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Mainstream Science Suggests Ethylmercury is More Toxic than Methylmercury

New and old research support the caveat that “safe” levels of ethylmercury exposure might indeed be dramatically lower than the EPA’s RfD. A 2012 Italian study , for instance, showed that ethylmercury-containing Thimerosal diminished the viability of human cells in the lab at a concentration one-fiftieth that of methylmercury. By this measure, ethylmercury is 50 times as toxic as methylmercury to humans.

Japanese research on rats in 1968 showed that ethylmercury compounds, such as ethylmercuric chloride from which Thimerosal is made, clear the body more slowly than other mercury compounds including mercuric chloride and phenylmercuric chloride.

A book chapter in 1972 by Staffan Skerfving, an emeritus professor at Lund University in Sweden, reviewed literature on methylmercury versus ethylmercury, noting several instances where compounds of the latter appeared more toxic than the former in animal studies.

For example, ethylmercury chloride killed off half of a test population of mice—a classic “LD50” (lethal dose) study—within a week at a concentration of 12 milligrams of mercury per kilogram of body weight; methylmercury chloride’s LD50, meanwhile, lethal to half the mice was 14 milligrams. This study suggested that ethylmercury was twice as toxic.

Further examples abound. Pig studies by Tryphonas and Nielsen in 1973 showed that ethylmercury “proved much more toxic” than methylmercury. Meanwhile, another 1973 study that emerged from a 1971 international conference found the toxicity of ethylmercury compounds comparable to or even greater than that of methylmercury, as well as more persistent in the brain.

An advisory committee at the conference reported that the International Committee on Maximum Allowable Concentration for Mercury and Its Compounds grouped ethylmercury with methylmercury, and observed that accounts of human intoxication with ethylmercury have usually described neurological and other symptoms similar to those of methylmercury. The report noted that in studies of patients transfused with a commercial product of human plasma containing 0.01 percent Thimerosal, as well as in studies of mice injected with an ethylmercury solution, the increased level of inorganic mercury added to the mercury already existing in the body resulted in a “longer biological half-life of total mercury than that reported for methylmercury injection.”

Why do the CDC and WHO Report that Ethylmercury Exposure is Safe?

The WHO’s conclusion that ethylmercury is safer because of its “short” half-life may be based on observations that ethylmercury disappears from blood samples quicker than methylmercury. However, this tendency may be evidence not of ethylmercury’s comparative safety, but of its greater danger if, as science has suggested, ethylmercury is not leaving the body but simply migrating more rapidly to the organs, including the brain. Indeed, studies have shown that an ethylmercury compound’s short residence in the blood stems from its ability to more easily pass into the organs, where it can remain for long periods and possibly cause injury.

For example, Blair in 1975 dosed squirrel monkeys with intranasal saline or Thimerosal daily for six months, finding that, compared to the saline group, mercury concentrations in the Thimerosal group were significantly raised in the brain, liver, muscle, and kidney, though not in the blood. Although there were no signs of toxicity in the animals, Blair concluded that the “accumulation of mercury from chronic use of thiomersal-preserved medicines is viewed as a potential health hazard for man.”

Beyond a possibly greater capacity to have inorganic mercury accumulate in organs, Thimerosal also passes more easily from a mother’s bloodstream through the placenta into a developing baby than does methylmercury. That was the evaluation made in a 1983 review study by A. Leonard. In addition, a 1995 study demonstrated that both ethylmercury and methylmercury cause mutagenic changes at similar concentrations in bacterial cells.

