The researchers found traces of osmium everywhere - including in snow from the Sierra Nevada mountains in California. Credit: Richard Broadwell / Alamy

The planet has been covered with a fine layer of osmium thanks largely to efforts to clean up car exhausts, according to a global survey of rainwater.

Although there's no reason to think that this tiny amount of osmium is doing harm, some scientists are growing concerned about rising levels of platinum-group metals such as osmium, and think we should be keeping a closer eye on them. Even if they don't turn out to be a problem, measuring the levels of these metals means scientists could use these pollutants as tracers to study environmental processes such as ocean circulation.

Since the 1970s, many cars have been fitted with catalytic converters to keep nitrogen oxides and carbon monoxide out of the air. This cuts down on smog and has huge health benefits. But catalytic converters created a demand for platinum, which has its own environmental impact. The smelting of platinum can release metals into the air, for example — particularly osmium tetroxide.

Platinum-group elements are also released from catalytic converters during driving, and are deposited on roadsides. Recent research suggests these metals can dissolve in water, and be absorbed by plants and animals, more readily than previously appreciated. That suggests platinum group metals "are a greater risk than once thought", says Clare Wiseman, coordinator of the Environment and Health programme at the University of Toronto's Centre for Environment in Canada. And they are not confined to the roadside: platinum and related metals have been found in Greenlandic ice from the 1990s at levels of up to 120 times higher than was present 7,000 years ago1. Their production is on the rise for use in high-tech components including fuel cells.

Now a report in the Proceedings of the National Academy of Sciences2 points to a global spread of osmium.

Osmium, osmium everywhere

The team publishing today's survey, led by Mukul Sharma of Dartmouth College in Hanover, New Hampshire, found a tiny amount of osmium in rainwater or snow in all their samples, from the United States to India, the Netherlands and the Antarctic. The amounts ranged from 0.25 to 23 femtograms (10-15 of a gram) per gram of water (about the same as 1 kilogram of coal as compared with all the coal on Earth). These tiny concentrations are the reason that environmental osmium has not been studied much before; Sharma and his group had to develop new techniques to measure it.

To determine the source, the researchers looked at the ratio of different isotopes, or atomic weights, of osmium in their samples. Their findings pointed to ore processing, car exhaust, cosmic dust and perhaps volcanic emissions as possible culprits. However, they reckon that cosmic dust contributes an insignificant amount, car exhaust emissions are small, and volcanoes probably produce osmium with a slightly higher isotopic ratio, leaving ore processing as the main probable contributor.

More than 200,000 kilograms of platinum are produced each year; if all the osmium in those ores winds up in the air during smelting, then every gram of rainwater would contain 9 femotgrams of osmium — an estimate that roughly matches the new results.

Cleaning up

The osmium could be removed by scrubbers that also extract sulphur dioxide from emissions. Such scrubbers are used in most base-metal smelters, and in the first stages of platinum production, but not always throughout the entire platinum process. In South Africa, where about three quarters of platinum is produced, the Air Quality Act only came into force in 2004, and the job of cleaning emissions is not yet done.

It's not clear, however, that the lack of scrubbers in platinum production is a problem, in terms of osmium release. "It's not a health issue in the way lead was; lead was very serious," says Sharma. "The osmium concentrations are so low we have trouble analysing them. It's more a curious thing for the people who are studying it."

Sharma has obtained a 200-year-old ice core, and plans to next determine how osmium pollution has changed over time. If there is a sharp point at which it was introduced, for example, researchers could use the compound as an environmental tracer.