There's good news about worldwide mercury pollution. The amount of the toxic metal circulating through the atmosphere has dropped by 17 percent since the late 1980s. A recent study shows mercury concentration hit that low in the mid 1990s and has held there ever since.
Still, Franz Slemr, who led the study, isn't cheering.
When atmospheric chemist Dr. Slemr and colleagues published their work in Geophysical Research Letters last month, they added a disturbing caveat. They don't know what is going on. That 17 percent drop seems too large to be attributed mainly to efforts to curb industrial pollution. The scientists' bafflement emphasizes the need to understand better the sources - not just the amount - of atmospheric mercury, according to the study announcement.
Slemr, at Germany's Max Planck Institute for Chemistry in Mainz, notes that mercury pollution is a global threat to human health, as "some 5,000 metric tons of atmospheric mercury are currently deposited worldwide every year." Once on the ground or in the water, he adds, it can be "transformed into methyl mercury, one of the most toxic compounds." In the ocean, this poison works its way up the food chain from microscopic plants to top predators like tuna and dolphins, becoming increasingly concentrated.
Last month, Tetsuya Endo and associates at the Health Science University in Hokkaido, Japan, reported that mercury in legal whale and dolphin meat on the Japanese market often exceeds the government safety level, sometimes by large amounts. Their findings, in the journal Environmental Science and Technology, aggree with two earlier studies.
What's more, last month, the World Health Organization and the United Nations' Food and Agriculture Organization cut in half their recommendations on how much mercury people can safely consume in their food. And though, in the US, the government's Food and Drug Administration has not yet toughened its warnings, the Environmental Protection Agency and 10 states have issued advisories on canned tuna, known to generally contain methyl mercury.
Finding and dealing with global mercury sources will be hideously complex. Scientists don't know all the natural sources, although volcanoes are a major suspect. Man-made sources include coal and biomass burning, waste incineration, and some industrial processes. But even if all sources of new mercury were shut off, Slemr points out that Earth would still be left with 200,000 metric tons of the stuff that have accumulated since Roman times. Some of this inventory circulates through air, land, and sea at all times.
No place is exempt: The atmosphere carries mercury everywhere. Cathy Banic with Canada's Meteorological Service in Toronto and colleagues confirmed this last month in the Journal of Geophysical Research. Their data tracking airborne mercury at various altitudes are some of the best evidence yet of gaseous mercury circulating globally.
How mercury gets into soil and water is another partially solved mystery. Some studies have shown that air pollutants such as smog can convert gaseous mercury to a water-soluble form. These findings suggest that only a global effort to minimize general air pollution could limit mercury washout.
Even identifying and eliminating man-made sources may prove tricky. Industrialized countries, including the US, are phasing out a chlorinemaking process that involves mercury. This leaves the question of what to do with the leftover metal. Some want to take it out of service completely, but the question of how and where to store it is still unresolved. Landfills are out: Mercury already there too readily escapes.
Mercury is fast becoming a major international environmental issue. The struggle to resolve the complex political, diplomatic, and economic issues involved will make the effort to phase out chemicals that destroy Earth's protective ozone layer seem like a piece of cake.