In 1971, Iraq imported grain that had been treated with methylmercury to clean it for use. Farmers who ate bread made from this grain suffered from mercury poisoning. Pregnant women also ate the bread, and some of their babies suffered epilepsy and mental delays. It was one of the worst, single-source, mercury poisonings in history. About 6,500 people were hospitalized and about 450 died. As this episode demonstrated, mercury can be harmful. It is because of this understanding of the dangers of mercury that concerns arose about use of a mercury-containing chemical, called thimerosal, in some vaccines. 

Thimerosal contains ethylmercury, not methylmercury, which is an important distinction. But people still wondered why thimerosal was in vaccines at all. Thimerosal was used as a preservative to prevent contamination of vaccines sold in multi-dose vials. In the late 1990s, thimerosal was removed from routinely recommended childhood vaccines used in the U.S.

Thimerosal in childhood vaccines has never been shown to cause harm. Keep reading to find out more.

About mercury

Mercury is a naturally occurring element found in the earth's crust, air, soil and water. Mercury is released into the environment from volcanic eruptions, weathering of rocks and burning of coal. 

Elemental, or metallic, mercury can be found in several products, including thermometers, novelty jewelry, and dental fillings. When this type of mercury is vaporized, it becomes more toxic.

Another form of mercury that people can be exposed to is methylmercury. Methylmercury forms when bacteria use inorganic mercury in the environment. Eating fish exposes people to methylmercury. Fish, and some aquatic plants, accumulate methylmercury. Larger fish that eat other fish tend to have greater quantities. In large quantities, methylmercury can be toxic.

Ethylmercury is another form of mercury. This is the form contained in thimerosal. Thimerosal is used as a preservative in vaccines.

Because the words methylmercury and ethylmercury vary by only a single letter, it may be difficult to believe they could be very different chemicals. But they are. A similar example is methanol and ethanol (also known as methyl alcohol and ethyl alcohol). Methanol is added to some types of gasoline and antifreeze. It is toxic if consumed. On the other hand, ethanol is what we drink at a party. 

Thimerosal, a mercury-containing chemical used in vaccines

Thimerosal contains ethylmercury. Studies comparing ethylmercury and methylmercury suggest that they are processed differently in the human body: 

• Ethylmercury is broken down and excreted more quickly than methylmercury. 
• Because of its fast processing, ethylmercury is much less likely to accumulate in the body and cause harm.

Learn more about thimerosal by watching this short video, part of the Talking about Vaccines with Dr. Paul Offit video series.

Health effects of mercury

Mercury can harm the nervous system, digestive tract and immune system. It can also damage organs and tissues, like the kidneys, lungs, skin and eyes. 

When pregnant women are exposed to mercury, it can affect development of the unborn baby. Young children are also at greater risk of damage from exposure to mercury.

Symptoms from neurological damage can include:

• Tremors
• Inability to sleep
• Memory loss
• Issues with communication between muscles and nerves
• Headaches
• Cognitive delays

Kidney damage can result in a range of issues, including high protein levels in urine and kidney failure.

Because of the known health effects of mercury exposure, particularly in young infants, government agencies have set exposure limits for babies up to 6 months of age as shown in the table below. These limits were informed by what is known from exposure to methylmercury in diet. However, as described, ethylmercury (the type contained in vaccines) breaks down more rapidly than methylmercury.

Cumulative exposure limits for infants < 6 months of age based on percentile of body weight

Table originally published in Offit PA and Jew RK. “Addressing Parents’ Concerns: Do Vaccines Contain Harmful Preservatives, Adjuvants, Additives, or Residuals.” Pediatrics. 2003, Dec. 6; 112(6): 1394-1401.

* Note: 

• 1 milligram = one-thousandth (1/1,000) of a gram 
• 1 microgram = one-millionth (1/1,000,000) of a gram
• 1 gram = weight of one-fifth of a teaspoon of water or about one raisin 
• See more about these quantities on the “Types of Vaccine Ingredients” page.

Thimerosal in vaccines and health

Some people have worried that the thimerosal in vaccines could cause mercury poisoning either in the unborn children of pregnant women who receive thimerosal-containing influenza vaccine or in children who receive thimerosal-containing vaccines. But this possibility has been studied, and none of the studies have suggested reason for concern. The thimerosal contained in vaccines was not harmful.

Even though the data did not cause alarm, thimerosal was removed from childhood vaccines in the U.S. during the late 1990s.

Removal of thimerosal from vaccines

An amendment to the Food and Drug Administration (FDA) Modernization Act was signed into law on Nov. 21, 1997. The amendment gave the FDA two years to "compile a list of drugs and foods that contain intentionally introduced mercury compounds and ... [to] provide a quantitative and qualitative analysis of the mercury compounds in the list..." 

At the time the FDA Modernization Act was passed, infants were recommended to receive three different vaccines that contained thimerosal: 

• Diphtheria-tetanus-acellular pertussis (DTaP) 
• Hepatitis B 
• Haemophilus influenzae type b (Hib) 

Infants receiving all these vaccines could have been exposed to a cumulative dose of mercury as high as 187.5 micrograms (ug) by 6 months of age. The cumulative dose exceeded guidelines recommended by the Environmental Protection Agency (EPA) (see table above). 

