(CNN) Thimerosal is best known as a preservative used in some vaccines to keep them from becoming contaminated.
The preservative has gotten a lot of attention over the years, particularly since it was removed from childhood vaccines in 2001.
Several studies have shown that the preservative is safe, but not everyone has been convinced. Here's what you need to know about what it is (and what it isn't).
It is the most widely used preservative in vials of vaccines used multiple times, a mercury-based organic compound that can prevent bacteria and fungus from growing.
The antiseptic is also used in some drug products.
Vaccine makers started using preservatives in the 1930s after they found that contamination could become a problem with multi-dose vaccines. Doctors learned that the hard way in 1928, when 12 children died after getting vaccinated for diphtheria.
An investigation found that the multi-dose vaccine had been contaminated with living staphylococci. The children had been injected with the diphtheria vaccine and a staph infection.
Scientific committees recommended that vaccine makers use preservatives, and some did. It wasn't until 1968 that the United States started requiring manufacturers to use them in most multi-use vaccines.
Your body easily eliminates the thimerosal. Unlike chemicals that might stay in your body for a long time, it is quickly removed from the blood and excreted in your waste.
Thimerosal does not build up in your system like other mercury-based compounds can.
Hundreds of studies have shown that it is extremely safe for humans. Several comprehensive reviews have shown there is no evidence of harm caused by low doses.
In animals, some studies have shown central nervous problems, coma and death, although the same has not been found in humans.
The most common side effect is a mild rash or redness at the injection site. There may also be a little swelling. All of these symptoms disappear quickly.
On rare occasions, some people have had allergic reactions to the preservative.
All routinely recommended vaccines for children in the US are available in a thimerosal-free formulation or contain only a trace amount.
Thimerosal was taken out of most of the vaccines young children get in 2001, with the exception of the flu vaccine, which contains small amounts. You can ask your doctor for a flu shot that is thimerosal-free.
Some vaccines -- including measles, mumps and rubella; chicken pox; polio; and pneumonia -- were never made with thimerosal.
Thimerosal is an ethylmercury; the mercury that can be found in fish is a methylmercury. Though they are only one letter apart, the substances are different.
Mercury is an element found in the Earth's crust. We are all exposed to mercury as we live on the surface of the planet. It's in our water, in our soil and in the air.
Methylmercury is created when mercury comes into contact with some bacteria. Doctors warn pregnant women to avoid some fish because it can be contaminated with this form.
Unlike ethylmercury, which can pass through your body quickly, methylmercury can linger and accumulate. If enough accumulates, it can be toxic.
In 1999, the National Vaccine Advisory Committee held a meeting to discuss the safety of thimerosal. Some who testified worried that babies were unable to eliminate the mercury from their systems. Followup studies of infants showed that they "excreted significant amounts of mercury in stool after thimerosal exposure," meaning it was removed from the body quickly, compared with methylmercury.
Based on the recommended childhood vaccination schedule, there was concern that some babies could be exposed to a higher cumulative level of mercury in the first six months of life from these shots.
At the time, babies got diphtheria-tetanus-acellular pertussis, Haemophilus influenza type b and hepatitis B shots. If an infant got all three, the accumulation went beyond the EPA guidelines for methylmercury.
There are no existing guidelines on ethylmercury exposure.
Out of an abundance of caution, the public health branches of the US government and the American Academy of Pediatrics released joint statements that encouraged manufacturers to reduce or even eliminate the preservative as quickly as possible.
Scientists have studied the chemical closely, and there is no scientific evidence to support a connection between the use of thimerosal and autism or any other serious disorder or disease.
A study of autism rates before thimerosal and after it was removed from most vaccines found that autism rates went up after the preservative was removed.
"Something that has the word 'mercury' in it is never going to sound good to some people," said Dr. Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia.
Offit said he gets calls and emails on a daily basis from worried parents who are referred to him by their doctors. They tend to be worried about vaccine safety in general, but some questions also arise about thimerosal.
"Even trace quantities puts people off, and people talk about a zero tolerance toward any amount of mercury, but it can't apply here," Offit said. "You are exposed to low levels of mercury all this time. You'll be exposed to 10 times more mercury in the water you drink than what you will get in a vaccine."