Issue Date: October 13, 2008
More On Mercury Toxicity
WE OFFER the following comments on Martin Saunders' assertions that some forms of mercury are not dangerous and that health and environmental agencies are overly concerned about mercury exposures from spilled mercury (C&EN, Sept. 1, page 6). Saunders does not consider the environmental fate of the various forms of mercury people may ingest or encounter; all mercury released to the environment has the potential to become methylmercury and contaminate fish.
If the systemic dose of mercury is similar, the toxicity of most mercuric compounds is similar, even though the target tissues may not be the same. The Environmental Protection Agency reference for safe chronic exposures for mercury vapor and methylmercury are essentially identical. The apparent difference in toxicity comes from the different systemic availability of different forms of mercury depending on the route of exposure: Methylmercury is available by ingestion; elemental mercury vapor is readily absorbed when inhaled; and dimethylmercury is easily absorbed through ingestion, inhalation, or even dermally, so it is often considered the most toxic form of mercury. Saunders says that calomel is not very toxic, but acrodynia was a relatively common and deadly childhood disease until it was discovered that calomel use caused it (Am. J. Dis. Child. 1966, 112, 147).
The EPA reference concentration (RfC) for mercury vapor exposure is based on occupational studies where adult workers were exposed to levels equivalent to about 30 times the RfC when averaged 24/7. These workers showed subtle effects of exposure including hand tremor, increases in memory disturbance, and autonomic dysfunction. The RfC is intended to be protective of adults as well as sensitive individuals including children and fetuses. Chronic instances of exceeding the RfC should be taken seriously.
Saunders states that mercury vapor is heavy and will stay close to the floor. However, slight movements of room air can mix mercury vapor with air, after which there is no tendency for mercury to settle downward. Mercury vapor dispersion is nicely shown in a video: wbgustream.bgsu.edu/bgsu/epa/index-fl.html. We looked at a large gymnasium with a mercury source at the floor and found only a 10 to 20% difference between mercury vapor concentrations at the floor and at the 30-foot ceiling.
Surprisingly, exposure to mercury at the site of a mercury spill isn't usually the greatest concern. A larger concern is that the mercury will be tracked or otherwise taken to some place where significant exposure may occur. If it is clear after a spill that dangerous incidental exposures cannot occur, cleanup efforts can be minimal. However, major efforts may be required if the mercury permeates a building due to delayed or improper cleanup.
The focus on the important message in the original article should not be lost: Reductions of mercury use, exposure, and environmental release are important activities that have a positive impact on public health.
Minnesota Department of Health
Minnesota Pollution Control Agency
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