The Facts

When the FDA and American Academy of Pediatrics decided to recommend the removal of thimerosal from childhood vaccines in 1998, it was based on 1) a recent lowering of standards for exposure to mercury along with 2) a recalculation of the total exposure to mercury contained in childhood vaccines, several of which had only been mandated in the prior 10 years. When the scientists were making their recommendations, they knew they were acting with incomplete information. However, due to uncertainty and a desire to be overly cautious, they acted to remove thimerosal from childhood vaccines.

The decision turned out to be very conservative. This is because it was based upon data associated with methyl mercury exposure, on which the scientists had relatively good data concerning how it was metabolized and how long it stayed in the body. However, the type of mercury in thimerosal was ethyl mercury, a type of mercury science thought was less damaging than methyl mercury, but on which they had less information. Therefore, being cautious, they decided to use methyl mercury levels as an inexact proxy. Since that decision, several studies have been done on ethyl mercury. And, it has been found that ethyl mercury is cleared from the body and the brain significantly faster than methyl mercury, so the late-1990s risk assessments turned out to be overly conservative.

Detailed human studies, including the second study for Pichichero et al, found a blood half life of under 4-7 days, or about one tenth the half life for methyl mercury, confirming earlier findings by the same group. In that study, baseline levels of ethyl mercury were restored after 11 days. Pre-vaccination levels of blood mercury in 6 month olds were not higher than those in 2 month olds, suggesting that exposure to thimerosal containing vaccines does not resulting accumulation of mercury in blood as might have been expected if the half life was similar to that of methyl mercury.

Pichichero’s estimate of mercury blood half life was confirmed in animals studies comparing the intramuscular administration of ethyl and methyl mercury in infant rhesus macaque monkeys (this type of experiment is not permitted in humans). That study showed much shorter half life for ethyl mercury in the range shown by Pichichero in human studies (an initial elimination half life of 2.1 days and terminal elimination half life of 8.6 days). Also, Magos et al compared exposure to these 2 types of mercury in rats and found that methyl mercury is actively transported across the blood brain barrier, whereas ethyl mercury is only passively transported. This means ethyl mercury is not as neurotoxic in animals.

What this Means?

When the US government reset acceptable levels of mercury exposure which ultimately resulted in the elimination of thimerosal from childhood vaccines, it did so based on calculations of risk from methyl mercury – a known and potent toxin. Even so, the acceptable levels were set with a 10 times safety factor, meaning the acceptable levels of methyl mercury exposure were 10 times less than levels shown to cause the mildest detectable neurological harm.

And, this new acceptable mercury exposure upon which decisions about vaccines were made did not account for the fact that science was pretty sure ethyl mercury was less potentially harmful than methyl mercury. In fact, what the recent evidence on ethyl mercury tells us is that there is another 10 times safety factor built into the ethyl mercury calculation simply from relative half life, not to mention toxicity; thus there is a 100 times safety factor built into this calculation for ethyl mercury.