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Introduction:

Persistent organic pollutants (POPs) are
organic carbon based chemicals that are intentionally or non-intentionally
produced or rereleased to the environment. The United Nations Environment
Program (UNEP) defines POPs as “..Chemical substances that persist in the
environment, bio accumulates through the food web and poses a risk of causing
adverse effects to human health and the environment”. Some are pesticides,
industrial products or un-intended by products of resulting from industrial
process or combustions. They can resist environmental breakdown via biological,
chemical and photolytic processes, some remaining in the environment for
exceptionally long periods of time.

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POPs are known to be long-range
transport pollutants that results in global pollution. They become distributed
in the environment as result of natural processes like wind and ocean currents.
They have been measured on every continent irrespective of their origin. They
have been recorded in high levels in the arctic area of both the hemispheres.
POPs are not presented in one phase. They are constantly shifting through
different phases and dependent upon the chemicals physiochemical properties.

POPs are hydrophobic and lipophilic.
Though they are not soluble in water, they have a tendency to remain in
fat-rich tissues. This means once released into the environment, they are not
easily expelled. They are likely to partition into lipids and accumulate in
organs and adipose tissues. Over the time period, they accumulate
(bioaccumulation) into the living organism and magnify in the food chains. In
humans it has been well documented that the lipophilic nature of POPS can
result in transport to a developing foetus.

 

Stockholm
Convention

            POPs are in use for more than 100
years with some discovered as the late 1800s. The chronologies for the POPs are
provided in appendix A. In the early 60s the scientific advancements in
analytical techniques such as electron capture detectors led to the detection
of compounds such as Dichlorodiphenyltrichloroethane (DDT), Dichlorodiphenyldichloroethylene
(DDE) in the environment. Initial actions were taken focusing on individual
specific industry based chemicals.

            International collaboration began by
1970s, as research confirmed the transboundary migration of pollutants. In
1970, the active international co-operation was signalled by United Nations
conference on the human environment in Stockholm. This brought in the signing
of the Convention on Long Range Transboundary air pollution (CLRTAP) by 34
governments.

            In 1995, United Nations Environment
Programme (UNEP) called for a global action to be taken on the POPs, resulting
in series of meetings of the Intergovernmental Negotiating Committee (INC) for
treaty on persistent organic pollutants. These negotiations led to the
Stockholm convention, a UN treaty signed over by 90 countries and European
Union in May 2001. It was ratified and entered into force on 17 May, 2004 and
by late 2008 there were over 180 participants (http://chm.pops.int)

            Under the convention, the parties
are required to implement measures to reduce and eliminate the release of an initial
12 POPS. They were infamously named as the “dirty dozen”. Of these eight are
pesticides and two are industrial chemicals as well as used pesticides and two
are intentionally formed by products. The above 12 POPs were linked to cancer
and nervous, reproductive and immune systems.

            According to the article of the
Stockholm convention, the objective is to protect human health and the
environment from persistent organic pollutants.

            The conventions requires each
country participating in it to take measures to, (Stockholm convention on POPs,
text and annexure)

·        
Prohibit
and/or take the legal and administrative measures necessary to eliminate, the
production and use as well and import and export of chemicals listed in Annex A
(article 3)

·        
Reduce
or eliminate releases from unintentional production (article 5)

·        
Reduce
or eliminate releases from stockpiles and waste. Ensure they are managed safely
and in an environmentally sound manner (article 6)

·        
Implement
the developed plans through periodic review and update of the plans (article 7)

·        
Target
additional POPs by following the detailed procedure mentions in Annexes A,B
and/ C (article 8)

 

Role
of Canada in the Stockholm convention

           

 

Dirty
Dozen

            POP substance have three main chemical
characteristics in common (i) one or more cyclical ring structures of either
aromatic or aliphatic nature (ii) a lack of polar functional groups (iii) a
variable amount of halogen substitutions usually chlorine (chap 8 POPs, author
viktor). The following are the initial POPs identified causing adverse effects
on humans and the ecosystems.

