Persistent Organic Pollutants POPs

The discovery and extensive use of organochlorine pesticides such as aldrin, chlordane, DDT, dieldrin, endosulfan, endrin, heptachlor, lindane and toxaphene began during and after World War II. Although chemists were aware that these compounds are very stable, there was little concern about possible long-term environmental effects. In 1962 the book Silent Spring by Rachel Carson raised public concern, drawing a link between the use of organochlorine insecticides and declining bird populations; as a result, in the following years intense research was carried out on the environmental fate and biological effects of what we now call POPs. Since then many books have been published on POPs (e.g. Edwards 1973; Hutzinger et al. 1974; Kurtz 1990; Mackay 1991; Howard 1991; Mackay et al. 1992; Beek 2000).

Many thousands of chemicals fall into POPs, and certain "families" such as PCBs include more than 200 compounds, which differ from each other by level of chlorination and substitution position. POPs are persistent and may have a half-life of years to decades in soil and sediments, and of several days in the atmosphere; they are therefore prone to long-range atmospheric transport. POPs are hydrophobic and lipophilic, and partition strongly in organic matter, particularly in fatty tissues. Because they are slowly metabolised, they accumulate in organisms and food chains and may have an adverse impact on human health and the environment (Jones and de Voogt 1999). As a complex array of POPs bioaccumulate simultaneously in all ecosystems, it is usually difficult to establish if a biological effect in a species is due to a particular chemical, family of chemicals, their metabolites or many chemicals acting synergistically. While early ecotoxicological studies concentrated mainly on eggshell thickness of birds of prey and reproductive potential of fish-eating birds and marine mammals (e.g. Ratcliffe 1970; Reijnder 1986), during the last decade there has been increasing concern over possible adverse effects of POPs on humans and wildlife. Evidence suggests that POPs, together with other chemicals, are immunotoxic, endocrine disrupters and tumour promoters (e.g. Vallack et al. 1998).

Because of vapour pressure under ambient temperature, POPs may volatilise from waters, soils and vegetation into the atmosphere, where they are unaffected by breakdown reactions and are transported for long distances before re-deposition. The cycle of volatilisation and deposition may be repeated many times, and POPs assume a global-scale redistribution according to the theory of global distillation and cold condensation in polar or mountainous regions (Wania and Mackay 1993). Concern about their potential toxicity, propensity for long-range atmospheric transport and global-scale redistribution has resulted in international measures to ban or restrict the production and use of POPs. Diplomats from 122 countries drew up a treaty in Johannesburg in December 2000 for the control of the production, import, export and use of toxic and persistent chemicals. The agreement was formally signed at a conference in Stockholm in May 2001, and this treaty will become international law after the legislatures of 50 countries ratify its terms (http://www.chem.unep.ch/pops/). Twelve chlorinated compounds or classes of compounds which were used or are still in use, such as chlorinated pesticides, hexachlorobenzene (HCB), polychlorinated biphenils (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), have been selected and regulated by the Stockholm Convention. However, other persistent organic substances of known anthropogenic origin have been detected in remote regions of the Earth (e.g. poly-chlorinated paraffins, polychlorinated terphenyls, and polychlorinated naphthalenes; Beek 2000; Paasivirta 2000; Ballschmiter et al. 2002). Two semi-volatile hologenated compounds (a group of mixed bromochloro C10 compounds and a heptachlorinated C9 compound), showing the same characteristics as anthropogenic POPs, are probably of natural marine origin (Vetter et al. 1997; Tittlemier et al. 1999; Vetter et al. 2000). Thus, as in the case of trace metals, it is rather difficult to establish accurate global emission inventories for POPs. Moreover, owing to the large number of substances with different physico-chemical properties and the temperature-dependent behaviour of individual compounds, their input and behaviour in the environment cannot be generalised. As a result, most available emission inventories (e.g. Li 1999; Bailey 2001; Breivik et al. 2002) or global-scale models on the long-range transport of POPs (e.g. Strand and H0w 1996; Wania and

Mackay 1999; Scheringer et al. 2000) usually refer to selected isomers/con-geners.

