Mercury in unequivocally a neurotoxin. It is especially damaging to the developing brain. But it’s the dose that makes the toxin, and so a low enough exposure even to something known to be potentially harmful may not be. Further, the body has mechanisms for dealing with toxins, and toxins in the body may not be reaching the cells they can potentially damage in significant amounts. Therefore if we want to know if a potential toxin is actually causing any harm to people we need to do some type of epidemiological study – correlating exposure to possible adverse outcomes. All the studies in petri dishes and with cell cultures just won’t answer the question of harm.
The question of whether or not mercury in vaccines has caused neurological harm, specifically autism, has been largely answered. Numerous studies have shown no association between the amount of mercury exposure from vaccines and the risk of developing autism. A separate mercury-related question, however, is whether or not there is any risk of harm from mercury exposure from seafood. Mercury is methylated by bacteria into methymercury, and through them gets into the food chain in the oceans. Fish that eat other fish then concentrate the mercury in their tissues, and so predatory fish and sea mammals tend to have high concentrations of methymercury.
This has led to some precautionary recommendations, including that pregnant women should not eat tuna or other fish with high mercury levels. This makes sense, but what is the actual risk? The precautionary principle can also cut both ways. Fish contain many high-quality nutrients important for a developing brain, such as polyunsaturated fatty acids. Removing this food source from the diet of pregnant women may have unintended negative consequences.