Persistent organic pollutant

A persistent organic pollutant (POP) is an organic compound that is only slowly degraded in the environment, so that it tends to persist and even to accumulate rather than being broken down by photochemical or biological processes. Accumulation can occur either in the environment (eg, in sediments) or in the tissue of animal or plant species (bioaccumulation).

Public concern about persistent organic pollutants was first raised in the early 1960s with the publication of the book Silent Spring (1962) by U.S. author Rachel Carson (1907–64), which led to the banning of DDT for agricultural use in the United States in 1972. The U.S. and several other countries have regulated POPs on a case-by-case basis since then. A more systematic approach to the regulation of POP production and use can be found in the Stockholm Convention on Persistent Organic Pollutants, signed in 2001, and the European Union REACH Regulation (EC No 1907/2006).

Initial concern over persistent organic pollutants centred on organochlorine insecticides, industrial organochlorine compounds such as polychlorinated biphenyls (PCBs) and byproducts such as dioxins. Other classes of POPs include brominated flame retardants, perfluorooctyl compounds and nitro compounds such as musk xylene.

Definitions

A persistent organic pollutant could simply be defined in terms of its persistence in the environment, which would include substances such as crude oil. However, it is more normal also to take account of the potential for bioaccumulation and any toxicity associated with the substance. The Stockholm Convention Secretariat gives the following general definition:

Persistent Organic Pollutants (POPs) are organic chemical substances, that is, they are carbon-based. They possess a particular combination of physical and chemical properties such that, once released into the environment, they:

• remain intact for exceptionally long periods of time (many years);
• become widely distributed throughout the environment as a result of natural processes involving soil, water and, most notably, air;
• accumulate in the fatty tissue of living organisms including humans, and are found at higher concentrations at higher levels in the food chain; and
• are toxic to both humans and wildlife.

The European Union distinguishes between PBT substances (persistent, bioaccumulative, toxic) and vPvB substances (very persistent, very bioaccumulative). All substances listed in the Stockholm Convention are vPvB under the EU classification, and also have evidence of toxicity and long-range environmental transport.

	Stockholm Convention	REACH Regulation
Persistence	half-life in water > 2 months; or half-life in soil > 6 months; or half-life in sediment > 6 months.	P	half-life in marine water > 60 days; or half-life in river or esturine water > 40 days; or half-life in marine sediment > 180 days; or half-life in river or esturine sediment > 120 days; or half-life in soil > 120 days.
		vP	half-life in water > 60 days; or half-life in sediment > 180 days; or half-life in soil > 180 days.
Bioaccumulation	bioconcentration factor for aquatic species > 5000; or direct evidence of bioaccumulation in biota; or other reasons for concern, such as high bioaccumulation in nonaquatic species, high toxicity or ecotoxicity. In the absence of other data, log Kow > 5.	B	bioconcentration factor for aquatic species > 2000.
		vB	bioconcentration factor for aquatic species > 5000.
Long-range environmental transport (LRET)	evidence of LRET potential by air, water or migratory species, from either measured concentrations distant from the source of pollution; or modeling results. For transport by air, the half-life in air should be > 2 days.	not taken into account
Toxicity	evidence for the potential for damage to human health or to the environment, either direct or through laboratory studies.	no observed effect concentration (NOEC) for aquatic species < 0.01 mg/l; or substance is classified as carcinogenic (1A or 1B), germ cell mutagenic (1A or 1B) or toxic for reproduction (1A, 1B or 2) under the CLP Regulation; or substance is classified as STOT (repeated exposure) 1 or 2 (oral, dermal, inhalation of gases/vapours, inhalation of dust/mist/fume) under the CLP Regulation.

Safe disposal of POPs

Just as POPs are, by definition, persistent in the environment, they are also difficult to dispose of safely. For organochlorine compounds in particular, the only practical procedure is high-temperature incineration, but this must be done under strictly controlled conditions to minimize the formation of dioxins and dibenzofurans, POPs in their own right. Such disposal is costly even in countries that have access to the necessary technology.

Many organizations, not least the Food and Agriculture Organization (FAO) of the United Nations, have pointed out the problems with the banning of several organochlorine insecticides in the 1970s and 1980s by many industrialized countries. Pesticide stocks in industrialized countries became a financial liability for their holders, and many such stocks ended up being exported to countries where their use was still legal, often countries which themselves could not manage the chain from production to residue disposal.

External links

• The POPs from the Stockholm Convention Secretariat
• Persistent Organic Pollutants at the United Nations Environment Programme
• POPs Portal at the United Nations Industrial Development Organization
• Obsolete Pesticides at the Food and Agriculture Organization
• POPs – Persistent Organic Pollutants from the European Commission