Mercury is a very evocative metal. Though once thought to be the elixir of youth, the silvery liquid is now associated with Carroll’s confused and eccentric Hatter. Mercury poisoning is a damaging and permanent condition arising from exposure to mercury in high amounts which causes nerve damage and effects the way the brain works. Naturally, exposure to mercury is actively avoided and prevented – especially for expecting mothers, who are advised not to eat seafood during their pregnancy due to the higher levels of mercury in certain types of fish.
From this information, it’s unsurprising that popular opinion is strongly against the use of mercury in medical applications such as preservatives in vaccines. The idea of injecting such a potent poison into our children is unsettling, and mercury has now been removed from vaccines in most of the western world. But is the mercury that poisoned the hatters and the people of Minamata the same as the mercury used to preserve medicines?
Dose is hugely important in determining whether something will kill or cure you. Anything and everything is toxic in the right dose – chocolate is thought to only be toxic to dogs, but the (naturally occurring) theobromine that causes the harm can and does do the same to humans. Clearly the amount of mercury present makes a big difference, as well as the length of exposure (hatters typically worked their trade for decades before they felt the effects). But even more important than dose is the type of mercury to which the person is being exposed.
The different ‘types’ of mercury lies right at the heart of what chemistry is: the study of how different arrangements of atoms and molecules interact together. Elemental – or pure – mercury is relatively safe, as the toxicity comes from breathing in the vapours. When the mercury is heated the vapours are given off more strongly and are easier to breathe in. Pure mercury isn’t very good at absorbing through the skin, but the process of breathing in the vapours allows some atoms of mercury into the blood stream. Poisoning by inhalation generally takes chronic exposure to heated mercury, such as experienced by the hatters.
When mercury has other atoms attached to it to form a molecule, the behaviour of the mercury compound can completely change. Dimethylmercury, which has two additional carbon atoms, is toxic at very small doses and passes through skin, latex, and most standard laboratory equipment. Once exposed, dimethylmercury will very quickly travel across the blood-brain barrier and is difficult for the body to metabolise or remove. This makes dimethylmercury considerably more dangerous than pure mercury, and more difficult to treat.
This, then, is the major difference between mercury in thermometers and mercury in vaccines. Known as thiomersal, this compound is metabolised and removed from the body much quicker than dimethylmercury, and isn’t very good at crossing the blood-brain barrier. The dose of mercury is very small in a vaccine; enough to kill bacteria and fungus in the vaccine, but not enough to do any harm to the human.
When encountering any element or chemical, ‘natural’ or otherwise, it’s important to consider not only the dose, but the environment around it. Chemicals are around us all the time, and any confusion and fear is being exploited in the name of making profits. Sense about Science has recently released a handy guide, Making Sense of Chemical Stories, which clearly and plainly lays out everything you’ve ever wanted to know about chemical misconceptions.
And for the record, the mad Hatter doesn’t even show the symptoms of mercury poisoning. His madness is probably a consequence of being subjected to endless cups of tea.