Catalytic converters have been an environmental success, but not an unqualified one. The tailpipe devices reduce the toxicity of auto emissions and cut down on the formation of smog, but they also output greenhouse gases such as carbon dioxide. Now a new study faults widespread adoption of catalytic converters for another environmental issue, albeit one without a clear immediate impact: osmium pollution.

Osmium is an extremely hard metal that exists in seawater and in the Earth's crust—and, crucially, in ores extracted and processed for platinum, which is commonly used in catalytic converters. (Platinum also finds use as a catalyst in fuel cells, another green technology.) Smelting those ores can produce osmium tetroxide, a toxic chemical, but that's not the real issue with osmium pollution, says lead study author Cynthia Chen, a graduate student in the department of Earth sciences at Dartmouth College. "I don't think we need to worry about it from a human health standpoint," Chen says, as osmium's concentrations, even as elevated by platinum production, are extremely small.

What is potentially concerning, she adds, is that it appears that humans have upset the geochemical cycle of osmium by extracting it from the Earth and releasing it into the atmosphere. "We've disturbed the natural budget of this element," Chen says. In other words, osmium provides a small reminder of the ways in which human activities can transform the face of the planet.

The element has several isotopes (that is, forms with different numbers of neutrons in the atomic nucleus), and by sampling the ratios of those isotopes, researchers can help parse natural from human sources. In their study, published online this week by the Proceedings of the National Academy of Sciences USA, Chen and her co-authors looked at the ratio of osmium 187 (Os 187) to osmium 188 (Os 188).

That isotope ratio differs substantially from the deep ocean, where the isotopes exist in nearly equal quantities, to platinum-containing ores, where Os 188 outnumbers Os 187 by roughly 5 to 1. The isotope ratio found by Chen and her co-authors in precipitation around the globe was rich in Os 188, like that found in ores, and the ratio in surface water was skewed in that direction as well when compared to deep-water samples. Industrial processes, primarily the smelting of ores to extract platinum and related elements, seem to be unbalancing the planet's osmium budget, the researchers conclude.

"It is ironic," Chen and her colleagues wrote, "that, whereas the phasing out of leaded gasoline and use of catalytic converters has removed [lead] and other pollutants from the environment, the processing of [platinum] used in catalytic converters is polluting the earth's surface" with osmium.

Most of that processing takes place in Russia and South Africa. If the plants in those countries scrubbed their exhaust gases for sulfur dioxide, the study's authors say, the osmium emissions would be curbed as well. Such scrubbing is already used in smelting plants for metals such as copper, nickel, zinc and lead.

Photo of platinum ore: Wikimedia Commons