Autism is one of the most severe and disruptive of all childhood disorders. It is a communicative disorder that interferes with an individual's ability to form social relationships, as well as to communicate with others. Autism Spectrum Disorders (ASD's) officially belong to a group of illnesses known as 'developmental disabilities'. They are characterized by problems with social and communication skills of varying degrees (Schechter & Grether, 20). Autistic people also commonly display unusual ways of learning, reacting to different sensations and paying attention. Sufferers also tend to repeat certain behaviors and have difficulties when required to change their usual daily activities. ASD's are commonly said to start in childhood and last for the person's whole life, but as you will see below, there are many new theories as to the cause(s) of these disorders and many offer the hope of effective treatments (Gerber & Offit, 457). Recent reviews in environmental health have suggested that early exposure to hazardous substances may underlie some cases of neurodevelopment disorders, including ADHD, learning disabilities, and speech/language difficulties (Baker, 247). In 1999, Thimerosal used as a vaccine preservative was identi-ed as a widespread source of organic mercury exposure in infants. Mercury (Hg), a heavy metal, is considered highly neurotoxin (Blaxill et al., 791). The amount of mercury in vaccines, while small, exceeded USEPA safety guidelines on a cumulative basis. Certain individuals may exhibit severe adverse reactions to low doses of Hg which are otherwise largely benign to the majority of those exposed. Some individuals with idiopathic autism spectrum disorder may represent such a sensitive population (Nelson & Bauman, 676).

Characteristics of autism that are under study because of the association between autism and the thimerosal are many because of the following reasons: "ASD traits are known to arise from mercury exposure, onset of ASD symptoms is temporally associated with administration of immunizations, the reported increase in the prevalence of autism in the1990s closely follows the introduction of two mercury containing vaccines, and the elevated mercury has been detected in biological samples of autistic patients" (Baker, 247). But what exactly is the role of thimerosal in all this controversy? Thimerosal (49.55% mercury by weight) is a highly toxic mercury compound used as a preservative in some Over The Counter and prescription drugs, including most flu shots given to pregnant women, infants, children, adults, and the elderly (Downey, 925). It has been assumed that Thimerosal breaks down into ethyl mercury in body and expose thiol group. This has high affinity for sulfhydral groups and readily exchanges to bind with available sulfhydral groups. Chemically, Thimerosal has ethyl mercury group that gets covalently bonded to sulfhydral group (Nelson & Bauman, 676).

On April 19, 2007, Dr. Larry L. Needham, Chief, Organic Analytical Toxicology Branch, CDC, announced to the U.S. National Academy of Sciences' Institute of Medicine that Thimerosal was among the "Chemicals Linked to ASD." Thus, Geier and Geier (2007) provide the first clinical case-series of ASD patients that confirmed this causal role for Thimerosal-preserved drugs in patients having a regressive ASD diagnosis. This clinical study also found a significant dose-response relationship between the severity of the ASD symptoms and the total mercury dose these children received from Thimerosal-preserved drugs. Thus, these initially normally developing children suffered mercury toxic encephalopathy's that manifested with clinical symptoms consistent with their regressive ASD diagnosis (Baker, 252). Hence, mercury poisoning should be considered as a cause for those children exhibiting the symptoms of an ASD in any differential diagnosis designed to assess underlying causes. Today, though any parent or other healthcare provider can easily confirm whether, or not, an autistic child is mercury poisoned by having a simple urinary porphyrin profile analysis (UPPA) test run. Much of this information has been made regularly available through the internet (Kortum, 21).

The role and impact of thimerosal and autism has been questioned over and over again about its plausibility as a hypothesis. In 1999, the U.S. Public Health Service and the American Academy of Pediatrics (AAP) called for the reduction or elimination of the ethyl mercury preservative Thimerosal from vaccines, saying that the cumulative amount of mercury in infant vaccines exceeded U.S. Environmental Protection Agency (EPA) guidelines for methyl mercury (Dardennes et al., 1143). Since 2002, Thimerosal-containing vaccines have been largely eliminated for administration to infants less than 6 months of age in the developed world, except for the In-uenza and diphtheria -- tetanus vaccines in the U.S. And the routinely recommended diphtheria -- tetanus -- peretanus -- pertussis vaccine in the UK (Downey et al., 926). Clinical manifestations of mercury toxicity vary greatly depending...

