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 Organisation, mercury is one of the top 10 chemicals of public health concern and is highly toxic. Most of the worry is centred 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. A WHO vaccines expert says not being able to use mercury “is not a viable option.” He 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.

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