by Dr. Bo Wahlström

Persistent organic pollutants are hazardous chemicals, with moderate to low volatility, that resist degradation and tend to accumulate in living tissues. Their persistence in various media facilitates their transport over long distances to remote regions where they have never been used. In polar and other regions they have been found to present risks to human health and the environment.

The UNEP Governing Council Decision 19/13, adopted by Governments in February 1997 identified twelve persistent organic pollutants (DDT, aldrin, dieldrin, endrin, chlordane, heptachlor, hexachlorobenzene, mirex, toxaphene, polychlorinated biphenyls, dioxins and furans) for which international action should be initiated to protect human health and the environment.

The Governing Council also noted the need to develop science-based criteria and a procedure for identifying additional persistent organic pollutants as candidates for future international action, and requested the intergovernmental negotiating committee should establish an expert group to carry out this work.

The following factors are of primary interest for criteria development to identify further persistent organic pollutants:

Persistence: The ability to resist degradation in various media, e.g. air, soil, water, sediment, measured as half-life of the substance in the medium.

Bioaccumulation; the ability of a chemical to accumulate in living tissues to levels higher than those in the surrounding environment, expressed as the quotient between the concentration in the target tissue and the environmental concentration.

Toxicity; the ability of a chemical to cause injury to man or the environment.

Volatility; the ability of a chemical to vaporise into air, measured in Pascals. Substances with a volatility of <1000 Pascals are of primary concern.

Long-range transport; as evidenced e.g. by measurements of the chemical in biota in remote regions.

Bioavailability; based on field data or expert judgement has also been proposed as a criterion for identifying POPs.

Persistence reflects the ability of the substance to resist physical, chemical or biological degradation. There are various ways to estimate the persistence of a chemical.

In the testing of new and existing chemicals in industrialised countries for the assessment of hazard and risk the OECD Test Guidelines are used together with the Good Laboratory Principles. There are guidelines for determining the easy biodegradability of substances as well as for measuring their inherent biodegradability. However, these are primarily focused on separating chemicals that are easily biodegradable from those that are not. Persistent chemicals would be only a small subset of the not easily biodegradable ones, and may not be identified using existing methods.

Persistence is therefore often measured in the field as the ability of the compound to survive in a specified environmental medium sufficiently long for it to be either transported over long distances or to be sufficiently available for uptake by living organisms. UNECE LRTAP uses a half life in air of more than 2 days as a screening criterion for identifying candidate POPs. This criterion has also been used by the NAFTA CEC and by Canada in developing its Toxics Management Policy. Modelling shows that significant quantities of a substance would remain in air after 8-10 days for a substance with a half-life of two days or more. During that time the substance may be transported several thousand kilometres.

The present twelve POPs all exceed or are close to the two-day atmospheric half-life criterion. Collected data show that there are several other organic compounds in use which exceed this criterion. In the UNECE LRTAP as well as in NAFTA CEC it is used as a first screening criterion. It should be used together with presence of the substance in remote regions as demonstrated by monitoring of biota.

Chemicals with long persistence times in water, soil or sediments have a high potential for accumulation in the medium and also for uptake by living organisms. The pattern of application is of great importance. Substances that are applied once annually, e.g. some pesticides, may have half-lives of several months without accumulating in soil over the years, in spite of repeated applications. At the same time the opportunity for uptake in biota is increased. Most substances are not applied in this way. Releases and emissions occur over the year, often from diffuse sources. Under such circumstances accumulation in soil or sediments may occur. However, the principal concern is related to uptake by organisms and transport in food chains and food webs.

As with other properties organic chemicals are distributed in a continuum from very reactive to extremely persistent ones. There is no clear demarcation line between persistent and non-persistent chemicals. Half-lives in soil, water or sediments between 2 and 6 months have been suggested as criteria for identifying candidate POPs. Factors such as temperature, pH and amount and content of biological fraction greatly influences persistence in the field, as well as processes such as photolysis and hydrolysis.

The twelve POPs mentioned by the UNEP Governing Council Decision 19/13 all have very long half-lives in water and soil. For some POPs, e.g. DDT and mirex, the half-lives are several years.

Bioaccumulation measures the potential for a chemical to concentrate in living tissues. While POPs are diluted by dispersion during the long-range transport, bioaccumulation counteracts this effect and redistributes POPs within an ecosystem. Many natural substances, e.g micronutrients, are also bioaccumulated to concentrations several orders of magnitude above that in the surrounding medium, usually by active energy-dependent mechanisms. The POPs presently under discussion for international action are all characterised by their lipid affinity and low water solubility. After uptake, which proceeds by physical processes, they concentrate in tissues with high lipid content, e.g. fatty tissues or nerve cell sheaths.

Bioaccumulation can best be measured in intact organisms in the laboratory or in the field. It is usually expressed as the BioConcentration Factor (BCF) or Bioaccumulation Factor (BAF). Bioaccumulation measured in this way confirms that uptake takes place and integrates accumulation with biodegradation by the organism. Values between 1,000 and 5,000 have been proposed, e.g in UNECE LRTAP and NAFTA CEC, as a criterion for identifying candidate POPs.

