Dioxins: An overview

Herbicides including Agent Orange were sprayed by United States forces for military purposes during the Vietnam War (1961‐ 1971) at a rate more than an order of magnitude greater than for similar domestic weed control. In 1974, the US National Academy of Sciences published estimates of the extent and distribution of herbicides sprayed. Here we present revised estimates, developed using more-complete data. The spray inventory is expanded by more than seven million litres, in particular with heavily dioxin-contaminated herbicides. Estimates for the amount of dioxin sprayed are almost doubled. Hamlet census data reveal that millions of Vietnamese were likely to have been sprayed upon directly. Our identification of specific military herbicide targets has led to a more coherent understanding of spraying. Common errors in earlier interpretations of the spray data are also discussed. Between 1961 and 1971 herbicide mixtures, nicknamed by the coloured identification band painted on their 208-litre storage barrels, were used by United States and Republic of Vietnam forces to defoliate forests and mangroves, to clear perimeters of military installations and to destroy ‘unfriendly’ crops as a tactic for decreasing enemy food supplies 1 . The best-known mixture was Agent Orange. About 65% of the herbicides contained 2,4,5trichlorophenoxyacetic acid (2,4,5-T), which was contaminated with varying levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin

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The extent and patterns of usage of Agent Orange and other herbicides in Vietnam

Disruption of retinoid transport, metabolism and signaling by environmental pollutants.

The association between parental exposure to Agent Orange or dioxin and birth defects is controversial due to inconsistent findings in the literature The principal aim of this study was to conduct a meta-analysis of relevant epidemiological studies that examined this association and to assess the heterogeneity among studies Relevant studies were identified through a computerized literature search of Medline and Embase from 1966 to 2002; reviewing the reference list of retrieved articles and conference proceedings; and contacting researchers for unpublished studies A specified protocol was followed to extract data on study details and outcomes Both fixed-effects and random-effects models were used to synthesize the results of individual studies The Cochrane Q test and index of heterogeneity (I2) were used to evaluate heterogeneity and a funnel plot and Eggers test were used to evaluate publication bias In total 22 studies including 13 Vietnamese and nine non-Vietnamese studies were identified The summary relative risk (RR) of birth defects associated with exposure to Agent Orange was 195 [95% confidence interval (95% CI) 159--239] with substantial heterogeneity across studies Vietnamese studies showed a higher summary RR (RR 5 300; 95% CI 219--412) than non-Vietnamese studies (RR 5 129; 95% CI 104--159) Sub-group analyses found that the magnitude of association tended to increase with greater degrees of exposure to Agent Orange rated on intensity and duration of exposure and dioxin concentrations measured in affected populations Parental exposure to Agent Orange appears to be associated with an increased risk of birth defects (authors)

http://vn-agentorange.org/edmaterials/ije_paper.pdf

Association between Agent Orange and birth defects: systematic review and meta-analysis

Analytical techniques for the determination of polychorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF) and dioxin-like PCBs (DLPCB) are reviewed. The focus of the review is on recent advances in methodology and analytical procedures. The paper also reviews toxicology, the development of toxic equivalent factors (TEF) and the determination of toxic equivalent quantity (TEQ) values. Sources, occurrence and temporal trends of PCDD/PCDF are summarized to provide examples of levels and concentration ranges for the methods and techniques reviewed.

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Christopher T. Hatfield

Papers

Dioxin reservoirs in southern Viet Nam--a legacy of Agent Orange.