Oceans' mercury a natural matter

May 10, 2008

The Sun's article on mercury in wetlands - and rivers, lakes and estuaries - focused on a real problem that, in extreme cases, has led to serious human consequences from consumption of fish ("Marshes produce mercury hazard," May 4).

Mercury pollution in those waters is almost entirely a consequence of human activity. However, tuna and swordfish do not feed in coastal waters, and their high mercury content is entirely the result of bio-concentration up the food chain in the open ocean.

Cans of tuna more than 100 years old have been opened and analyzed for mercury. They contain, within the limits of experimental error, about as much mercury as tuna or swordfish caught yesterday does.

That is because the mercury pollution in the oceans is almost all of natural origin. It comes from volcanic eruptions, from mercury in the ocean ridge and from mercury leaching from rocks.

The total amount of mercury in the oceans is at least hundreds, and perhaps thousands, of times the amount of mercury ever mined.

The level of mercury in the oceans varies somewhat, but is about 0.1 part per billion. However, there is a lot of water, and therefore a lot of mercury, in the oceans.

The adverse effects of methylmercury consumption are the same regardless of its source.

But it is useful to know the source to know what control and remediation methods are appropriate.

R.L. Hall

Towson

The writer is a retired toxicologist who has served on a number on National Academy of Sciences panels on food safety.

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