Routinely recommended childhood vaccines in the U.S. have not contained thimerosal since the late 1990s. By 2025, when the Secretary of Health and Human Services required removal of thimerosal from vaccines, only multi-dose vials of influenza vaccine used in the U.S. contained thimerosal. However, in other countries, cost or convenience often dictate use of multi-dose vials. In multi-dose vials, use of a preservative, like thimerosal, remains important, particularly given that thimerosal previously used in vaccines has never been shown to be unsafe.

Thimerosal in vaccines and autism

Because mercury can cause neurological harm, some people worried that the thimerosal in vaccines was contributing to increases in autism. However, a vast amount of evidence has shown that thimerosal-containing vaccines don't cause autism:

• A study found that more children in Sweden and Denmark were diagnosed with autism after removal of thimerosal from vaccines in those countries. The study was published in the American Journal of Preventive Medicine in 2003 by Paul Stehr-Green.
• A review of the medical records of 1,000 autistic children diagnosed between 1971 and 2000 found that despite the removal of thimerosal from vaccines, the incidence of autism diagnoses increased between 1992 and 2000. The study was published by Kreesten Madsen in Pediatrics in 2003.
• A study of Danish children between 1990 and 1996 found that the number of children with autism increased after thimerosal was removed from vaccines. This study was published by Anders Hviid in 2003 in the Journal of the American Medical Association.
• A study of more than 14,000 children in the United Kingdom found no evidence of increased autism relative to exposure to thimerosal. The study by Jon Heron was published in 2004 in the journal Pediatrics.
• Another study of the records of more than 100,000 children who received different amounts of thimerosal did not find evidence that exposure to thimerosal led to neurodevelopmental disorders. This study by Nick Andrews was published in the same issue of Pediatrics as the Heron study.
• A review of more than 200 studies related to thimerosal and autism concluded that there was no relationship to cause concern. This work was completed in 2004 by the Institute of Medicine (IOM).
• A study of 28,000 children born in Canada between 1987 and 1998, a period during which there were varied amounts of thimerosal contained in recommended vaccines, found no relationship between the quantity of thimerosal a child was exposed to and development of autism. The study was published in Pediatrics in 2006 by Eric Fombonne and colleagues.
• Another study compared results of 40 separate tests performed on 1,000 children exposed to thimerosal-containing vaccines. It found no link between quantities of thimerosal and development of autism. The research was done by scientists at the Centers for Disease Control and Prevention and published in The New England Journal of Medicine in 2007.
• A study of rates of autism diagnosed in California between 1995 and 2007 found that despite removal of thimerosal from vaccines, rates of autism continued to increase. The study was published by Schechter and Grether and published in the Archives of General Psychiatry.

Several other pieces of information are also reassuring when considering thimerosal exposure and autism:

• In the 1971 Iraq experience (described earlier), babies of pregnant women who ate bread made with the grain treated with methylmercury were diagnosed with epilepsy and mental delays, but they weren’t at increased risk of autism.
• The symptoms of mercury poisoning are different from the symptoms of autism as shown in studies of the head size, speech patterns, vision, coordination and sensation of children poisoned by mercury.
• Methylmercury is found in low levels in water, infant formula and breast milk. Although it is clear that large quantities of mercury can damage the nervous system, there is no evidence that the small quantities contained in water, infant formula and breast milk do. An infant who is exclusively breastfed will ingest more than twice the quantity of mercury than was ever contained in vaccines and 15 times the quantity of mercury contained in the influenza vaccine.

What’s known about the causes of autism

First, like cystic fibrosis or sickle-cell disease, autism has a genetic basis. Researchers found that when one identical twin had autism, the chance that the other twin had autism was about 90%. For non-identical twins, the chance was less than 10%.

Second, although autism has a genetic basis, environmental factors can also cause the disease. For example, children whose mothers took thalidomide, a medication that used to be prescribed for nausea during pregnancy, had birth defects, including malformed ears and shortened limbs. They also had a greater likelihood of developing autism compared with babies born to mothers who never took thalidomide. Thalidomide clearly caused autism, but only if mothers took it early in pregnancy. If mothers took thalidomide in the second or third trimester of pregnancy, their babies weren't at increased risk of autism.

The thalidomide experience showed that early in pregnancy was a vulnerable time when a drug could cause autism. The same has been found related to rubella virus. Babies born to mothers who suffered a rubella infection early in pregnancy developed birth defects involving the eyes, ears, brain and heart. They were also at greater risk of developing autism, but like thalidomide, this risk was associated with exposure to rubella early during pregnancy. Babies don't develop autism if they are infected with rubella soon after birth. These findings suggest that a virus or a drug can cause autism, and that there is a vulnerable time early during pregnancy when the baby is at risk. During the second or third trimester of pregnancy, or after the child is born, the window for environmental factors causing autism has apparently closed.

Women in the U.S. also occasionally received mercury when they were pregnant. It happened when doctors found that the mother's blood type was not compatible with their baby's blood type. To prevent this blood mismatch from hurting the baby, mothers were given RhoGAM, a product that used to contain thimerosal as a preservative. Consistent with the observation in Iraq, babies exposed to thimerosal in RhoGAM did not have a greater risk for autism than babies whose mothers never received RhoGAM. Although thalidomide and rubella virus can cause autism in pregnancy, scientific evidence clearly indicates that mercury doesn't.

Read more about autism and vaccines.