Researchers have found a clue to the enigma of how asbestos triggers cancer, and it may lead to a way to screen asbestos substitutes for their cancer-causing potential, a scientist says.

Test-tube experiments showed that asbestos fibers can bind to genetic material outside a cell and insert the material into the cell, damaging the genetic material in the process.If that happens in people, it may help explain why exposure to asbestos causes an increased risk of lung cancer and of a cancer of the chest lining called mesothelioma, researcher Edward Johnson said Thursday.

But an asbestos expert said she questioned how relevant the test-tube findings are for human cancer.

The work is reported in the new issue of the Proceedings of the National Academy of Sciences by Johnson and his colleagues at the Mount Sinai School of Medicine in New York.

It dealt with deoxyribonucleic acid, called DNA, which is the substance that makes up genes. Every cell of the body contains a set of genes, which help direct its activities.

Cancer can result if genes are damaged or if certain ones are activated inappropriately.

In a series of test-tube experiments, researchers found that asbestos fibers can bind to DNA outside a cell and introduce the DNA to the cell's interior. Some of the DNA inserted itself among the cell's own genes, and at least some of it reproduced itself.

A known cancer-causing gene in the introduced DNA was found to be active after entering cells, and a second gene remained active 18 days after entering.

Johnson suggested that in people, the introduced DNA may trigger cancer when it is inserted among a cell's genes. Or the DNA itself may be damaged in such a way that it could trigger cancer, since asbestos was found to damage the DNA it inserted, he said.

The insertion may also trigger the cell's natural mechanisms for DNA repair, which may end up making mistakes in repair that lead to cancer, Johnson said.

The work was done with monkey kidney cells to aid in testing for reproduction of DNA. The experiments were not set up to show whether the cells would become cancerous.