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9 Risk to human health


P Goll and B McCarton

Effects in Man
Regulatory limits
Exposure from foodstuffs
Conclusions
References

P Goll and B McCarton

The World Health Organisation considers DDT safe to people and the environment when applied in the home for mosquito control. There has been little research to support this claim.

Acute poisoning occurs when DDT is deliberately or inadvertently administered at a dose exceeding 10 ppm body weight.4 2 The central nervous system is affected, resulting in convulsions within 30 min to 6 hours and possibly unconsciousness. However, recovery is swift and without sequelae if appropriate remedial measures are taken.

Chronic exposure to DDT after weaning appears harmless. Workers occupationally exposed to 0.25 ppm body weight/day for 25 years have shown no adverse clinical effects. Storage in adipose tissue is apparently innocuous and there in no unequivocal evidence that exposure has tumorogenic, carcinogenic or teratogenic effects. DDT can cause hepatic tumours in rats but the incidence of hepatic cancer in humans in the USA has declined steadily since 1930, despite exposure to DDT and other suspect pesticides.

Babies tend to be born with slightly lower blood levels of DDT than their mothers2 and no adverse effects have been found.3 However, lactation is the most effective excretory mechanism of DDT and the mother sheds DDT residues at this time. DDE levels in breast milk have been linked with hyporeflexia in infants just after birth.3 Hyporeflexia was defined if more than 4 out of 20 reflexes were low or absent as assessed by the Brazelton Neonatal Behavioral Assessment Scale. Hyporeflexia increased significantly from 3.4~O at 0-0.9 ppm DDE in milk (milk fat, n = 59) to 14.1% at 6 ppm DDE and more (n = 64). The longer-term consequences of hyporeflexia are unknown but the risks should be set against those from contracting malaria due to inadequate mosquito control, or from diarrhoeal diseases due to bottle-feeding instead of breast-feeding. In 1988, for example, there were over one million clinical cases of malaria and 276 deaths reported in Zimbabwe and the true situation was undoubtedly much worse (R. Liverton and T. Freeman personal communication). Malaria and diarrhoea, and more recently AIDS, are the predominant causes of death in sub-Saharan Africa, especially in children under 5 years.4

Rectangular Limits

Despite the apparent lack of effects in adults, regulatory limits for the intake of DDT have been set. The Acceptable Daily Intake, or ADI, is the amount of a compound which can be consumed daily, over a life-time, without risk to health. ADIs are set by the Joint World Health Organisation/Food & Agriculture Organisation Meeting on Pesticides (JMPR). The level is determined on the basis of animal experimentation and informed judgement. The maximum concentration administered daily to the test animal not producing a toxic effect is the No Observed Effect Level (NOEL). An arbitrary safety factor of 10-500 x, but generally 100 x, is applied to the NOEL, expressed in mg/kg body weight/day, to give the ADI for Man.

The Maximum Residue Limit, or MRL, is the residue concentration which determines whether a foodstuff is fit for human consumption. MRLs are set by the the JMPR Codex Committee on Pesticide Residues to allow a wide safety margin between possible intake from a 'normal' diet and the ADI.

ADIs and MRLs are reviewed periodically as new information appears and temporary levels are set where information is lacking. However, they have been criticized for their reliance on animal experimentation—and its questionable relevance to Man—and for the use of arbitrary safety factors.

The following levels have been set for DDT and its metabolites (i.e. SDDT):

NOEL

0.25 ppm body weight/day

ADI

0.02 ppm body weight/day

MRL in milk

0.05 ppm

cereal grains

0.10 ppm

eggs

0.50 ppm

fruit and vegetables

1.0 ppm

carcass meat

5.0 ppm

fish

5.0 ppm

Temporary levels set in 1989

Exposure from foodstuffs

Fish

Residue levels were measured in commercial catches of six fish species from two sites on the lake (Table 9.1).

Samples were too small to allow statistical comparison between sample sites, although the trend suggests higher residue burdens in fish from Charara. Residue levels are broadly similar to those recorded in Berg's recent study of DDT contamination in fish from Lake Kariba, although levels in Tigerfish and Kapenta at Msampa are higher than those recorded by Berg. Assuming 3.6% lipid, levels are also higher than was found in kapenta from the Ume mouth in 1984.


Table 9.1 Residue levels in fish fillets and kapenta from Lake Kariba

On a worst sample basis, and on the assumption that Tigerfish muscle and whole kapenta contain 0.8% and 3.6% lipid respectively, residue levels in food will be 2.9 and 0.24 ppm S DDT respectively, below the MRL of 5 ppm. Similarly, consumption of 200 g of Tigerfish daily would result in an intake of 0.13 mg S DDT, well below the ADI for a 60 kg person of 1.2 ma. Other foodstuffs are unlikely to increase the total daily intake significantly. However, the use of DDT to preserve fish from post-harvest infestation, as happens in Zambia,~3 would result in the MRL being exceeded.
Breast milk

Levels of DDE in breast milk commonly exceed 6 ppm in eastern and southern Africa where DDT is used to control mosquitos (Table 9.2) and infants are at risk from hyporeflexia. Relatively low contamination levels in Southern Province, Zambia, and one sample batch from KwaZulu, South Africa reflect the absence of mosquito control operations there. The incremental effect of tsetse fly control to the Zimbabwean and Kenyan values is unknown, but is probably relatively low unless fish are a significant part of the diet.


