventually settle to the bottom. TCDD attached to this organic matter may enter the aquatic food chain. Small aquatic organisms absorb TCDD that is attached to sediment and organic matter in bodies of water. Larger fish then consume these smaller organisms and accumulate TCDD in their fatty tissues. Human consumption of contaminated fish is thus a potential route of exposure (ATSDR, 1998).
Root uptake and translocation to upper plant parts is very minimal. The ATSDR described a laboratory study in which plants were grown in soil heavily contaminated with TCDD: " the amount of 2,3,7,8-TCDD applied to these soils was many thousands of times greater than that which would occur in soils from herbicide applications containing a few ppm 2,3,7,8-TCDD as an impurity. Even upon exposure to these high concentrations in the soil, significant amounts of 2,3,7,8-TCDD could not be measured in plants " (ATSDR, 1998). The vast majority of TCDD found in plants is due to absorption of airborne TCDD that settles on the plant surface. This has been demonstrated in studies of fruits and vegetables that show TCDD concentrations in the outer peel that are up to ten times greater than the TCDD concentration in the pulp (ATSDR, 1998).
Herbicide testing was conducted at Eglin Air Force Base in Florida from 1962 to 1970, when roughly 75 000 kg of 2,4-D and 76 000 kg of 2,4,5-T (the ingredients of Agent Orange) were aerially sprayed on an area of less than 3 square kilometres. It is estimated that 3.1 kg of TCDD contaminant was released in this area. Because of the extent of the testing, each hectare on the Eglin test grid received at least 1300 times more TCDD than a hectare sprayed with Agent Orange in Vietnam. Much of the vegetation on the test site had been removed, allowing an opportunity to study ground-based residues that would not be affected by interception of the sprayed herbicides by the forest canopy (Young et al., 2004b).
Small but detectable levels of TCDD (in the parts per trillion range) were found in some soil samples 20 years after the last application of herbicide. It was estimated that the vast majority of TCDD that reached the ground had been degraded by sunlight within 24 hours of spraying and would not have persisted in the environment. For the small amounts of TCDD that were detected, the majority of TCDD was confined to the top 15 cm of soil, indicating that there was little travel of TCDD deeper into the soil. In the years that followed the herbicide testing, vegetation growth gradually returned to normal, indicating that there was no persistent herbicide effect. Examination of animal species that lived in close contact with the soil did not reveal significant health effects (Young et al., 2004b).
In summary, very large quantities of herbicides were applied to the Eglin Air Force Base test site, far more than would be applied during typical aerial spraying, and far more that were used during the herbicide tests at CFB Gagetown June 14-16, 1966, and June 21-24, 1967 (see " How much Agent Orange was sprayed at CFB Gagetown? "). These herbicides were applied over areas where vegetation had been removed, thereby maximizing the amount of herbicide that reached the ground. No long-term negative effects on vegetation or wildlife were detected. Although TCDD could be detected in the soil years after the testing, the amounts present were extremely small and did not appear to have a significant adverse effect on the environment (Young et al., 2004b).
Agent Orange is not a significant source of TCDD in the environment. Most of the TCDD released into the environment comes from combustion sources, such as municipal and medical waste incineration, backyard burning of household waste such as plastics, cement kilns, forest and brush fires, and burning of fuel for agricultural purposes and home heating. TCDD can be formed in metals operations, such as aluminum smelting, steel production and scrap metal recovery. TCDD can also be formed as a byproduct in the manufacture of chlorine-bleached wood pulp and chlorinated chemicals (ATSDR, 1998; Hays et al., 2003; Sullivan et al., 2001; Travis et al., 1991).
