Thiomersal (also known as thimerosal) is a mercury based preservative used in some vaccines and other pharmaceutical products such as antiseptics and contact lens solution. It is used to prevent bacterial and fungal growth. It has also been used during vaccine production both to inactivate certain organisms and toxins and to maintain a sterile production line.

Mercury is a naturally occurring element found in the earth’s crust, soil, water and the air. Mercury is released into the environment by volcanic eruptions, weathering of rocks and the burning of coal. Once released mercury can find its way through the food chain via fish and other animals. At high levels it can be toxic. Mercury can have harmful effects on the central nervous system, skin and kidneys, cases of the toxic effects of mercury have been from methylmercury ( more toxic ), not ethylmercury ( less toxic ).

Not all mercury is created equally and some forms pose a greater health risk than others. For example mercury vapour is extremely dangerous whereas amalgam, used in dental fillings, has not been shown to pose a health risk.

Mercury becomes harmful only after it reaches a certain level in the body. The toxicity depends on the amount of mercury consumed in relation to body weight and can accumulate over a period of time. Infants are therefore at greater risk than adults because they are smaller.

When thiomersal-containing vaccines are in the childhood vaccination schedule, the maximum number of doses of thiomersal-containing vaccines a 6 month old child might have received was as follows:

• 3 doses of diphtheria-tetanus-pertussis vaccine,
• 3 doses of hepatitis B vaccine, and
• 3 doses of Hib vaccine.

This would have resulted in a total intake of 175 µg of ethylmercury, which is equivalent to about 1.9 µg/kg body weight per week, for an average-sized baby. This level is well below the World Health Organization (WHO) limit for methylmercury. Studies measuring mercury levels in the blood of infants given thiomersal-containing vaccines have indicated that their blood concentrations of mercury did not rise above designated levels, except possibly transiently in a premature infant less than 1kg in weight.