The Herbicide 2,4,5-T and Dioxin Contamination

The now superseded herbicide 2,4,5-T was a selective systemic herbicide belonging to the synthetic auxins (group O). Absorbed by the roots, bark, and foliage, it was translocated through the phloem and was used for killing: woody weeds, brush, undergrowth and treatment of tree stumps and shoots to prevent re-growth. An amine formulation was also used for weed control in rice.

Dioxin contamination was brought to prominence during the Viet Nam – American war, with the widespread use of the defoliant ‘Agent Orange’ by the US military. The late Arthur Galston did some of the early research to develop the herbicide, but having discovered the consequences, later helped raise awareness of the environmental impact of its military use.  Named after the colour of its drum container, agent orange contained a 1:1 mixture of the herbicides 2,4,5-T and 2,4-D.  The former included residual amounts of the dioxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which was derived from the manufacturing process and known have carcinogenic properties, with a positive trend for all cancers amongst US male chemical workers with increasing cumulative exposure. For a US Government collection of documents, click here.

It is believed that any remaining dioxin contamination is now very localised in Viet Nam.  There have been a number of recent studies on residual levels of TCDD in sprayed areas and near old US air bases. Sweeny (2007) provides a summary which includes:

  • Studies by Hatfield (2006, 2007) have confirmed elevated concentrations around bases, in human serum and animal fat.
  • Residual levels in soil of southern Vietnam are now generally below background of industrialized countries in North America & Western Europe.
  • Remaining areas of significant soil and sediment contamination are around some former US Airbases.
  • Distribution of dioxins point to Agent Orange assource of contamination in and around airbases.

If soil were contaminated, are crop plants likely to absorb and translocate dioxins to harvested leaves/fruit?

The UK Environment Agency has evaluated models for predicting plant uptake of chemicals from soils and many such models rely heavily the use of a chemical’s partition coefficient (Kow or LogP) as a measure of bio-accumulation. Kow coefficients for dioxins are typically in the 4.3 – 8.4 range (see Lancaster data-base) and for TCDD the values has been given as 6.8 (Yoon et al.[1]) so any dioxin residues in the soil are unlikely to be translocated substantially by plants.  The greatest risk of exposure is probably would probably occur if disturbing (e.g. initially cultivating) contaminated soil, with a potential risk or residues in root crops.

Documented long-term ecological damage

A forest which was extensively sprayed with Agent Orange, then subsequently left to regenerate, is the area surrounding Cat Tien National Park. To this day the Park has extensive bamboo and grassland cover and trees have yet to grow back.

[1] Boo Ok Yoon, Shin Koyanagi, Takao Asano, Mariko Hara and Akon Higuchi (2003)  Removal of endocrine disruptors by selective sorption method using activated carbons.  J. Mass Spectrom. Soc. Jpn. 51, 168-173 referring to W. Y. Shiu, W. Doucette, F. A. P. C. Gobas, A. Andren, and D. Mackay  (1988) Environ. Sci. Technol., 22, 651.


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