Only two well documented mercury exposure patterns lie close to the developmental window proposed in the autism -- mercury hypothesis: congenital Minamata disease (CMD) and Pink disease or acrodynia. Each of these disorders involves unique mercury exposure patterns. CMD results from fetal exposures via cross-placental transfer of relatively high doses of methyl mercury ingested by the mother through contaminated ?sh. Acrodynia results from direct trans-dermal and mucosal exposures in infants and small children (often via teething powders) to inorganic mercury, speci-cally, mercurous chloride in calomel (Nelson & Bauman, 677). The "typical and characteristic manifestations" subsequent to the known mercury exposures in CMD and acrodynia bear little resemblance to the vague manifestations of "mercurism" that Nelson and Bauman describe.
Emerging evidence supports a ?nding of elevated mercury exposure and unusual mercury metabolism in autistic children. Reliable evidence of increased mercury exposure will not be found. They are premature in their assessment (Baker, 252). One could only relate studies similar to the levels of mercury found in vaccines to the hypothesis of it having a causal relationship with autism (Gerber & Offit, 458). An unpublished study did just this by observing young children with Higher levels of exposure to Thimerosal-containing vaccines and compared it to another published study using the Vaccine Adverse Events Reporting System (VAERS) database (Downey et al., 927). The VSD study found a relative risk of autism of 2.48 in infants receiving 62.5 mcg or more of ethyl mercury by three months of age. This also correlates with increased levels of prenatal exposure to mercury in autistic children that have been found, resulting from both higher numbers of maternal amalgam-llings and higher probability of receiving Thimerosal-containing Rho D. immunoglobulin injections (Gerber & Offit, 458). Any such prenatal mercury exposures occur against a background of elevated mercury blood levels in women of child-bearing age, with over 8% of women in a recent study showing blood mercury readings in excess of the EPA's allowable levels. Lower levels of mercury have also been found in the first baby haircuts of autistic children as compared to controls suggesting reduced excretion rates, since the autistic group had elevated mercury exposures as compared to controls. High levels of mercury have been detected in the urine of autistic children following chelation therapy with DMSA (Al Anbar at al, 820).

A review of medical literature has shown that exposure to mercury, whether organic or inorganic, can give rise to the symptoms and traits de-ning or commonly found in ASD individuals (Jordan, 223). Mercury can cause impairments in social interaction, communication difficulties, and repetitive and stereotyped patterns of behavior, which comprise the three DSM-IV autism diagnostic criteria. Additionally, mercury can induce features prominent in ASD such as sensory abnormalities, emotional/psychological changes, movement disorder, impairments in abstract or complex thinking, severe sleep disturbances, and self-injurious behavior. Males are more affected than females in both conditions (Dardennes et al., 1141). Physiological abnormalities are more common in ASD populations and known to be caused by mercury exposure include gastrointestinal problems, autonomic nervous system disturbance, unusual EEG activity, immune system alterations, and irregularities in neurotransmitter systems, and non-speci-c brain lesions.

Autism was ?rst described in 1943 among children born in the 1930s. Thimerosal was ?rst added to childhood vaccines in the 1930s prior to 1970. Classic autism was estimated to occur in approximately 1 in 2000 children, while the average prevalence reported by studies from 1970 to 1990 is 1 in 1000 (Schechter & Grether, 21). This period was a time of increased immunization in the developed world. By 1995, the National Institutes of Health reported an autism prevalence of 1 in 500 children, and in 2000 the CDC identi-ed approximately 1 in 250 children with classic autism in one New Jersey town (Schechter & Grether, 21). It was in the early 1990s that the Thimerosal-containing HiB and Hepatitis B vaccines became part of the…