The dangers of mercury poisoning came to the fore in the 1960s and 1970s when at least 2,265 people were poisoned in Japan after eating fish from Minamata Bay where a chemical maker had been dumping mercury since the 1930s.

Although there have been no more instances of mass poisoning, problems persist.

Chronic, low-level exposure to mercury is known to cause permanent damage to the brain, nervous system and kidneys. Pregnant mothers and unborn children are particularly at risk.

"Mercury never breaks down in the environment. It is a basic element and it will always persist," said Jim Willis, UNEP chemicals director.

"Slowly but surely we have come to have a potentially significant global problem associated with mercury."

Mercury is a heavy metal that has been widely used in thermometers, dental fillings, fluorescent lamps and other electrical equipment. Once released into the environment, it can travel thousands of miles (kilometers), contaminating soil, water systems and the atmosphere, gathering in greater concentrations in cold areas such as the Arctic, Antarctic and mountain ranges.

Although it is released naturally from rocks, soil and volcanoes, up to three-quarters of emissions are believed to come from human activities, primarily burning coal in power stations, factories and homes.

Broken thermometers that are thrown out with household trash and some forms of gold mining also lead to mercury pollution.

Most people are exposed through eating fish, leading some governments to issue guidelines on the safe level of fish consumption.

However workers in industries that use mercury and their families are at increased risk. The substance also is used in certain rituals, such as by followers of the Santeria religion who sprinkle mercury around their homes.

Many governments already have regulations to control mercury emissions, and several have succeeded in reducing emissions by as much as 75 percent in the past 10-20 years.

The experts' report will be considered by environment ministers at a meeting of UNEP's Governing Council next February in Nairobi, Kenya.

That could lead to the start of formal negotiations on an international treaty, though not all governments are in favor of such moves.

"All governments are concerned about mercury and want the right policy response, but some of them may be concerned about whether the treaty option is premature," Willis said.