Clean-air campaigners can't always pin down the source of a pollutant, by no means an easy task at best. Now, researchers at the Massachusetts Institute of Technology have devised a way to solve the problem. They are studying micro-structures in soot that should, in theory, trace each particle's history and source.

The MIT team runs electron micrographs of soot through computers to generate details such as spacing among graphite layers of the carbon particles, which are peculiar to a particular source. With a large enough database, says team leader Adel F Sarofim, such signatures could one day even identify the specific carbons that, say, triggered a patient's lung cancer.