Compiled by the Programme for the Promotion of Chemical Safety,
the Division of Control of Tropical Diseases,
and the Food Safety Unit
of the World Health Organization
Click to download: Persistent Organic Pollutants, WHO, 23K/22K, English
In view of the requirement to examine the sources, benefits, risks and other considerations relevant to production and use of twelve persistent organic pollutants (POPs) (task c), and to evaluate the availability, including costs and effectiveness of preferable substitutes, where applicable (task d), the Programme for the Promotion of Chemical Safety, Central Unit of the International Programme on Chemical Safety (IPCS), the Division of Control of Tropical Diseases (CTD), and Food Safety Unit (FOS) of the World Health Organization (WHO), within their respective mandates are offering a contribution to the discussions concerning the assessment of realistic response strategies, policies and mechanisms for reducing and/or eliminating emissions, discharges and losses of POPs.
Persistent organic pollutants (POPs) are grouped on the basis of a common characteristic, i.e. persistence in the environment, longer than that required for their intended use. All twelve POPs listed are chlorinated compounds, nine of them having been developed as pesticides. One group - polychlorinated biphenyls (PCBs) are industrial chemicals and the two remaining groups - dioxins and furans, numbering over 200 isomers - are unwanted by-products of various technological processes but were never produced commercially, and have no intended use.
None of the nine chlorinated hydrocarbon pesticides is presently recommended for the use in agriculture, although many incidents of their misuse on food and cases of acute toxic exposure are still being reported. In public health programmes, only a few persistent chlorinated hydrocarbon pesticides are still being used in various countries, especially DDT in vector disease control, chlordane in termite control and aldrin in locust control, but their use is in constant decline, either because of voluntary withdrawal, or due to legal action.
All twelve POPs have been toxicologically evaluated by IPCS and IARC, and some details are given in Table 1.
The greatest part of human exposure to the listed POPs is attributed to the food chain, and only in selected populations of chemical workers does occupational exposure prevail. Most of the POPs have been monitored by the GEMS/Food programme since 1976 and data on levels of residues of organochlorine pesticides and PCBs have been submitted by twenty nations for a wide variety of foods.
Table 1: HUMAN HEALTH AND ENVIRONMENTAL RISK EVALUATION OF SELECTED POPS
ADI1(mg/kg WHO CLASS'N. IARC2 b.w.) by JMPR (Acute CLASS'N. EVALUATION toxicity) for BY carcino- genicity EHC3 LATEST JMPR4 IARC ALDRIN 0.0001 Ib 3 No.91 1994 No.5 DIELDRIN 0.0001 Ib 3 No.91 1994 No.5 ENDRIN 0.0002 Ib 3 No.130 1994 No.5 DDT 0.02 II 2B No.9 1994 No.5 No.83 No.42 No.53 CHLORDANE 0.0005 II 2B No.34 1994 No.20 No.42 No.53 HEPTACHLOR 0.0001 II 2B No.38 1994 No.50 No.20 No.53 HEXACHLORO- WITHDRAWN Ia 2B - 1978 No.20 BENZENE MIREX NOT EVALUATED OBSOLETE 2B No.44 - No.5 No.20 No.42 TOXAPHENE NO ADI II 2B No.45 1973 No.20 (CAMPHECHLOR) PCBs N/A* N/A 2A No.2 N/A No.7 suppl. No.140 No.18 DIOXINS N/A N/A 2B No.88 N/A No.7 suppl. (TCDD) No.15 FURANS N/A N/A 2B No.88 N/A No.7 suppl. No.15
* N/A = Not applicable; 1 ADI = acceptable daily intake;
2 IARC = International Agency for Research on Cancer;
3 EHC = Environmental Health Criteria;
4 JMPR = Joint Meeting on Pesticide Residues.
The contamination of food, including breast milk, by POPs is a worldwide phenomenon. A number of incidents of acute toxic effects in humans, including death, have occurred as a result of contaminated food. Edible oils and foods of animal origin are most often involved. Food contaminated by POPs can also pose chronic health risks, including cancer, but the long-term implications of low level exposure is not fully known. In some countries, levels of certain POPs in breast milk have approached or exceeded tolerable levels, as specified by WHO. The latest compilation of reports on levels of DDT and its residues in breast milk from the Global Environmental Monitoring System/Food Contamination Monitoring and Assessment Programme (GEMS Food) and other sources is presented in Annex 1. Levels in breast milk can provide an assessment of the integrated exposure of women to POPs which is largely due to food contamination. Breast milk is also the sole source for most infants and, consequently, levels of POPs are an important safety concern.