Persistent organic pollutants are transported to polar regions mainly in the atmosphere, and the main forces driving their transport, temporary deposition and remobilisation are POP concentrations, the physical properties of each compound (i.e. Henry's constant, vapour pressure, and the octanol-water partition coefficient), temperature gradients and weather conditions. The range of transport increases with volatility. HCHs and HCB are usually the dominant compounds in the polar atmosphere, while less volatile compounds such as PCBs, DDTs and dieldrin tend to condense closer to their sources. Only a small fraction of less volatile POPs is transferred from tropical to polar ecosystems, and their inter-hemispheric exchanges are restricted by the presence of Hadley cells over the tropics, which produce a strong upward flux in the tropics and a subsiding flux in the subtropics (Levy 1990). Ocean currents are potentially important in inter-hemispheric mixing, but major currents circulate in an anticlockwise direction in the Southern Hemisphere and in a clockwise direction in the Northern Hemisphere. For instance, there is no evidence of inter-hemispheric mixing of PCBs, and it has been estimated that PCB concentrations in the Northern Hemisphere oceans are fourfold those in Southern Hemisphere oceans (Tanabe and Tatsukawa 1986).

The main sources of POPs in the Southern Hemisphere are urbanised areas and those with intensive agriculture, and tropical and subtropical regions where spraying is used for disease vector control. According to several authors (e.g. Mowbray 1986; Forget 1991), the demand and use of many POPs in the 1990s was increasing in tropical Asian countries and Southern Pacific islands, and considerable quantities of PCBs were used in older electrical devices and deposited as landfill in some developing countries (Iwata et al. 1994). Available data indicate that DDT has been widely used in the Southern Hemisphere, and South America has historically been the heaviest user of DDT, toxaphene and lindane. In contrast, the use of HCH (in contrast to its g isomer, lindane) has been largely restricted to the Northern Hemisphere.

The distribution of POPs in eastern and southern Asia and Oceania (Iwata et al. 1994) shows higher levels of HCHs (sum of a, b and g isomers) and DDTs (sum of p,p'-DDT, pp-DDD, pp-DDE, and pp-DDT) at lower latitudes than at higher ones. Concentrations of CHLs (sum of czs-chlordane, trans-chlor-dane, ds-nanochlor and trans-nanochlor ) and PCBs (sum of isomers and congeners) show less prominent latitudinal variations. The highest HCH concentrations (about 11,000 ng m-3) were detected in air from Calcutta (India) and Hue (Vietnam) and were attributed to their use in mosquito control programmes in tropical urban areas. DDT concentrations in some cities of India, Thailand, Vietnam and the Salomon Islands were in the same range as those reported for Brazzaville (Congo; Ngabe and Bidleman 1992), and 2-3 orders of magnitude higher than in Japan, Australia, European countries and the

USA. Atmospheric concentrations of PCBs in tropical urban air were comparable to those in European and North American urban air, and were much higher than values measured in the open ocean. In general, HCB and HCHs show a remarkable tendency to distribute globally, while DDTs, CHLs and PCBs show a lower potential for long-range transport from point sources at lower latitudes.

Data on ambient air levels of POPs in the Southern Ocean and Antarctica originate from cruises close to the continents (Tanabe et al. 1982; Kawano et al. 1985; Weber and Montone 1990; Bidleman et al. 1993), and from some sampling sites in the continent and sub-Antarctic islands (Risebrough et al. 1990; Larsson et al. 1992; Kallenborn et al. 1998; Montone et al. 2003). This section briefly reports on available data; for a more comprehensive review the reader can refer to the Antarctica Regional Report on persistent toxic substances (UNEP 2002a). Most shipboard sampling along transects to the Antarctic coasts measured total DDT concentrations: values were usually lower near the Antarctic coast and there was a remarkable decrease in average values from the early 1980s to the early 1990s. Very low concentrations (0.07-0.4 pg m-3), for instance, were measured in the period December 1994-April 1995 at Signy Island (Kallenborn et al. 1998), a site which is closer than Antarctica to potential sources of DDT. Atmospheric concentrations of chlordane, heptachlor, HCB and HCHs showed a distribution pattern similar to that of DDT, with lowest values at the Antarctic end of the transect. In general, their concentrations decreased during the 1980s and early 1990s (Fig. 35), but there are no recent measurements to ascertain whether this decline has progressed further.

In contrast to pesticides, some industrial POPs have been used in Antarctica; Risebrough et al. (1990), for instance, reported high PCB contamination at McMurdo Station on Ross Island. In the 1980-1982 period, Tanabe et al. (1983a) performed extensive sampling during voyages in the Indian and

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Total DDT Chlordane HCH

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