LONDON » Scientists are warning officials negotiating a global treaty on mercury that banning the deadly chemical completely would be dangerous for public health because of the chemical’s use in vaccines.

The ban option is one of several proposals on the table for a meeting later this month in Nairobi, but a final treaty isn’t expected until 2013.

According to the World Health Organization, mercury is one of the top 10 chemicals of public health concern and is highly toxic. Most of the worry is centered on mercury emissions from burning coal, gold mining and people eating mercury-tainted fish.

The problem is that a proposed ban might include thiomersal, a mercury compound used to prevent contamination and extend the shelf life of vaccines. It is used in about 300 million shots worldwide, against diseases including flu, tetanus, measles, diptheria and meningitis.

"Not being able to use mercury is not a viable option," said David Wood, a WHO vaccines expert.

Wood said there isn’t a viable alternative to thiomersal at the moment. If banned, pharmaceuticals would likely have to switch to preservative-free vaccines, which would complicate the supply chain and vaccination campaigns in poor countries, since the injections would have a much shorter shelf life. Costs would also spike since manufacturers would need to reconfigure their factories.

In 2009, the United Nations Environment Programme, or UNEP, began working on a legally binding global treaty on mercury. At the end of October, the third of five meetings to hammer out a treaty will take place in Nairobi.

"The document is a draft at the moment, so some of these proposals have to be taken with a grain of salt," said Tim Kasten, head of the chemicals branch at UNEP. Kasten said the amount of mercury in vaccines is so minute it doesn’t threaten the environment. He said there could be provisions to allow mercury for certain uses, such as in dental fillings and vaccines.

But according to an annex in the draft document, there is currently no "allowable use exemption" for mercury products in pharmaceutical products, putting vaccines in the same category as banned mercury-containing paints and pesticides.

"That would be a terrible idea," said Paul Offit, an infectious diseases expert at the University of Pennsylvania. "It would be another tragic example of us not being able to explain to the public where the real risk lies."

Thiomersal has mostly been removed from childhood vaccines in the U.S. and Canada. In some European countries, including Norway and Sweden, manufacturers have been encouraged to make thiomersal-free vaccines — and no other uses of mercury as a medical preservative are allowed.

Fears about thiomersal in vaccines were first raised after a flawed medical study in 1998 linked a common childhood injection to autism. But numerous studies since have found no sign the mercury compound is risky.

Experts hope countries won’t go overboard in their attempts to control the substance.

"Provided you know the risks and it’s handled properly, there isn’t a problem," said Andrew Nelson, a toxicology expert at the University of Leeds. "The health of so many millions of children benefit from vaccines containing mercury that an absolute ban is ridiculous."