From the data available from a large number of countries, it is clear that DDT has become widely disseminated in the food supply and is present in virtually every sample of breast milk tested. In most cases, the levels in breast milk are below the FAO/WHO Acceptable Daily Intake (ADI) of 20 µg/kg body weight. Furthermore, in countries which have reported over several years, the trend in the levels appears to be declining. However, in many developing countries, the mean levels of DDT complex in breast milk approach or exceed the FAO/WHO ADI. Because of the 10-fold safety factor (based on no-effects on humans) used in establishing the ADI and the fact that adult exposures are well within the ADI, such temporary exposures above the ADI are not considered a public health concern. It should be emphasized that the demonstrated significant benefits of breast feeding far outweigh any hypothetical risk posed by slightly exceeding the ADI. The same applies to dioxins, but the period of exposure (short) contributes little to total body burden. With PCBs and dioxins, neurodevelopmental effects may occur requiring balanced risk assessment. More details on dietary intake of selected POPs in food and in breast milk from the GEMS/Food programme are given in Annex 1.
Contamination of food may occur through environmental pollution of the air, water and soil, or through the previous use of organochlorine pesticides, which are now generally, but not totally, banned from agricultural use. However, introduction of persistent organics into the food chain is also known to occur through unauthorized use of certain pesticides on food crops. Food contamination by POPs can have a significant impact on food exports which are especially important for developing countries. At the international level, limits for residues of persistent organochlorine insecticides have been established for a range of commodities by the FAO/WHO Codex Alimentarius Commission and are recognized by the World Trade Organization as the international reference in assessing non-tariff barriers to trade.
Human acute exposure to polychlorinated dibenzo-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) occurs mainly in the occupational setting (e.g. herbicide production, industrial accidents, chemical fires) and through burning of garbage in dump areas in developing countries. Episodes of massive food contamination (e.g. with PCDFs) and of exposure under specific war circumstances (e.g. TCDD as contaminant in Agent Orange) (Kahn et al., 1988), are widely publicized. Amongst all dioxins, TCDD is the most widely studied. Exposure to TCDD occurs in the general population through ingestion of fatty fish from industrially polluted waters (Rappe et al., 1989, Svensson et al., 1991). Dairy products may also contain TCDD. Levels reported in the blood of occupationally or accidentally exposed individuals reflect exposures of limited duration, but exposure through food consumption may be lifelong and result in accumulation.
The acute toxicity of the twelve POPs discussed varies greatly, with an acute oral LD50 ranging between 0.002 mg/kg b.w. (for TCDD in guinea pigs) to 10 000 mg/kg b.w. (for hexachlorobenzene in rats). Acute toxicity of a toxic chemical is not the only determinant of its hazard to humans, as many of them have well documented or suspected irreversible, harmful effects, such as malignancy, teratogenicity, organ damage, etc. Some chlorinated hydrocarbon insecticides have been known to be the cause of many serious, acute poisonings, mainly through food contamination. This particularly refers to endrin, aldrin and dieldrin. According to data reported by twenty-three poisons centres from different parts of the world, cases of OC pesticide poisoning are still occurring, due mainly to aldrin, dieldrin, HCB and chlordane (see Annex 2). On the other hand, DDT has a recognized safety record during over half a century of intensive use. There have only been a few reports on the effects of accidental or suicidal poisoning, following ingestion of DDT. There is no convincing body of evidence on adverse effects of DDT following long-term occupational exposure (see WHO Report, 1993, Annex 3).
It should be pointed out that, should acute poisoning occur following an overexposure, no specific antidotes are available for any of the listed POPs.
None of the POPs discussed is classified by IARC as "Carcinogenic to humans" (IARC class 1); one of them (PCBs) is classified as "Probably carcinogenic to humans" (class 2A); eight are classified as "Possibly carcinogenic to humans" (class 2B), and the remaining three are "Not classifiable as to their carcinogenicity to humans" (class 3). These are endrin, dieldrin and aldrin, all of which are classified by WHO as highly hazardous (class 1b), on the basis of their acute toxicity to experimental animals.
Other suspected adverse effects of low doses of POPs for which evidence is being accumulated include porphyria, disturbance of hormonal function and suppression of the immune system in exposed individuals.
Deleterious effects of many POPs on beneficial species are well documented. This particularly applies to aquatic organisms. As an example, endrin is highly toxic to fish, with most LC50 values below 1 µg/l.
* For details see Annex 4.
Environmental aspects of DDT use in public health programmes, and malaria control in particular, have been the subject of much controversy for many years. The impact of DDT on the environment has been greatly reduced since its use in food production was banned in most countries many years ago.
Many countries rely on the use of DDT for the control of malaria and visceral leishmaniasis. Its use is, however, declining due to vector resistance to DDT, lesser availability of DDT which meets WHO specifications, and the adverse publicity concerning DDT.
In the early seventies, considerably more acutely toxic organophosphorous and carbamate insecticides were introduced in vector control operations, which resulted in a considerable increase in incidences of acute poisoning, both accidental and occupational. Resistance to them has also developed in a number of malaria vectors.
In recent years, pyrethroids started to replace both chlorinated hydrocarbons and organophosphorous compounds. However, there is more and more evidence that resistance is also increasing to pyrethroids; meanwhile malaria is on the increase in many parts of the world.
The issue is complicated even further by development of parasite (Plasmodial) resistance to the available antimalarial drugs, and the fact that no effective malaria vaccine is yet available, for operational use.
