Ice core sampling in the Himalayas has shown that international agreements are effective in reducing the prevalence of toxic persistent organic pollutants.

Scientists from the U.K.'s Lancaster University, working with colleagues from China and Germany, collected and analyzed samples from snow and ice cores that had been laid down over 30 years. They wanted to see how residues of Perfluoroalkyl substances (PFASs) in the environment have changed over time.

Chemical residues are often carried thousands of miles on the prevailing winds before being deposited in the ice, building up year on year.

The ice cores came from two sites: one on the eastern end of the Tibetan Plateau where the prevailing wind comes from Asia, and the other on the western end with prevailing winds from Europe. As a result, the researchers were able to examine the effect of changes in policy and production of these chemicals in the different regions over time.

"PFASs are used in many everyday products such as fabric linings, non-stick pans and firefighting foams," said Dr. Crispin Halsall from Lancaster University's Environment Centre.

"They are very persistent in the environment and some can accumulate in living things. Many of these compounds are thought to be toxic to humans: a recent study has shown very strong evidence that one, PFOA, retards human fetal growth."

The ice cores on the western side showed deposits of PFASs increasing in the 1980s as they were used more and more in European industry, but then declining as industrial output changed and industry agreed, largely voluntarily, to phase out certain persistent organic pollutants.

By contrast, the level of residues on the eastern side is still increasing as Asia becomes more industrialized, but the nature of the PFASs has changed.

Dr. Halsall explained: "Some of the long carbon chain compounds are particularly toxic and industry has responded by producing shorter chain compounds, and we are starting to see that in the ice cores. Good practice in industry is having an impact.

"It provides the evidence that you can change things and reduce the prevalence of toxic chemicals through consensus and an international framework. That is very encouraging."