IDENTIFICATION AND USE: Thiomersal (INN), or thimerosal (USP), is an organomercury compound. This compound is a well-established antiseptic and antifungal agent.

The pharmaceutical corporation Eli Lilly and Company gave thiomersal the trade name Merthiolate. It has been used as a preservative in vaccines, immunoglobulin preparations, skin test antigens, antivenins, ophthalmic and nasal products, and tattoo inks.

ANIMAL EXPOSURE/TOXICITY: researchers assigned 41 newborn monkeys to one of three exposure groups. Seventeen of the monkeys were injected with vaccines spiked with thimerosal for a total mercury dose of 20 micrograms per kilogram (μg/kg) at ages 0, 7, 14, and 21 days, mimicking the typical schedule of vaccines for human infants. At the same ages, another 17 monkeys received 20 μg/kg methylmercury by stomach tube to mimic typical methylmercury exposure. A third group of 7 monkeys served as unexposed controls.

The researchers drew blood from all monkeys prior to any exposure and at other points prior to sacrifice, which occurred 2, 4, 7, or 28 days after the last dosing on day 21. Total mercury concentrations were measured in blood samples, and total and inorganic mercury concentrations were measured in brain samples. Organic mercury concentrations were calculated from those values.

The initial absorption rate and tissue distribution of mercury was similar in both exposed groups. However, total mercury progressively accumulated in the blood of methylmercury-exposed monkeys and remained detectable 28 days after the last dose. Among thimerosal-exposed monkeys, total mercury in blood declined rapidly between doses, and the researchers estimated clearance to be 5.4-fold higher than in the methylmercury group. In the thimerosal group, the half-life of total mercury in blood was 6.9 days, compared to 19.1 days for the methylmercury group.

Brain concentrations of total mercury were approximately 3–4 times lower in the thimerosal group than in the methylmercury group, and total mercury cleared more rapidly in the thimerosal group (with a half-life of 24.2 days versus 59.5 days). However, the proportion of inorganic mercury in the brain was much higher in the thimerosal group (21–86% of total mercury) compared to the methylmercury group (6–10%). Brain concentrations of inorganic mercury were approximately twice as high in the thimerosal group compared to the methylmercury group. Inorganic mercury remains in the brain much longer than organic mercury, with an estimated half-life of more than a year. It’s not currently known whether inorganic mercury presents any risk to the developing brain.

Given these findings, the researchers caution that risk assessments for thimerosal based on studies using blood mercury measurements may not be valid, depending on the design of the study. Further, the observed differences in distribution and breakdown of mercury compounds between exposed groups indicate that methylmercury is not a suitable model for thimerosal toxicity.

The researchers emphasize, however, that the risks associated with low-level exposures to inorganic mercury in the developing brain are unknown, and they describe other research linking persistent inorganic mercury exposure with increased activation of microglia in the brain, an effect recently reported in children with autism. They recommend further research focused specifically on the biotransformation of thimerosal and its neurotoxic potential.

HUMAN EXPOSURE/TOXCITY: delayed-type hypersensitivity reactions from thimerosal exposure are well-recognized. Identified acute toxicity from inadvertent high-dose exposure to thimerosal includes neurotoxicity and nephrotoxicity. Limited data on toxicity from low-dose exposures to ethylmercury are available, but toxicity may be similar to that of methylmercury. Chronic, low-dose methylmercury exposure may cause subtle neurologic abnormalities.

Depending on the immunization schedule, vaccine formulation, and infant weight, cumulative exposure of infants to mercury from thimerosal during the first 6 months of life may exceed EPA guidelines. Dr. Hooker, a PhD scientist, worked with two members of Congress to craft the letter to the CDC that recently resulted in his obtaining long-awaited data from the CDC, the significance of which is historic. According to Hooker, the data on over 400,000 infants born between 1991 and 1997, which was analyzed by CDC epidemiologist Thomas Verstraeten, MD, “proves unequivocally that in 2000, CDC officials were informed internally of the very high risk of autism, non-organic sleep disorder and speech disorder associated with Thimerosal exposure.

Mercury has no positive role in the human body; in fact a safe level of mercury exposure is very difficult to determine. It can be present in the environment in several different forms, and while all forms of mercury are toxic to humans, the pattern of toxicity varies with its chemical form, the route of exposure, the amount, the duration and timing of exposure, and the vulnerability of the person exposed. For example, pure elemental mercury (also known as quicksil-ver or Hg) is liquid at room temperature.

If ingested, quick-silver has very low toxicity because it is not absorbed by the gastrointestinal tract and is eliminated completely in the stool. If quicksilver is agitated or heated, however, the liquid mercury becomes a vapor which is readily absorbed by inhalation and is highly toxic to the lungs and central nervous system. The nervous system is the primary target of mercury toxicity, but, depending upon the specific exposure, the kidneys, liver and lungs are also important targets.

High doses of mercury can be fatal to humans, but even relatively low doses of mercury containing compounds can have serious adverse impacts on the developing nervous system, and have recently been linked with possible harmful effects on the cardiovascular, immune and reproductive systems. Mercury and its compounds affect the central nervous system, kidneys, and liver and can disturb immune processes; cause tremors, impaired vision and hearing, paralysis, insomnia and emotional instability.

During pregnancy, mercury compounds cross the placental barrier and can interfere with the development of the fetus, and cause attention deficit and developmental delays during childhood. Fetuses and young children are actively developing and therefore most at risk from health effects including neurological damage, resulting in behavioral problems and learning disabilities.

ADVERSE EFFECTS: neurological effects: Low doses of methyl mercury in pregnant women have been shown to have impacts on the fetus. In a major review of mercury health studies the US National Academy of Sciences stated: ‘Chronic, low-dose prenatal methylmercury exposure from maternal consumption of fish has been associated with …poor performance on neurobehavioral tests, particularly on tests of attention, fine-motor function, language, visual-spatial abilities (e.g. drawing) and verbal memory.

Cardiovascular effects: Two recent epidemiological studies found associations between exposure to low levels of methylmercury and adverse cardiovascular effects. The US National Academy of Sciences concludes that additional studies are needed to better characterize the effect of methyl mercury exposure on blood pressure and cardiovascular function at various stages of life. The European Commission also notes recent evidence suggesting that mercury from fish and seafood may promote or predispose the development of heart disease.