Toxicology
Thiomersal is very toxic by inhalation, ingestion, and in contact with skin (EC hazard symbol T+), with a danger of cumulative effects.
When applied to human nerve cells it changes cell membrane permeability and induces programmed cell death.
It is also very toxic to aquatic organisms and may cause long-term adverse effects in aquatic environments (EC hazard symbol N).
In the body, it is metabolized or degraded to ethylmercury (C2H5Hg+) and thiosalicylate.
Few studies of the toxicity of thiomersal in humans have been performed. Animal experiments suggest that thiomersal rapidly dissociates to release ethylmercury after injection; that the disposition patterns of mercury are similar to those after exposure to equivalent doses of ethylmercury chloride; and that the central nervous system and the kidneys are targets, with lack of motor coordination being a common sign. Similar signs and symptoms have been observed in accidental human poisonings. The mechanisms of toxic action are unknown. Fecal excretion accounts for most of the elimination from the body. Ethylmercury clears from blood with a half-time of about 18 days, and from the brain in about 14 days. Inorganic mercury metabolized from ethylmercury has a much longer clearance, at least 120 days; it appears to be much less toxic than the inorganic mercury produced from mercury vapor, for reasons not yet understood.
Risk assessment for effects on the nervous system have been made by extrapolating from dose-response relationships for methylmercury.[10] Methylmercury and ethylmercury distributes to all body tissues, crossing the blood-brain barrier and the placental barrier, and ethylmercury also moves freely throughout the body.[11] Concerns based on extrapolations from methylmercury caused thiomersal to be removed from U.S. childhood vaccines, starting in 1999. Since then, it has been found that ethylmercury is cleared from the body and the brain significantly faster than methylmercury, so the late-1990s risk assessments turned out to be overly conservative.[10] A 2008 study found that the half-life of blood mercury after vaccination averages 3.7 days for newborns and infants, much shorter than the 44 days for methylmercury.[12]
[edit] Allergies
Thiomersal is used in patch testing for people who have dermatitis, conjunctivitis, and other potentially allergic reactions.
A 2007 study in Norway found that 1.9% of adults had a positive patch test reaction to thiomersal;
a higher prevalence of contact allergy (up to 6.6%) was observed in German populations.
Thiomersal-sensitive individuals can receive intramuscular rather than subcutaneous immunization,
so contact allergy is usually clinically irrelevant.
Thiomersal allergy has decreased in Denmark, probably because of its exclusion from vaccines there.
It was voted Allergen of the Year in 2002 by the American Contact Dermatitis Society.
Autism
There is no convincing evidence that thiomersal is a factor in the onset of autism.[17] Despite this, many parents, and some scientists and doctors, believe there is a connection.[18] Parents may first become aware of autistic symptoms in their child around the time of a routine vaccination, and parental concern about vaccines has led to a decreasing uptake of childhood immunizations and an increasing likelihood of measles outbreaks
More than 5,000 U.S. families have filed claims in a federal vaccine court alleging autism was caused by vaccines, most implicating thiomersal;
the majority of these claims are still being adjudicated
The U.S. federal government agreed to award damages in one case, to a girl with a mitochondrial enzyme deficiency
who developed autistic-like symptoms after receiving a series of vaccines, some of which contained thiomersal.
Many parents view this ruling as confirming that vaccines cause regressive autism;
however, most children with autism do not seem to have mitochondrial disorders, and the case was conceded without proof of causation.