Table 9.2 DDT residue levels (geometric mean ppm milk fat) in breast milk reported from eastern and southern Africa.

Mosquito control reduces mortality and morbidity in Man due to malaria. DDT remains a cheap and effective insecticide against mosquitos in Zimbabwe but residue levels in human breast milk are amongst the highest recorded anywhere. High residue levels pose no known risk to adults, but DDE levels in breast milk commonly exceed those causing hyporeflexia in infants. A study of the risks, set within the context of cost-benefit studies of mosquito control and breast feeding, should be a priority. Adoption of the precautionary principle—and a switch from DDT to deltamethrin— would be a popular decision, as deltamethrin is also effective against other household pests, such as bed bugs. Operational cost comparisons indicate little difference between using pyrethroids or DDT (T. Freeman personal communication 1992).

References

1. Hayes, W.J. and Laws, K.R. (eds) (1991) Handbook of Pesticide Toxicology. Vol. 2. Classes of pesticides. London: Academic Press.

2. World Health Organization (1979) DDT and its Derivatives. Environmental Health Criteria 9. Geneva: World Health Organisation.

3. Rogan, W.J. Gladen, B.C., McKinney, J.D. Carreras, N., Hardy, P., Thullen, M., Tinglestad, J. and Tully, M. (1986) Neonatal effects of transplacental exposure to PCBs and DDE. Journal of Pediatrics, 109: 335-341.

4. Feachem, R.G. and Jamison, D.T. (eds) (1991) Diseases and Mortality in Sub-Saharan Africa. IBRD, The World Bank OUP.

5. Paynter, O.K. and Schmitt, R. (1979) the ADI as a quantified expression of the acceptability of pesticide residues. In: Pesticide residues. A contribution to their interpretation, relevance and legislation. (Frehse, H. and Geissbuhler, H., eds) Zurich: International Union for Physical and Applied Chemistry.

6. Pieters, A.J. (1979) The setting of MRLs in food—their role and their relation to residue data. In: Pesticide residues. A contribution to their interpretation, relevance and legislation. (Frehse, H. and Geissbuhler, H., eds) Zurich: International Union for Physical and Applied Chemistry.
7. Tincknell, R.C. (1979) Types of pesticide residue data in foods and their characteristics. In: Pesticide residues. A contribution to their interpretation, relevance and legislation. (Frehse, H. and Geissbuhler, H., eds) Zurich: International Union for Physical and Applied Chemistry.

8. Frehse, H. and Geissbuhler, H. (eds) (1979) Pesticide residues. A contribution to their interpretation, relevance and legislation. (Frehse, H. and Geissbuhler, H., eds) Zurich: International Union for Physical and Applied Chemistry.

9. Food and Agricultural Organisation (1985) Pesticide residues in food—1984. FAO Plant Production and Protection Paper No. 62.

10. Berg, H., Kiibus, M. and Kautsky, N. (1992) Pesticides in Lake Kariba. Ambio, 21: 444-450.

11. Mhlanga, A.T., Taylor, R.D. and Phelps, R.J. (1986) HCH and DDT residues in the freshwater sardine (Kapenta) at the Ume river mouth, Kariba. Zimbabwe Science News, 20: 46-49.

12. Matthiessen, P. (1984) Environmental Contamination with DDT in western Zimbabwe in Relation to Tsetse Fly Control Operations. Final report of the DDT Monitoring Project. London: Tropical Development and Research Institute, Overseas Development Administration.

13. Vaz, R. (1989) Determination of Source of Pesticide Residue Contamination in Zambian Dairy Products. Interim consultancy report. TCP/ZAM/8853(A). Rome: Food and Agriculture Organisation.

14. SEMG (1987) Environmental Monitoring of Tsetse Control Operations in Zimbabwe 1986 Saarbrucken: Scientific Environmental Monitoring Group.

15. Department of Research and Specialist Services (1987) Unpublished data.

16. Kanja, L., Skare, J.U., Nafstad, I., Maitai, C.K. and Lokken, P. (1986) Organochlorine pesticides in human milk from different areas of Kenya 1983-1985. Journal of Toxicology and Environmental Health, 19: 449-464.

17. SEMG (1987) Unpublished data.

18. Bouwman, H. (1991) DDT levels in Serum, Breast Milk and Infants in Various Populations in Malaria and Non-malaria Controlled Areas of Kwozulu. Tygerburg, South Africa: Medical Research Council.

19. Schofield, C.J. (1992) DDT and malaria vector control. International Pest Control, May/June, 88-89.