Dr Richard Liroff from the international conservation group WWF is concerned about levels of DDT and its breakdown product, DDE.
More than 120 countries have been negotiating a treaty to ban or severely limit the use of a series of chemicals classed as Persistent Organic Pollutants (Pops).
These chemicals have been linked to cancer, sterility and birth defects and are now banned by many governments.
Pops differ from other pollutants because they are extremely difficult to break down. They can persist in the environment for many years, and can be absorbed by plants and animals.
The Johannesburg conference will consider the fate of 12 such chemicals, and draw up measures to eliminate or reduce their use.
Some of them are by-products of industry, others are man-made pesticides, used as treatments for crops and in timber preservation or to attack pests directly.
Health link
By far the most controversial is DDT, an organic pesticide that has been used for more than 50 years to control malaria-carrying mosquitoes.
Its use has been largely curtailed after studies suggested it was a factor in a wide range of medical conditions.
Dr Richard Liroff from the international conservation group WWF is concerned about levels of DDT and its breakdown product, DDE.
He said: "Tests of DDT on laboratory animals indicate that DDT can affect their nervous systems, can weaken their immune systems, their defences against disease, and can have an impact on their reproductive systems.
"There is some human evidence which gives grounds for concern, there have been studies in North Carolina and in Mexico which suggest that the higher the levels of DDE in a woman's body the shorter the period of time when she's able to breast feed her children.
"There are plenty of Mexican studies which suggest that kids who are being breast fed are being dosed with high levels of DDT and DDE , levels that exceed standards set by the World Health Organisation."
Need for DDT
But in recent years cases of malaria have increased - a phenomenon largely blamed on the reduction in DDT use.
Dr Amir Attaran director of international health research at Harvard University says the risks of DDT have been exaggerated.
Despite the opposing views, a consensus is said to have emerged that DDT should not be phased out without the development of effective alternatives for controlling malaria.
Dr Liroff said: "There is very broad agreement among everybody involved in negotiations that the Pops treaty should provide a specific exemption allowing the use of DDT for malaria control.
This reflects the fact that while there are alternatives to DDT that have been successfully used in a large number of countries, there is still some uncertainty about the usefulness of those alternatives in the particular countries that are still using DDT for malaria control.