How does mercury get into the fish and where does it come from?
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Mercury has lots of natural sources, but most of the mercury found in fish comes from the burning of coal, run-off from mining, and industrial waste. It gets into water that washes into the sea where plankton and small fish absorb it. It’s fairly easy to absorb and very difficult to flush out, so it tends to accumulate in living things exposed to it. When larger things eat smaller things, they get even more.
Eventually people eat the fish that’s absorbed mercury and they absorb the mercury themselves. The human body will get rid of the mercury it absorbs, but slowly; it cuts the amount of mercury in your body in half every 50 days or so.
That means, if you eat 100g of fish containing 100 micrograms of mercury, you’ll absorb 100 micrograms of mercury. After 50 days, 50 micrograms will be in your body. 50 days later, it will be 25 micrograms; then 12.5, 6.25, … A year after eating that small piece of fish, you’ll have about 0.625% of the mercury that was in that fish still in your body. If you don’t eat seafood very often, perhaps it’s not a big deal. You won’t notice much in the way of health effects. If you seafood frequently, however, you will build up more and more mercury in your body over time and can get it to levels where it starts to affect your health.
Of course, there are animals that rely on fish as food, and they are constantly building up mercury in their bodies and begin to see toxic effects at some point.
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