SCIENTISTS have discovered what they believe to be the world's most cancer-causing substance in diesel fumes. The discovery in Japan raises new questions about the environmental impact of diesel and its use by environmentally-conscious fleet operators.

Researchers found the compound 3-nitrobenzanthrone in exhaust fumes from trucks and in the air above Tokyo. They also discovered that the substance caused abnormal changes in the cells of mice. The scientists also carried out tests, reported in New Scientist, to see how many mutations the compound caused on the DNA of bacteria.

In a test on salmonella it caused 20% more mutations than the next most carcinogenic compound - also found in diesel.

Kyoto University's Hitomi Suzuki, who led the experiment, said that although both substances were only found in tiny amounts they were so toxic that they could be linked to the recent increase in the number of cases of lung cancer in areas which suffered vehicle congestion. Suzuki also noted, however, that main culprits for producing hazardous emissions were heavily-laden lorries and called for tight limits on the loads trucks were allowed to carry.