Dioxins in Sydney Harbour

Data collected by the NSW Department of Environment, Climate Change and Water (Australia) in 2008 shows that large tracts of sediments in Sydney Harbour are more contaminated with dioxins than Tokyo Bay or New York Harbour, and are among the most contaminated in the world.
In the remediation area in Homebush Bay, the location of the 2000 Sydney Olympic Games, the dioxin levels were 610 picograms per gram of sediment compared with 2.3 for a "clean" environment. These high levels of dioxins in areas where fish feed means the warning not to eat fish caught west of the Sydney Harbour Bridge, and to eat only 150 grams a month of fish caught east of the bridge, will most likely have to remain in place for decades.

The source of the contamination is the former Union Carbide site where dioxins were used in the manufacture of the now-banned pesticide 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) and the defoliant Agent Orange which was used during the Vietnam War.

Dibenzo-p-dioxin is made up of two benzene rings joined by two oxygen bridges, so it is an aromatic diether.
The name dioxin actually refers to the central dioxygenated ring. So, if other elements are substituted for the hydrogen atoms in the molecule, they too would be referred to as dioxins. This means there are many different compounds that can be referred to as dioxins!

In common use, the term dioxins usually refers to polychlorinated dibenzodioxins (PCCDs) which are a group of chlorinated dibenzo-p-dioxin molecules. In PCCDs the chlorine atoms can attach on 8 different places on the dibenzo-p-dioxin molecule (shown as 1,2,3,4 and 6,7,8,9 on the structure above). The toxicity of PCDDs depends on the number and positions of the chlorine atoms. The most toxic dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin, and is well known as the contaminant of Agent Orange.


Dioxins are known to build up in the human body over time, mostly in fatty tissues.
Exposure to high levels of dioxins causes a severe form of persistent acne known as chloracne and leads to an increased risk of tumours.
Exposure to low levels of dioxins results in an elevated risk of sarcoma, a type of skin cancer.

Reference:
http://www.smh.com.au/environment/water-issues/the-poison-that-got-away-20101029-177i0.html?autostart=1

Further reading
http://www.ausetute.com.au/massconv.html
http://www.ausetute.com.au/partspm.html
http://www.ausetute.com.au/namctut1.html
http://www.ausetute.com.au/namhaloa.html
http://www.ausetute.com.au/mmcalcul.html
http://www.ausetute.com.au/percentc.html
http://www.ausetute.com.au/namisane.html

Study Questions

1. Convert 610 picograms per gram to
• micrograms per gram
• nanograms per gram
• micrograms per milligram
• milligrams per kilogram
• parts per million
• parts per billion
2. Write the molecular formula for each of the following:
   • dibenzo-p-dioxin
   • 2,3,7,8-tetrachlorodibenzo-p-dioxin
3. Calculate the percentage composition of dibenzo-p-dioxin.
4. Calculate the mass of chlorine present in 610 picograms of 2,3,7,8-tetrachlorodibenzo-p-dioxin.
5. Indicate the location of ether functional groups on a molecule of dibenzo-p-dioxin.
6. Give the structural formula of as many different structural isomers of 2,3,7,8-tetrachlorodibenzo-p-dioxin as possible.
7. Name each of the molecules you have drawn in question 6.