Being aware of the need for chemical insecticides as an essential tool in integrated pest management programmes in controlling vector-borne diseases, WHO is encouraging the development of new classes of pesticides through the WHO Pesticide Evaluation Scheme. Unfortunately, this development is much slower than insect resistance development, and there is a fear that the few available insecticides may not be sufficient in the near future especially taking into consideration the implications of resistance and cross-resistance in the vectors. It is for this reason alone that WHO is not discouraging the use of DDT in malaria control, on condition that it is:
Countries continuing to use DDT should assure that the pesticide is not diverted for unauthorized uses. To this end, DDT should, in general, be restricted to government-controlled programmes.
A WHO Study Group meeting on vector control (1993) looked into risk versus benefit of using DDT in vector control, the conclusions of which were presented to the WHO Executive Board. A position paper emerged and is attached as Annex 3.
It is to be predicted that, because of great reduction in DDT production worldwide (only a few countries are still producing it - see Annex 2 for information), the rapid development of insect resistance and possible appearance of suitable alternatives, DDT use may cease, without a significant legal intervention.
It should be emphasized that any alternative to DDT or other persistent organic pesticides for public health vector control should meet the three basic criteria: effectiveness, safety and acceptable cost.
Regardless of a country's decision to ban, or severely restrict the use of some, or all, persistent organic pesticides, there is hardly any country not facing the problem of disposal of some remaining stocks.
The strict requirements for proper disposal of these chemicals by incineration at very high temperatures create an enormous burden for a developing country, both economically and technologically. There are very few adequate incinerators in the world, and almost all of these are located in developed countries. Legal aspects concerning transboundary movement of toxic chemicals make the problem of safe disposal more complicated and costly. It should be considered, on a case-by-case basis, whether perpetual storage of unwanted POPs awaiting proper incineration would be a safer alternative than a feasible, but not entirely appropriate disposal (e.g. burying below topsoil).
A recent session of the Economic and Social Council requested U.N. Agencies to intensify their cooperation and assistance on environmentally sound management of toxic chemicals and hazardous wastes, bearing in mind safe disposal of unwanted chemicals. For this reason, any activity in the area of implementation of chemical safety programmes should address the problem of safe disposal of POPs as a priority issue.
The decision on whether a chemical should be banned, or its use restricted, should be based on its proven harmful effect on humans, non-target organisms and the integrity of the environment, and not solely on its persistence in the human body or in the environment.
The persistence of a given chemical should be assessed in view of the environmental conditions (temperature, humidity, insulation, etc.) as these factors influence the half-life of many toxic chemicals.
Each of the chemicals listed is able to produce untoward effects of varying degrees, both on humans and on the environment. Some of them are of a serious nature, causing irreversible damage, at a rather low level. Some others may have an effect which might be outweighed by the benefit desired from its use, and therefore the known risk may be acceptable under certain conditions.
None of the pesticides listed among the selected POPs is to be recommended for food production.
The development of alternatives to pesticides among the selected POPs still effectively used in some areas of pest control (vector-borne diseases, termite and locust control, etc.) should be given high priority by industry and international organizations. In addition to efficacy against a specific pest, safety for human health and the environment, as well as the cost/efficacy ratio, should be given even higher consideration.
Based on the data available, the exposure of the adult population through the food chain to the POPs considered does not appear to represent a significant public health concern. However, certain countries experiencing high levels of POPs in their food supplies may wish to consider interventions to assure the safe use of POPs, particularly as pesticides. Levels of POPs in human milk have resulted in exposures approaching the safe or tolerable intakes established for exposure over a lifetime. The health significance of this time-limited exposure should be assessed and efforts to reduce maternal exposure to such POPs, or to limit excretion of POPs in milk, should be considered.
Where levels of some POPs in breast milk at levels approach or slightly exceed tolerable levels, breast feeding should not be discouraged since the demonstrated significant benefits of this practice greatly outweigh the small hypothetical risk that POPs may pose.
Countries, but especially those employing older industrial technologies (including the use of older chemicals), should establish basic monitoring and assessment programmes to assess human exposure. In addition, exposure estimates for infants and children should be conducted, especially for those contaminants to which the young are particularly susceptible. Simple and inexpensive methods are available for most POPs with the exception of dioxins and furans. These programmes can supply the essential information necessary for developing effective intervention strategies and for efficient management of health and environmental resources.
Kahn PC, Gochfeld M, Nygren M, et al. Dioxins and dibenzofurans in blood and adipose tissue of Agent Orange-exposed Vietnam veterans and matched controls. JAMA, 1988, 259:1661-7.
Kappe C, Bergqvist PA, Kjeller LO. Levels, trends and patterns of PCDDs and PCDFs in Scandinavian environmental samples. Chemosphere, 1989, 18:651-8.
Svensson B-G, Nilsson A, Hansson M, et al. Exposure to dioxins and dibenzofurans through the consumption of fish. New England Journal of Medicine, 324, 1:8-12 (1991).
WHO Report: Use of DDT in vector control - Conclusions of the WHO Study Group on Vector Control for Malaria and other Mosquito-Borne Diseases, Geneva, 16-24 November 1993. WHO/MAL/95.1071, WHO/CTD/VBC/95.997.