What is thimerosal? 
Thimerosal is an additive that has been used in biologics and vaccines to kill bacteria used in certain vaccines and prevent bacterial contamination in certain containers. Thimerosal contains four main ingredients, ethyl mercuric chloride, thiosalicylic acid, sodium hydroxide and ethanol. Ethyl mercuric chloride is essentially an organic mercury. Some of the vaccines that contain thimerosal include Hepatitis B, diphtheria, pertussis, acellular pertussis, tetanus and Hib vaccines

Why is it potentially dangerous? 
Thimerosal contains some mercury, a toxic element that, if taken in high enough doses, can result in long-term immune, sensory, neurological, motor, and behavioral dysfunctions. Health-related organizations such the National Vaccine Program fear that infants who received up to 15 mercury-containing vaccines by the time they six months old may be exposing themselves to dangerous amounts.

Among the disorders feared may result in are autism, attention deficit disorder, and speech and language deficiencies. One of the biggest concerns for parents and health officials is the link between thimerosal-containing vaccines and autism.

What is autism?
Autism is a neurological disorder that typically appears within the first two years of a child's life. The disorder is marked by problems with the sensory, neurological, motor, and behavioral functions of an individual. Autism is the third most common child development disorders in the United States.

What is mercury?
Mercury is a naturally occurring element that occurs in three forms---inorganic salts, as a metallic element, and organic compounds. Thimerosal contains the organic compound form of mercury. The level of toxicity from mercury exposure is dependent on many variables, including the type of mercury involved, how much of the substance enters the body, how it enters the body, and how old the individual is when they are exposed.

The most common ways that we come into contact with mercury is through consumption of certain fish that we eat, as well as mercury contained in silver dental fillings (silver fillings are being used less and less).

Why is mercury dangerous? 
Organic mercury is dangerous if too much of it enters our bodies either when they we ingest it orally or through the skin. Mercury toxicity affects the central nervous system, as well as the kidneys and the immune system. The mercury found in thimerosal (methyl mercury), goes to all the body tissues but seems to concentrate in the blood and in the brain.

Fetuses, infants, and children under the age of six months are much more susceptible to the toxic effects of mercury exposure. The reason being that their brains are developing rapidly and can develop serious developmental and neurologic problems with enough exposure to mercury.

What is a vaccine?
A vaccine is an injection of a weakened or killed pathogen, such as a bacterium or virus, or a portion of the pathogen's structure that, once inside the body, stimulates the creation of antibodies or cellular immunity against the particular pathogen. Once the body has the antibodies for an infectious disease, the person is essentially immune to the disease. Someone can get this immunity through an active or passive "route." A vaccine is not able to cause severe infection.

Can vaccines cause side effects? 
Vaccines can cause side effects in a small number of people and especially to infants. Side effects can range from skin redness and warmth (caused by the injection) to more serious effects, including damage of the central nervous system, or death. However, the possibility of such a serious event is extremely small. The risk of contracting a vaccine-preventable disease is far greater than dying from an immunization.

Are other substances added to vaccines? 
Vaccines contain certain chemicals that stabilize the vaccine and inactivate viruses or bacteria. They also help to add how potent the vaccine is. Chemical additives are, for the most part, very slight. Below are some of the more common substances added to vaccines.