Delegations from some 140 countries agreed Saturday to adopt a ground-breaking treaty limiting the use and emission of health-hazardous mercury, the UN said, although environmental activists lamented it did not go far enough.

The world's first legally binding treaty on mercury, reached after a week of thorny talks, will aim to reduce global emission levels of the toxic heavy metal also known as quicksilver, which poses risks to human health and the environment.

"This was a herculean task ... but we have succeeded," Achim Steiner, UN under-secretary general and head of the UN environment programme (UNEP), told reporters in Geneva.

The treaty has been named the Minamata Convention on Mercury, in honour of the Japanese town where inhabitants for decades have suffered the consequences of serious mercury contamination.

The text will be signed in Minamata in October and will take effect once it has been ratified by 50 countries -- something organisers expect will take three to four years.

Large amounts of the heavy metal are released from small-scale gold mining, coal-burning power plants, metal smelters and cement production.

"It is quite remarkable how much mercury in a sense has entered into use in our lives.... We've been creating a terrible legacy," Steiner said.

"Mercury accumulates in the food chain through fish... It is released through coal fired power stations and it travels sometimes thousands of kilometres. It affects the Inuit in Canada just as it affects the small-scale artisanal gold miner somewhere in southern Africa," he said.

Serious mercury poisoning affects the body's immune system and development of the brain and nervous system, posing the greatest risk to foetuses and infants.

The treaty sets a phase out date of 2020 for a long line of products, including mercury thermometers, blood pressure measuring devices, most batteries, switches, some kinds of fluorescent lamps and soaps and cosmetics.

It however provides exceptions for some large medical measuring devices where no mercury-free alternatives exist yet.

In a controversial move, it also excluded vaccines that use mercury as a preservative, since the risk from these vaccines is considered low and for many developing nations removing them would entail losing access to vaccines altogether, Tim Kasten, head of UNEP's chemicals division explained.

Amid pressure from dentist groups, the treaty also did not provide a cut-off date for the use of dental fillings using mercury, but did agree that the product should be phased down.

Non-governmental groups at the talks meanwhile lamented that the treaty fell short in addressing the greatest sources of mercury in the environment: small-scale gold mining, which directly threatens the health of the some 10-15 million people working in this field and contaminates water and air, and emissions from coal-buring power plants.

"We're disappointed," Joe DiGangi, a senior advisor with an environmental umbrella group called IPEN, told AFP, saying that "the two biggest sources of mercury have only weak controls on them."

For coal-fired power plants, the treaty calls only for control and reduction of mercury emissions "where feasible", which is "vague and very discretional," he said.

As for small gold mining activities, using mercury will still be allowed, meaning imports and exports of the metal for this process will be legal, and governments will only be required to control the activity if they deem it "more than insignificant -- whatever that means," DiGangi said.

UNEP's Steiner acknowledged the criticism but stressed that the treaty "is a dynamic instrument," insisting it would evolve over time to address all the areas of concern.

Switzerland and Norway, which initiated the process a decade ago, had along with Japan pledged an initial $3.0 million to get things started.

Once up and running the treaty will provide funds to help transition away from mercury-linked products and processes through the UN's existing Global Environment Facility (GEF), and probably also a second mechanism, organisers said.