Representatives of about 100 governments are to begin five days of talks aimed at reducing the environmental risks from highly toxic and long-lasting chemicals.

The meeting, being held at the Nairobi headquarters of the United Nations Environment Programme (Unep), is the second round of talks on what are known as POPs - persistent organic pollutants.

The first round, held last year in Canada, reached broad consensus on the way forward. The Nairobi meeting hopes to finalise a treaty by next year.

Unep Director Dr Klaus Toepfer said: "The time is right for accelerated global action to protect human health and the environment from these extraordinarily dangerous chemicals.

"I am optimistic that by the year 2000 these talks will produce a legally binding regime that will prevent the terrible mistakes and tragedies of the past from ever happening again, and help repair the damage already done."

Whales are particularly threatened by the toxins

Unep describes POPs as "highly toxic pollutants that harm human health and erode the ecological support system on which life depends. They pose a serious risk because of a unique combination of factors".

As their name suggests POPs are a persistent threat, remaining in the environment for a very long time and accumulating in the fatty tissue of living organisms.

Through a process called "bio-magnification" the concentrations of the chemicals increase as they move up the food chain. They pose a particular threat to many birds and fish, to mammals such as polar bears and whales, and to humans.

They also travel long distances, often found in areas remote from their source. Unep calls them "a global problem requiring a global solution".

Many POPs are in fact no longer produced, but the problem has not gone away because large quantities of chemicals still exist.

Unep says there is a growing number of obsolete and uncontrolled stockpiles of pesticides and toxic chemicals.

"The problem is particularly acute in developing countries. Dump sites and toxic drums from the 1950s, 1960s and 1970s are now decaying, leaching chemicals into the soil and poisoning water resources, wildlife and people."

Three-pronged attack

The negotiators will be trying to agree the terms of a treaty to control three categories of POPs - pesticides, industrial chemicals, and unintended by-products of combustion and industrial processes.

Twelve POPs are on the first list for action, although others will be tackled later.

The 12 are aldrin, chlordane, DDT, dieldrin, dioxin, endrin, furans, heptachlor, hexachlorobenzene, mirex, PCBs (polychlorinated biphenyls), and toxaphene.

The meeting will decide how to reduce emissions, and how to eliminate the use of some chemicals entirely.

Dilemmas for delegates

That may present the negotiators with dilemmas. DDT, for example, has proved an invaluable weapon against malaria, despite its risks.

Malaria now kills three million people annually, and causes 500 million acute clinical cases.

Of those deaths, 5,000 a day are children under five years old - a death rate of four children every minute.

But DDT is known to disrupt sex hormones and other chemical messenger systems in the human body, leading medical experts to oppose the use of the chemical for controlling malaria.

Greenpeace says that POPs are passed from mother to child in the womb, and that they are linked to cancers, learning disabilities, behavioural abnormalities, changes to the immune system and reproductive disorders.

It produced a list of POP "hotspots", including

• Dzerzhinsk, a Russian city, where soil samples showed dioxin contamination 1,880 times above the level permitted in Russia

• Malir, a pesticide stockpile in Pakistan, sited 150m from a school

• Homebush Bay in Sydney, near the site of the 2000 Olympic Games, polluted with dioxins from a Union Carbide factory closed more than 20 years ago