As humans we come across mercury surprisingly often in our day-to-day lives. The silvery liquid metal is hiding in thermometers, lighting, and batteries. It’s poisonous and there are concerns that it is also becoming increasingly common in fish. Surveys have shown that two-thirds of the British public take their fish and chips with salt and vinegar, and over a third like theirs with a side of mushy peas. Unsurprisingly, there were no recorded requests for a portion of mercury.
Mercury consumption can do nasty things to you. It can cause sensation, speech and co-ordination difficulties, and in very severe cases, paralysis and death. Consequently, official bodies such as the US Food and Drug Administration (FDA) now issue guidelines warning of the potential effects of consuming a lot of fish.
A silver cloud looming over the oceans
As strange as it sounds, the mercury in the ocean gets there from the air. Burning fossil fuels, gold mining and cement production all cause mercury to go airborne. A small amount comes from volcanic eruptions. The atmospheric mercury levels are very low, so there’s no need to fret over the prospect of giant silver clouds unleashing torrential apocalyptic mercury rain (for now). It does however eventually come to rest on land, rivers or oceans. And once in our waterways it can be ingested by fish, algae and other organisms.
Certain forms of algae convert mercury into a particularly toxic form called methylmercury. This poison then moves up the food chain, being excreted into the water by the algae and ingested by plankton. Algae and plankton are eaten by small fish, e.g. salmon, pollock, and perch. These fish are then in turn eaten by larger fish like trout and tuna. Finally, higher predators such as you, me and Bruce the shark, eat these fish. The biggest fish accumulate methylmercury faster than they can remove it (a phenomenon known as bioaccumulation) and it is possible that mercury poisoning might drive some species to the brink of extinction.
For us humans, our mercury exposure will depend on the amounts and types of fish we eat. The people at most risk are young children, pregnant and nursing mothers, and from an economic standpoint, the owners of your local fish and chip shop.
Generally speaking, few of us eat enough fish to get poorly from it but there is health advice for reducing mercury exposure:
- Shark, swordfish and marlin: do not eat these if you are pregnant or trying to get pregnant. All other adults, including breastfeeding women, should eat no more than one portion per week. These fish can contain more mercury than other types of fish, and can potentially damage a developing baby’s nervous system.
- Oily fish: if you are trying for a baby, pregnant or breastfeeding, you should have no more than two portions of oily fish a week. (A portion is about 140 grams.)
- Canned tuna: if you are trying for a baby or are pregnant, you should have no more than four cans of tuna a week. This is because tuna contains higher levels of mercury than other fish. If you are breastfeeding, there is no limit on how much canned tuna you can eat.
How can we be mercury free?
Despite these measures, it will probably take many years before these changes filter through the oceans, plankton and fish – and there may be quite a wait before we see any benefits. Hopefully, with advances in technology we will see the near elimination of mercury release in industry.