by Dr J. Stober

In response to the 1995 UNEP Governing Council Decision on POPS, a consultant=s report was prepared under the auspices of the International Programme on Chemical Safety (IPCS) and submitted to the second session of the Intersessional Group of the IFCS held in 1996. The assessment report compiled existing information on the chemistry and toxicology of the 12 POPs specified in the UNEP GC Decision. This presentation, for the most part, is based on that report.

Persistent organic pollutants (POPs) have been defined in earlier presentations today. POPs are organic compounds that, to a varying degree, resist photolytic, biological and chemical degradation. POPs are often halogenated and characterised by low water solubility and high lipid solubility, leading to their bioaccumulation in fatty tissues. They are also semi-volatile, enabling them to move long distances in the atmosphere before deposition occurs.

Humans can be exposed to POPs through diet, occupation, accidents and the environment, including the indoor environment. Exposure to POPs, either acute or chronic, can be associated with a wide range of adverse health effects, including illness and death.

Human acute exposure to dioxins and furans can occur, for example, in the occupational setting - herbicide production, industrial accidents or chemical fires -, and through burning of garbage in dump areas.

According to recent data from poisons centres from different parts of the world, cases of organochlorine pesticide poisoning are still occurring, and are mainly due to aldrin, dieldrin, HCB and chlordane. It should be pointed out that, should acute poisoning occur following an overexposure, no specific antidotes are available for any of the 12 specified POPs.

The greatest part of human exposure to the 12 specified POPs is attributed to the food chain. Contamination of food may occur through environmental pollution of the air, water and soil, or through the previous use or unauthorized use of organochlorine pesticides on food crops. Episodes of massive food contamination have been reported. Some chlorinated hydrocarbon insecticides have been known to be the cause of many serious, acute poisonings. This particularly refers to endrin, aldrin and dieldrin.

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.

A notable episode involving the effects of HCB on humans involves the ingestion of HCB treated seed grain. The patients who ingested the treated seed experienced a range of symptoms including photosensitive skin lesions, hyper pigmentation, hirsutism, colic, severe weakness, porphyrinuria, and debilitation. Mothers who ingested the seeds passed the HCB to their children by placental transfer and through maternal milk. Children born to these women developed a condition called pink sore (pembe yara), with a reported mortality rate of approximately 95%.

Such frank expressions of effects are not as common in the case of exposure to lower concentrations derived from the environment and the food chain. Food contamination by POPs can also pose chronic health risks, including cancer, but the long-term implications of low level exposure is not fully known. Studies collectively demonstrate that overexposure to certain POPs may be associated with a wide range of biological effects. The developing fetus and neonate are particularly vulnerable to POPs exposure due to transplacental and lactational transfer of maternal burdens at critical periods of development.

Acute toxicity of a toxic chemical is not the only determinant of its hazard to humans, as many of them have document or suspected irreversible, harmful effects, such as malignancy, teratogenicity, organ damage, etc. Some studies link dietary intake of POPs to important reductions in the lymphocyte population, while other studies suggest that children with high organochlorine dietary intake may experience rates of infection some 10-15 times higher than comparable children with lower intakes.

Laboratory investigations and environmental impact studies in the wild have implicated POPs in endocrine disruption, reproductive and immune dysfunction, neurobehavioural and developmental disorders and cancer. Levels of certain POPs in breast milk have been reported to approach or exceed tolerable recommended international levels; this may be a particular concern for nursing mothers. More recently some POPs have also been implicated in reduced immunity in infants and children and the concomitant increase in infection, also with developmental abnormalities, neurobehavioural impairment and cancer and tumour induction or promotion. Some POPs are also being investigated as a potentially important risk factor in the etiology of human breast cancer.

Laboratory studies provide convincing supporting evidence that selected organochlorine chemicals may have carcinogenic effects and act as strong tumour promoters. The International Agency for Research on Cancer has classified PCBs as 'probably carcinogenic to humans' - eight (8) others on the specified list are classified as 'possibly carcinogenic to humans'. The remaining three (3) - endrin, dieldrin and aldrin are classified by WHO as 'highly hazardous' (class 1b) on the basis of their acute toxicity to experimental animals.

For some POPs, occupational and accidental high-level exposure is of concern for both acute and chronic worker exposure. The risk is high in developing countries where the use of POPs in tropical agriculture has resulted in a large number of deaths and injuries. In addition to other exposure routes, worker exposure to POPs during waste management is a significant source of occupational risk in many countries. Occupational, bystander and near-field exposure to toxic chemicals is often difficult to minimize, with obstacles to managing workplace exposure in part due to poor or non-existent training, lack of safety equipment, and sub-standard working conditions.

Humans encounter a broad range of environmental exposures and frequently to a mixture of chemicals at any one time. While much work remains to be done on the study of the human health impact of exposure to POPs, particularly in view of the broad range of concomitant exposing experienced by humans, the weight of existing scientific evidence has been judged sufficient to warrant immediate actions to reduce the risks to human health. There is general agreement that exposure to certain POPs can have the potential for a significant impact on human health either in the short or long term. High (over) exposures at the point of use of some POPs can lead to acute effects, including death, while at lower exposure levels long term effects can occur.