When mercury is belched into the air from power plants, smelters and incinerators, it returns to earth via rain, with bacteria and other natural processes converting it to organic methylmercury in lakes, rivers and oceans. Released into river systems that flow into the sea, the poison is widely dispersed through the world's oceans.

Scientists say that, as global warming melts the Arctic, mercury trapped in ice and sediments is being released back into the oceans. It builds up through the food chain as bigger creatures consume smaller ones, with the flesh of the biggest fish - among them tuna, swordfish, king mackerel and sharks - containing the highest mercury concentrations.

In the US, just under 5 million women, or one in 12 of the female population, have mercury in their bodies above the level considered safe by the US Environmental Protection Agency. The US and other governments have advised pregnant women, children and other groups considered vulnerable to mercury poisoning to limit fish consumption.

To tackle the mercury scourge, more than 140 nations meeting at the UNEP headquarters in Nairobi agreed last month to negotiate a binding global treaty to tighten controls on its use. The accord, backed by China, India and many Asian countries, came after the Obama administration announced it had reversed the US stance on the issue and was now in favour of a ban on mercury. The European Union has called for the ban to start by 2011. However, reaching agreement on the details of the treaty and then implementing it effectively will be difficult and might take years. It will involve finding cost-effective substitutes for mercury in products like thermometers, high-intensity lamps and liquid crystal displays, and in processes such as paper making and plastics production.

It will also involve tighter controls on the mining, storage, export and import of mercury. Most mercury mines are now closed and China only supplies its own market. So mercury comes from the leftover stockpiles of shuttered mines or the dozens of companies in Europe and the US that recycle the metal from old light bulbs, batteries or industrial waste, according to the UN and the Zero Mercury Working Group, a coalition of 40 organisations that campaigns to reduce mercury use.

International trade in mercury is poorly regulated. Mercury supplied for legal purposes is often diverted to gold mining, where it fetches a much higher price. In Indonesia, for example, use of mercury in gold mining is illegal. However, the price of gold has more than tripled since 2001 and mercury is the easiest way to separate gold from ore. Tens of thousands of remote mining sites have mushroomed, mostly in Asia, Latin America and Africa as the poor try to make money from the gold boom. Small-scale gold mining uses as much as 1,000 tonnes of mercury each year.

The UNEP estimates that 10 million miners and their families in countries such as Indonesia, India, Papua New Guinea, Brazil and Zimbabwe may be suffering from mercury poisoning or exposure. It says that on Mindanao, the main island in the southern Philippines, 70 per cent of gold miners may have chronic mercury intoxication.

A transition to a low-mercury world is a laudable goal. But it will require a high degree of international co-operation and government action.

While the terms of a new mercury treaty are negotiated, steps will be taken to raise awareness of the risks in using the poison in small-scale mining. The UN has already spent US$7 million in six developing countries to educate miners and local suppliers about mercury.

However, the miners like the liquid metal because it is relatively cheap, works fast and leaves the gold cleaner than traditional panning. The only readily available alternative is cyanide - also a poison that can ravage human health and the environment.