By EDUARDO GONZALES, MD
August 9, 2010, 11:35am
Q. I recently read an alarming article about mercury in fish. What are the toxic effects of mercury? How does mercury get into fish? Which fish contain high levels of mercury? Should people limit their intake of fish because of the risk for mercury poison
Q. I recently read an alarming article about mercury in fish.
What are the toxic effects of mercury?
How does mercury get into fish?
Which fish contain high levels of mercury?
Should people limit their intake of fish because of the risk for mercury poisoning?
Lina O., Manila
A. Mercury is a heavy metal that is poisonous to humans. Its toxic effects include damage to the brain, kidney, and lungs.
For fetuses (in wombs of mothers with high blood level of mercury), infants, and children, the primary adverse effect of mercury is impaired brain development.
Mercury is used extensively in a lot of fields. In medicine, it is an important constituent of some drugs and antiseptic agents; and, it is a vital component of some medical diagnostic equipment such as blood pressure apparatuses and clinical thermometers.
In agriculture, it is sometimes used as a fungicide. In industry, it is used in the manufacture of electrical equipment, paper, and some plastics and other chemicals.
It is likewise utilized in the extraction of some ores, such as gold, from mines.
Invariably, some of the mercury that is used for various purposes find its way into large bodies of water including oceans where they enter the food chain.
In the Philippines, the main sources of mercury pollution are mined mercury deposits; use of mercury in gold extraction (gold mining takes place in at least 20 provinces in the country); and, coal burning power plants (coal contains mercury as a natural contaminant).
Mercury in bodies of water is absorbed, usually in the form of methylmercury, by algae.
Algae comprise the staple food of many small fish that scavenge near shorelines.
These small fish are eaten by larger fish which in turn, serve as prey for still larger fish.
Thus, because the mercury in the prey transfers to the predator, the level of mercury accumulation among fish becomes higher the higher one goes in the food chain. Sharks, for example, which are on top of the food chain, have very high levels of mercury.
Tests that were conducted on shark fins on sale in Thailand a few years back showed that the fins contained levels of heavy metals (particularly mercury) that are 42 times the level considered safe for human consumption.
Aside from sharks, fish that are beginning to exhibit dangerously high levels of mercury in their tissues are tilefish, swordfish, and king mackerels.
By and large however, most types of saltwater and freshwater fish contain only minute levels of mercury and inasmuch as the body has the capability to excrete minute amounts of mercury—what the body cannot handle is abnormally high amount of mercury—the risk of a person getting slowly poisoned with mercury by eating fish is minimal.
In the Philippines, we do not have an effective monitoring system for mercury levels in fish, but it is probably safe to say that except in high risk areas (i.e., those communities and water bodies near gold and mercury mines) the fish that we commonly eat such as bangus, tilapia, tuna and tamban are still free from dangerous levels of mercury contamination. Other commonly eaten fish that are low in mercury include shrimp, canned light tuna, salmon, pollock, and catfish.
Filipinos should not limit their intake of fish—except perhaps for shark, swordfish, king mackerel, and tilefish—because of fear from possible mercury poisoning.
The fish that we commonly eat are safe to eat. In fact, we should increase our fish intake because aside from their high-quality protein and other essential nutrients content, fish, especially the deep sea species, are low in saturated fats and contain omega-3 fatty acids, factors which contribute to heart health and children’s proper growth and development.
(E-mail inquiries on health matters to: medical_notes@yahoo.com)
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