Nine dangerous chemicals used in farming and industry will be added to a list of banned substances whose presence in the environment causes serious health risks, more than 160 governments have agreed.

The nine pesticides and industrial chemicals join 12 substances targeted for elimination under the 2001 Stockholm Convention on Persistent Organic Pollutants (POPs).

Trade in some of the chemicals can amount to billions of dollars a year, but countries at the United Nations conference agreed they are so dangerous that alternatives must be found.

'Just five years after this convention came into force, we will have nine new chemicals added to the list of those that the world community agrees we need to control and ultimately get rid of,' said Achim Steiner, executive director of the UN Environment Programme (UNEP), which hosted the conference.

Donald Cooper, executive secretary of the Stockholm Convention, set out why the banned substances were exceptionally dangerous: They cross boundaries and are found everywhere, from the Tropics to polar regions; they persist for long periods in the atmosphere, soil and water, and take years to degrade; they accumulate in bodies; they accumulate in food chains.

The chemicals can damage reproduction, mental capacity and growth and cause cancer, he said.

'In most cases the question is not simply how do we control them, but how we eliminate them,' he told a news briefing.

The conference had been scheduled to wrap up on Friday, but officials wrangled into Saturday over the details.

Cooper said governments at various stages of economic development differed about how fast they should be phased out, especially when there are no alternatives.

One of the newly proscribed chemicals is a pesticide called Lindane. It has been replaced in agriculture, but in some countries it is still used to tackle head lice and so will be phased out over five years instead of the standard one year.

Perfluorooctane sulfonic acid (PFOS) appears in a wide range of products from electronics components to fire-fighting foam, and trade in it amounts to billions of dollars a year. With no alternatives to some of its applications, it will be restricted rather than eliminated immediately.

Steiner said the challenge was to resolve two conflicting objectives - harnessing the power of science and the chemicals industry and dealing with its negative impact. Sometimes the conflict is not obvious.

The inventor of the pesticide DDT won a Nobel Prize for Medicine in 1948 because of its efficacy in killing mosquitoes that transmit malaria.

DDT has since been found to be toxic and is banned as a pesticide for crops as one of the original 'dirty dozen' under the Stockholm Convention - but it is still widely used to fight malaria.

UNEP and the World Health Organisation have announced a plan to rid the world of DDT by 2010 by developing environmentally friendly ways to fight malaria.

In some cases banning the chemicals will have little economic impact as they are no longer used, and the question is of managing and disposing of them.

But even where they are still widely traded, the bans will create business opportunities in the search for alternatives.

'This is not an economic disruption, it is rather an investment in public health with perfectly compatible economic opportunities, but we have to give the right incentives,' he said.

The meeting also agreed to coordinate its work more closely with two other environment conventions - the Basel Convention on transporting hazardous waste and the Rotterdam Convention on trade in hazardous chemicals.

A conference of all three will be held in February 2010.-Reuters