The Twisted Saga of Pichichero

With these and other studies as background, an important study in humans took place in the early 2000s. The study, by Michael Pichichero of the University of Rochester Medical Center and published in The Lancet in 2002, lent some apparent scientific credence to the idea that ethylmercury is safer than methylmercury. Pichichero, who helped develop the HiB vaccine and previously received grants and honoraria as a consultant for other vaccine makers , did not declare these conflicts of interest in a statement in his paper, as required by The Lancet’s peer review rules. The Pichichero study assessed mercury levels in the blood, urine, and feces of forty infants ages six months or younger three to twenty-eight days after they had received Thimerosal-preserved vaccines (DTaP, HepB, and in some cases Hib). For comparison, twenty-one similar infants who received Thimerosal-free vaccines were also evaluated. Although infants who received Thimerosal-preserved vaccines had higher levels of mercury in their blood, urine, and feces than did the infants who received Thimerosal-free vaccines, the authors concluded that the levels of mercury detected were not greater than what is considered safe. Most of the mercury from the injected Thimerosal seemed to have left the children’s bloodstreams more rapidly than methylmercury found in the blood of those eating fish in previous studies; the researchers estimated a half-life of seven days for ethylmercury in the blood. Pichichero concluded that ethylmercury, therefore, did not remain in children’s bodies long enough to possibly cause damage.

Pichichero’s study immediately came under attack by internationally respected scientists in a 2003 letter to The Lancet by Neal Halsey , of the Institute for Vaccine Safety at Johns Hopkins Bloomberg School of Public Health, and Lynn Goldman, also of the Bloomberg School of Public Health. Halsey and Goldman pointed out that Pichichero and colleagues “did not measure the peak blood concentrations that occurred within hours after the injections.” The concentration listed for one child in the study of 20.55 nanomoles per liter was obtained five days post-vaccination. Assuming Pichichero’s own estimate of an ethylmercury half-life in the blood of seven days, the peak blood concentration for this child was 29.4 nanomoles per liter—exceeding the conventional safety threshold of 29.0 nanomoles per liter, and contradicting the study’s claim that “no children had a concentration of blood mercury exceeding 29 nmol/L.” The child in question had received 37.5 micrograms of ethylmercury rather than the possible maximum exposure of 62.5 micrograms. In the latter scenario, the child’s peak blood mercury concentration would have hit 48.3 nanomoles per liter.

Another child in the study registered a 7 nanomole per liter blood concentration 21 days post-vaccination; extrapolating backwards, this child’s peak mercury level might have reached 42 nanomoles per liter. Halsey and Goldman’s letter further pointed out that Pichichero seemed to have cherry-picked the children in the study—some already with no margin of safety for further mercury exposure—seemed to have come from a population with low background environmental and maternal exposure to methylmercury.

Soon after publication of Pichichero’s study, alarming new evidence emerged that ethylmercury lingers in the body. In an unpublished letter submitted to Pediatrics, Dr. Boyd Haley, then-chairman of the chemistry department at the University of Kentuck, and Mark Blaxill challenged Pichichero’s hypothesis that ethylmercury is quickly excreted. Pichichero and colleagues had measured the excretion levels of mercury in the stools of 22 healthy infants exposed to Thimerosal-containing vaccines. Pichichero’s estimated range for the infants aged two and six months was 23 to 141 nanograms per gram of stool (dry weight). Assuming the excretion rate reported by Pichichero, Blaxill and Haley demonstrated that it could take children with low excretion rates of mercury in their stool almost four years to eliminate a 187.5 microgram mercury burden from their bodies.

In 2006, Luis Maya and Flora Luna further debunked Pichichero’s conclusions. The authors pointed out that while Pichichero’s team had found ethylmercury to be excreted in appreciable quantities in the feces, the researchers did not study other body parts beyond the blood, such as the central nervous system. In agreement with Halsey and Goldman, Maya and Luna criticized Pichichero for neglecting to measure the peak serum levels of ethylmercury after the first hours of inoculation, though other investigations had documented substantially elevated blood concentrations in the first 48 to 72 hours after administration in pediatric vaccines. Maya and Luna also pointed out that the study was small and measured variables of pharmacokinetics (the actions of a drug within the body over time), so it was not designed to measure the biological effect of Thimerosal as a preservative.