Because of this past and continued production and release of TCDD, it is everywhere in our environment in very small quantities. Everyone is exposed to small amounts of TCDD every day, mostly through food (Birmingham et al., 1989; Gilman et al., 1991; Travis et al., 1991; Huwe, 2002). Cigarette smoke is also an important source of exposure (Muto et al., 1989). It has been estimated that the average Canadian or American adult takes in about 40-150 picograms of TCDD every day (ATSDR, 1998; Health Canada, 1994; Gilman et al., 1991). A gram is a relatively small amount: one teaspoon of water weighs about 5 grams and there are 450 grams in one pound. A picogram is a trillionth of a gram. Put another way, one picogram is one part per trillion of a gram (see "How much is a part per trillion (ppt)?" ). Therefore, 40 to 150 picograms of TCDD is a very small amount, but it is equivalent to about 100 billion molecules of TCDD (see " Calculation "). Every day, the average adult is exposed to about 100 billion molecules of TCDD.
In a study of industrial workers for whom an elevated cancer risk was observed (see " What are the health risks associated with large exposures to TCDD? "), it was noted by the authors that " excess cancer was limited to the highest exposed workers, with exposures that were likely to have been 100 to 1000 times higher than those experienced by the general population " (Steenland et al., 1999).
Recently, the World Health Organization (WHO) established a tolerable daily dioxin intake equivalent to 1 to 4 picograms of TCDD per kilogram of body weight per day ( http://www.who.int/mediacentre/factsheets/fs225/en/print.html ), which is similar to other international health assessments (Pohl et al., 2002). Health Canada's tolerable daily intake value is currently under review, but will likely be equivalent to the WHO value. Based on the assessment of health authorities, the tolerable daily intake is the amount of TCDD that people can be exposed to every day of their lives without harm. The tolerable daily intake is expressed in a manner that takes into account differences in body weight between different people. For example, the tolerable daily intake of TCDD for a 70 kilogram adult is 280 picograms (4 picograms of TCDD per kilogram multiplied by 70 kilograms) per day. For a 80 kilogram adult, the tolerable daily intake would be 320 picograms (4 picograms of TCDD per kilogram multiplied by 80 kilograms) per day.
Because of continuous low-level exposure to TCDD, we all have measurable amounts of TCDD in our bodies, referred to as a "background" level of TCDD. It is estimated that in North America, the average body burden of TCDD (the amount of TCDD contained within our bodies) is 3-7 parts per trillion (ppt), measured either in body fat or in blood lipids (Hays et al., 2003; Travis et al., 1991, ATSDR 1998).
In comparison, the average serum level of TCDD in a sample of Ranch Hand personnel (who handled and sprayed herbicides in Vietnam) was 49 ppt in 1987, many years after exposure had occurred. Because half the TCDD in our bodies is naturally eliminated every 7-12 years (TCDD's half-life), it was estimated that 2 to 4 half-lives had passed since the time of their exposure and that their serum TCDD levels around the time of exposure had probably been several hundred ppt (MMWR, 1988). Other Vietnam veterans who did not directly spray or handle herbicides in Vietnam had body burdens of TCDD that were no different than normal background levels, in the range of 2 to 4 ppt (Young et al., 2005). A serum TCDD study of US Army Vietnam veterans who served as ground troops concluded, " most US Army ground combat troops who did not handle or spray herbicides were not heavily exposed to TCDD in Vietnam " (CDC, 1988).
It is estimated that the maximum TCDD dose experienced by Ranch Hand veterans was only about one tenth of the maximum predicted dose of industrial workers (Akhtar et al., 2004). In a study of heavily exposed industrial workers (for whom evidence of associations between TCDD and cancer was observed), the average estimated serum level of TCDD at the end of exposure was 1589 ppt, with a maximum value of 210 054 ppt (Steenland et al., 2001). People who have serum TCDD levels greater than 12000 ppt develop a skin condition known as chloracne (Greene et al., 2003).
We are all exposed to TCDD. It can be measured in our bodies. The risk of health effects associated with TCDD is entirely dependent on the degree of exposure, or dose (see "Under what circumstances might Agent Orange or its ingredients lead to health effects?" ).