A study has found mothers tested in rural areas have pesticides, including DDT, in their breast milk.
Some of the women had 77 times the international limit for DDT residue in humans, while in some of the babies it was 12 times the World Health Organisation's acceptable daily intake.
These shock findings emerged as a new US study shows DDT in mothers is linked to delays in physical and mental development. The study is one of the first to link DDT with human developmental problems.
Banned in most countries because of its harmful effects on the environment, DDT is used in some parts of South Africa to kill malaria mosquitoes.
Pyrethroids, found in women's breast milk here, are used as agricultural pesticides.
DDT, one of the world's "dirty dozen" organic pollutants, stays in the environment for years after use, enters the food chain and gets stored in the fatty tissue of birds, animals and humans.
Barabara Sereda, from the Agricultural Research Council in Pretoria and one of the scientists who conducted the study, said DDT was present in water and soil and had been taken up by crops.
"The results are quite scary," she said.
The pesticides were found in the breast milk of 152 women at clinics at Jozini, Mkuze and Kwaliweni in northern KwaZulu-Natal.
DDT is used in Jozini and Mkuze to kill mosquitoes, but not Kwaliweni. Between 1995 and 2000 pyrethroids were used for malaria control in the two towns, and then DDT usage was resumed.
Mothers from Jozini have the highest residue, followed by Mkuze and Kwaliweni. Mothers in Jozini had life-long exposure to DDT from malaria control. Exposure for those in Mkuze was variable while moms in Kwaliweni had never lived in DDT-treated dwellings.
The scientists said the source of DDT in Kwaliweni should be investigated to determine whether water or fish were the source.
First-time moms had the highest levels, meaning first-borns got the highest amount of the pesticide in the breast milk. Scientists said the levels of DDT found in babies warranted strong concern, but believed the malaria threat outweighed the negative effects of DDT at these levels.
The peer-reviewed study has been accepted for publication by Environmental Pollution. It is available online.