·        
Pesticides
: Aldrin, Chlordane, DDT, Dieldrin, Endrin, Heptachlor, Hexachlorobenzene,
Mirex, Toxaphene

·        
Industrial
chemicals: Hexachlorobenzene, Polychlorinated biphenyls (PCBs)

·        
By-products:
Polychlorinated dibenzo-p-dioxins and Polychlorinated dibenzo furans
(PCDD/PCDF) and PCBs

 

POPs

Appearance

Characteristics

Aldrin
(C12H8Cl6)

White,
odourless crystals when pure; technical grades are tan to dark brown with a
mild chemical odour.

o  
Organochlorine insecticide
o  
Mainly used to kill termites and grasshoppers,
corn rootworm and other insect pests
o  
Due to its persistent nature aldrin is known to
bio-concentrate
o  
Carcinogen as well as a mutagen
o  
Human exposure is through dairy products ad meats
 

Chlordane(
C10H6Cl8)

Colourless
to yellowish-brown viscous liquid with an aromatic, pungent odour similar to
chlorine

o  
Broad spectrum insecticide used on agricultural
crops as well as used in control of termites
o  
Possible human carcinogen, recent studies have
linked with prostate and breast cancers
o  
Exposure mainly through air and it has been
detected in the indoor air and residences in the USA and Japan.

DDT
(C14H9Cl5)

Odourless
to slightly fragrant colourless crystals or white powder.

o  
Widely used during World war II to protect
civilians and soldiers from malaria, typhus and other diseases spread by
insects
o  
After war, used on crops and continued to be
applied against mosquitoes to control malaria
o  
Its metabolic products DDE and DDD magnify through
the food chain
o  
Long-term exposure associated with human health
and raising serious concerns about infant health

Dieldrin
(C12H8Cl6O)

A
stereo-isomer of endrin, Dieldrin may be present as white crystals or pale
tan flakes, odourless to mild chemical odour.

o  
Used to control termites and textile pests
o  
The pesticide aldrin converts into dieldrin, so
the concentration of dieldrin is higher than aldrin
o  
Linked to Parkinson’s disease and breast cancer

Endrin
(C12H8Cl6O)

White,
odourless, crystalline solid when pure; light tan colour with faint chemical
odour for technical grade.

o  
Used on cotton, maize and rice crops
o  
Does not accumulate in the fatty tissue but highly
toxic to fish
o  
Enters the atmosphere through volatilization
o  
Chemical properties of endrin such as low water
solubility and high stability in environment favours the long range transport
and has been detected in arctic fresh water

Heptachlor
(C10H5Cl7)

White
to light tan, waxy solid or crystals with a camphor-like odour.

o  
Primarily used to kill soil insects and termites
o  
Stable structure can remain in environment for a
long time
o  
Responsible for the decline of several wild bird
populations

Hexachlorobenzene
(HCB) (C6Cl6)

White
monoclinic crystals or crystalline solid.

o  
By-product of manufacture of industrial chemical
including carbon tetrachloride, perchlorethylene and penta chlorobenzene
o  
Volatile and expected to partition into atmosphere
o  
Bio concentrate in fat tissue of living organisms
o  
It is ubiquitous in the environment and has been
measure in food of all types

Mirex
(C10Cl12)

White
crystalline, odourless solid.

o  
Used to combat fire ants and termites
o  
Used as fire retardant in plastics, rubber and
electrical goods
o  
Stable and persistent pesticide

PCBs
(C12H10?xClx)

pale-yellow
viscous liquids

o  
209 possible PCBs exis among them 13 exhibit a
dioxin like toxicity
o  
Persistence in the environment corresponds to the
degree of chlorination
o  
They do not degrade easily, despite being banned
in 1970s, it still persist in the environment

Polychlorinated dibenzodioxins (PCDD)

 

o  
Unintentional by products due to incomplete
combustion and manufacture of pesticides
o  
75 different dioxins, seven are considered to be
of concern

Polychlorinated dibenzofurans (PCDFs)

 

o  
Extremely persistent.
o  
It has not showed any degradation in tests of
hydrolysis, photolysis or biodegradation at any environmental condition
tested. The only known condition whereby PFOS is degraded is through high
temperature incineration.

Toxaphene (C10H8Cl8)

Yellow, waxy solid with a chlorine/terpene-like
odour

o   Highly insoluble in water and has a half life in soil of up to 12
years.
o    It has been shown to bio concentrate
in aquatic organisms and is known to undergo atmospheric transport.

 

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