Facts: The seafood lobby uses this fable to suggest that consumers don’t need to be concerned about short-term high exposure to methylmercury. They assert that, as long as one’s average lifetime exposure is below 10 times the RfD (which the lobby likes to claim is the safe level), there is no risk associated with occasional high exposures. This claim is fundamentally wrong on scientific grounds, and potentially dangerous if consumers believe it.
The RfD is in fact based on “chronic” exposure, that is, exposure averaged over a fairly lengthy period. For many people, under most conditions, a single meal of high-mercury fish, now and then, is unlikely to be harmful. But based on both what is well known scientifically and what is not known, the “long term” is definitely not a lifetime, and there are reasons for caution, even about single high-mercury meals, for a pregnant woman.
The health risk of greatest concern is damage to the developing brains of babies in the womb and in young children. The period the Reference Dose applies to is therefore a pregnancy and the first few years of life. A woman’s mercury intake before she becomes pregnant and while she is breast-feeding can also affect the exposure of her fetus and newborn baby, so the critical time frame may extend to two years or so. But mercury exposure matters most in the womb and the first few years of life, when the brain is growing and developing most rapidly. Since the child is the vulnerable party, the woman’s lifetime exposure is largely beside the point.
Environmental health experts also have long recognized that exposure does not need to be constantly high to cause damage. There is substantial experimental evidence (in animals) showing that occasional short-term peaks of exposure, such as might occur from a single meal, or a few meals over the course of a week or two, of high-mercury fish, can significantly damage fetal development, if they occur during critical developmental stages. In other words, both the amount of exposure and the timing of exposure are crucial to whether damage occurs. Although there is much still unknown about the effects of short-term spikes of methylmercury exposure during human pregnancies, what is known on this topic provides an ample basis for advising women to avoid mercury exposure as much as possible before, during (and after, if they are breast-feeding) pregnancy.