Banning mercury -- a toxic chemical commonly found in fish, light bulbs, and thermometers -- could impede the production of health vaccines, scientists told officials negotiating a global treaty on the toxin.

The proposed ban is scheduled to be discussed by the United Nations Environment Program (UNEP) at a meeting later this month in Nairobi, although a final treaty isn’t expected until 2013.

According to the World Health Organization (WHO), products with small amounts of mercury such as light bulbs, batteries, and thermometers should be phased out of use.

But it’s the use of thiomersal, a mercury compound used to extend the shelf life of vaccines that has riled up so many scientists. The compound is currently used in some 300 million vaccines worldwide against diseases such as the flu, tetanus, hepatitis B, diphtheria, and meningitis.

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

Fears about thiomersal in vaccines were raised after a medical study in 1998 that linked a common childhood injection to autism. Since then, however, numerous studies have found no sign the compound is risky. Still, thiomersal has mostly been removed from childhood vaccines in the U.S. and Canada.

Creating exemptions for mercury in dental fillings and vaccines may be one way to get around the concerns, says Tim Kasten, head of the chemicals branch at UNEP, although no such exemption exists in the draft proposal now.

"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."