Among factors that influence the BCF are choice of species, study design, lipid content of organism and others.

Most man-made chemicals have not been studied in bioconcentration tests. Systematic testing of existing chemicals is on-going in several national, regional and international programs, e.g. within the US, EU and OECD. However, it will take years if not decades before even all high production volume chemicals have been tested to a sufficiently large extent. In the absence of data on bioconcentration from animal testing the octanol-water partition coefficient (Kow) has been used as a surrogate. It can be easily measured and even calculated on the basis of the molecular formula and structure and correlates fairly well with BCF. It must be noted, however, that it should primarily be used as a screening tool, since by itself it will not tell whether a chemical is actually taken up by the organism, or, if taken up whether it is actually accumulated. Some chemicals with a high Kow have molecular weights higher than 1000. Such large molecules are generally too large to pass biological membranes by passive diffusion and are therefore not bioavailable. Some chemicals with high Kows are extensively metabolised, e.g. pentachlorophenol and Benz(a)pyrene. A high Kow (> 1,000 ) should therefore always be confirmed by testing the BCF in an animal species.

This criterion has been used by the UN Economic Council for Europe (UNECE) in the negotiations for a POPs protocol under the Geneva Convention on Long Range Transport of Air Pollutants (LRTAP), the Commission on Environmental Co-operation (CEC) under the North American Free Trade Agreement (NAFTA), and others. Normally, chemicals with a volatility of less than 1000 Pascals are considered. The volatility criterion is applied together with persistence in air, and/or data on presence in remote regions. It should be noted that even chemicals with a low to very low volatility may be transported over long distances in sufficient quantities to cause risks to human health and the environment in remote regions. A good example is pp=-DDT, which has a vapour pressure of less than 10^-4 Pascal.

The best way to establish if long-range transport occurs is direct measurement of POPs, e.g. in monitoring programmes in remote locations such as the Arctic, isolated Pacific islands or mountain areas. Measurements in biota and human populations also present data that may be used in risk assessments.

The potential for long range transport may be assessed indirectly by persistence times in air, water or soil, and by factors such as volatility. However, the behaviour of persistent chemicals in the environment is dependent on a host of other factors, e.g. adsorption to particles, , soil binding, etc., which makes prediction difficult. The present twelve POPs are also chemically heterogeneous and their volatility varies by one million fold.

There are other means of long-range transport than through air or water. Migratory birds may accumulate substantial amounts of POPs in their winter quarters, e.g. during periods of pesticide spraying of crops. A significant fraction of the migrating birds die in their summer quarters in arctic regions every year, thus transferring their POPs content to the northern environment.

The long-range transport criterion is basically qualitative in character, and should be assessed on a case-by-case basis for each candidate POP substance. It should be used with some caution. There may be several explanations for the absence of monitoring data on a specific chemical, e.g. not included in existing monitoring programmes or no easily available reliable analytical methods.

There is at present no established criterion for toxicity in international negotiations, and it is unlikely that there will be one. While the other parameters are essentially one-dimensional, toxicity is multidimensional and multifaceted. Any assessment of toxicity requires an assessment of dose. Substances of moderate toxicity may cause concern because they are present in significant doses. Chronic and irreversible effects are also assessed differently from acute and transient effects. Toxicity is therefore essentially a qualitative parameter at the screening stage for presumptive POPs. For the twelve POPs presently under consideration for the global negotiations it is generally agreed that there are significant potential risks to man and the environment at present levels in exposed populations.

In applying the screening criteria for identifying further POPs other factors, such as dispersion mechanisms, patterns of use, influences of marine transport and tropical climates should be taken into account, as well as the need to conserve biodiversity and protect endangered species.

The criteria mentioned here are scientifically derived and based on scientific measurements. There are, however, no clear lines that separate out POPs chemicals from other chemicals. There is also a biological variability that necessitates expert judgement in assessing the basic data. The sensitivity of varied ecosystems must also be taken into account, as well as the circumstances of developing countries and countries in transition. Setting criteria for identifying further POPs should therefore be seen as one step in a procedure of international policy development, and an interactive process where both science and policy input is needed.

The UNEP Governing Council Decision 19/13 requests the International Negotiating Committee to establish, at its first meeting, en expert group that should include scientific and socio-economic expertise relevant to the POPs issue and be representative of countries in different stages of development and from different geographical regions, as well as participants from relevant nongovernmental and intergovernmental organisations.

It is foreseen that the expert group should work expeditiously and concurrently with the INC, in order for agreed criteria to be incorporated in the final convention. Countries need to consider how best to prepare themselves for participating in the expert group. Issues that might be considered are: availability of relevant expertise; national or regional projects in specific areas that might support the criteria development process; national or regional data bases on POPs; national or regional experience of use, dispersion and transport of POPs; known effects in local environment; etc. Countries in a region might also consider ways and means to increase understanding of the criteria development process by information sharing and co-operation at regional or subregional level.