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.
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.
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
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)
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
or eliminate releases from unintentional production (article 5)
or eliminate releases from stockpiles and waste. Ensure they are managed safely
and in an environmentally sound manner (article 6)
the developed plans through periodic review and update of the plans (article 7)
additional POPs by following the detailed procedure mentions in Annexes A,B
and/ C (article 8)
of Canada in the Stockholm convention
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.
: Aldrin, Chlordane, DDT, Dieldrin, Endrin, Heptachlor, Hexachlorobenzene,
chemicals: Hexachlorobenzene, Polychlorinated biphenyls (PCBs)
Polychlorinated dibenzo-p-dioxins and Polychlorinated dibenzo furans
(PCDD/PCDF) and PCBs
odourless crystals when pure; technical grades are tan to dark brown with a
mild chemical odour.
Mainly used to kill termites and grasshoppers,
corn rootworm and other insect pests
Due to its persistent nature aldrin is known to
Carcinogen as well as a mutagen
Human exposure is through dairy products ad meats
to yellowish-brown viscous liquid with an aromatic, pungent odour similar to
Broad spectrum insecticide used on agricultural
crops as well as used in control of termites
Possible human carcinogen, recent studies have
linked with prostate and breast cancers
Exposure mainly through air and it has been
detected in the indoor air and residences in the USA and Japan.
to slightly fragrant colourless crystals or white powder.
Widely used during World war II to protect
civilians and soldiers from malaria, typhus and other diseases spread by
After war, used on crops and continued to be
applied against mosquitoes to control malaria
Its metabolic products DDE and DDD magnify through
the food chain
Long-term exposure associated with human health
and raising serious concerns about infant health
stereo-isomer of endrin, Dieldrin may be present as white crystals or pale
tan flakes, odourless to mild chemical odour.
Used to control termites and textile pests
The pesticide aldrin converts into dieldrin, so
the concentration of dieldrin is higher than aldrin
Linked to Parkinson’s disease and breast cancer
odourless, crystalline solid when pure; light tan colour with faint chemical
odour for technical grade.
Used on cotton, maize and rice crops
Does not accumulate in the fatty tissue but highly
toxic to fish
Enters the atmosphere through volatilization
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
to light tan, waxy solid or crystals with a camphor-like odour.
Primarily used to kill soil insects and termites
Stable structure can remain in environment for a
Responsible for the decline of several wild bird
monoclinic crystals or crystalline solid.
By-product of manufacture of industrial chemical
including carbon tetrachloride, perchlorethylene and penta chlorobenzene
Volatile and expected to partition into atmosphere
Bio concentrate in fat tissue of living organisms
It is ubiquitous in the environment and has been
measure in food of all types
crystalline, odourless solid.
Used to combat fire ants and termites
Used as fire retardant in plastics, rubber and
Stable and persistent pesticide
209 possible PCBs exis among them 13 exhibit a
dioxin like toxicity
Persistence in the environment corresponds to the
degree of chlorination
They do not degrade easily, despite being banned
in 1970s, it still persist in the environment
Polychlorinated dibenzodioxins (PCDD)
Unintentional by products due to incomplete
combustion and manufacture of pesticides
75 different dioxins, seven are considered to be
Polychlorinated dibenzofurans (PCDFs)
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
Yellow, waxy solid with a chlorine/terpene-like
o Highly insoluble in water and has a half life in soil of up to 12
o It has been shown to bio concentrate
in aquatic organisms and is known to undergo atmospheric transport.