Cape Town — Delegates from 122 countries have finalised the text of a legally binding treaty that will require governments to minimise and eliminate some of the most toxic chemicals ever created.

The United Nations Environment Programme, which organised the negotiations in Johannesburg said that the "dirty dozen" persistent organic pollutants (POPs) range from industrial chemicals to insecticides.

They are linked to birth defects, cancer and developmental problems in children.

The agreement came at the end of five rounds of negotiations over two-and-a-half years.

The convention will be signed in Stockholm in May and become a legally binding treaty once it has been signed by 50 countries, a process that conference chairman John Buccini said would take four or five years.

"Persistent organic pollutants threaten the health and well- being of humans and wildlife in every region of the world," said John Buccini, the Canadian official who chaired the talks.

"This new treaty will protect present and future generations from the cancers, birth defects, and other tragedies caused by POPs."

Executive Director Klaus Toepfer of the United Nations Environment Programme applauded the strong international regime that has been established for promoting global action on POPs.

"This is a sound and effective treaty that can be updated and expanded over the coming decades to maintain the best possible protection against POPs," he said.

The treaty sets out control measures covering the production, import, export, disposal, and use of POPs. Governments are to promote the best available technologies and practices for replacing existing POPs while preventing the development of new POPs.

They will draw up national legislation and develop action plans for the implementation of the commitments.

POPs Review Committee will consider additional candidates for the POPs list on a regular basis. This will ensure that the treaty remains dynamic and responsive to new scientific findings.

A financial mechanism will help developing countries and countries with economies in transition meet their obligations to minimise and eliminate POPs.

New and additional funding and technical assistance will be provided.

Most of the 12 chemicals are subject to an immediate ban.

However, a health-related exemption has been granted for DDT, which is still needed in many countries to control malarial mosquitoes.

This will permit governments to protect their citizens from malaria - a major killer in many tropical regions - until they are able to replace DDT with chemical and non-chemical alternatives that are cost-effective and environmentally friendly.

Similarly, in the case of PCBs, which have been widely used in electrical transformers and other equipment, governments may maintain existing equipment in a way that prevents leaks until 2025 to give them time to arrange for PCB-free replacements.

Although PCBs are no longer produced, hundreds of thousands of tons are still in use in such equipment.

In addition, a number of country-specific and time-limited exemptions have been agreed for other chemicals.

Governments agree to reduce releases of furans and dioxins, which are accidental by-products and thus more difficult to control, "with the goal of their continuing minimisation and, where feasible, ultimate elimination".

Other national measures required under the treaty relate to reporting, research, development, monitoring, public information and education.

The meeting in Johannesburg was the fifth and final POPs negotiating session and was attended by some 600 participants.

The treaty will be formally adopted and signed by ministers and other plenipotentiaries at a Diplomatic Conference in Stockholm in May next year.

Governments must then ratify, and when 50 have done so the treaty will enter into force. The process normally takes several years.

Of all the pollutants released into the environment every year by human activity, POPs are among the most dangerous.

They are highly toxic, causing an array of adverse effects, notably death, disease, and birth defects, among humans and animals. Specific effects can include cancer, allergies and hypersensitivity, damage to the central and peripheral nervous systems, reproductive disorders, and disruption of the immune system.

These highly stable compounds can last for years or decades before breaking down. They circulate globally through a process known as the "grasshopper effect".

POPs released in one part of the world can, through a repeated (and often seasonal) process of evaporation, deposit, evaporation, deposit, be transported through the atmosphere to regions far away from the original source.

In addition, POPs concentrate in living organisms through another process called bio-accumulation.

Though not soluble in water, POPs are readily absorbed in fatty tissue, where concentrations can become magnified by up to 70,000 times the background levels.

Fish, predatory birds, mammals, and humans are high up the food chain and so absorb the greatest concentrations. When they travel, the POPs travel with them.

As a result of these two processes, POPs can be found in people and animals living in regions such as the Arctic, thousands of kilometres from any major POPs source.

Fortunately, there are alternatives to most POPs. The problem is that high costs, a lack of public awareness, and the absence of appropriate infrastructure and technology often prevent their adoption.

Solutions must be tailored to the specific properties and uses of each chemical, as well as to each country's climatic and socio-economic conditions.

The 12 initial POPs include eight pesticides (aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex, and toxaphene), two industrial chemicals (PCBs and hexachlorobenzene, which is also a pesticide), and two unwanted by-products of combustion and industrial processes (dioxins and furans).