Last summer a new advisory was issued by the Missouri Department of Health. The advisory warned pregnant women, women who may become pregnant, nursing mothers, and children under 12 years of age to avoid eating largemouth bass over 12 inches in length, due to mercury contamination. The shocking part of the advisory was that it covered all streams and lakes in the state, not just a handful of waterbodies. Missourians were left to ask "Why the sudden problem with mercury?"
Mercury is an element that is naturally found in rocks and minerals. Natural events such as volcanic eruptions and erosion account for the low background levels of mercury that we expect to find in the environment. These natural levels are generally not a cause for concern. Problems associated with mercury in our lakes and streams occur due to elevated concentrations associated with human activities. These activities include the burning of coal, incineration of materials that contain mercury, certain industrial processes and the disposal of some household items. It is estimated that humans account for 75% of mercury emissions worldwide.
Elemental mercury is often the form that is emitted into the atmosphere. This form is not very toxic, but it can travel great distances making it a threat to almost all waterbodies. Atmospheric deposition delivers mercury to our lakes and streams, where it can undergo a change into the highly toxic methyl mercury. Methyl mercury remains dissolved in the water and is incorporated into the tissue of aquatic organisms.
Mercury levels in the algae, zooplankton and even small fish are usually not of concern. It is when the mercury has had time to build-up or bioaccumulate in larger, long-lived organisms that problems arise. That’s why the advisory in Missouri focuses on largemouth bass 12 inches or longer. These fish are at the top level of the food chain and have had time to accumulate enough mercury in their tissues to make eating them a cause for concern.
The primary health impacts associated with mercury are on the development of the central nervous system and brain. Methyl mercury is completely absorbed into the blood and distributed to all tissue (including the brain). The mercury will readily pass through the placenta to the fetus where it may cause developmental neurological abnormalities. That is why the high-risk group encompasses pregnant women and women who may become pregnant. Children under the age of 12 are susceptible to mercury because their nervous systems are still developing. The mercury levels in bass over 12 inches in length is not high enough to be of concern to those not in the specified high-risk groups.
Mercury levels were not a worry to the high-risk groups a year ago. Federal Food and Drug Administration guidelines and EPA criteria were at 1 part per million mercury in fish tissue. A study published by the National Academy of Sciences in 2000, led to the mercury criteria being lowered to 0.3 parts per million. Based on the old criteria of 1 part per million, none of Missouri=s waters were unsafe. With the lowering of the criteria, 40 lakes and sections of streams were now above the new threshold.
So why there is a statewide advisory if there are only 40 lakes or sections of streams that are above the new criteria? The simple fact is that not all waters have been tested. The 40 impaired waters make up 29% of the 141 lakes or stream sections that have been tested. A lack of data, along with the knowledge that mercury contamination tends to be widespread, has led government agencies to err on the cautious side.
Missouri is not the only state dealing with mercury pollution. Forty-one states have fish consumption advisories associated with mercury, with 13 of those states issuing state-wide advisories. In fact, mercury is the most frequent basis for fish consumption advisories. The state is working on plans to decrease the amount of mercury in our waterways but it may be a while before the current advisory is lifted.
While the mercury advisory was announced this summer, a 1985 fish consumption advisory for chlordane was lifted. The use of chlordane, a termite pesticide, was banned in 1988 and concentrations of the contaminant in fish populations have dropped to safe levels.