Air Board Delays Vote on Hazards of Diesel Fumes

The crux of the debate is over the state's effort to quantify the cancer threat to all Californians.

Most scientists agree that diesel exhaust does increase cancer in many occupational settings in which people breathe high concentrations.

Diesel exhaust contains a mix of thousands of compounds, including more than 40 that have been declared carcinogenic.

But some scientists say that the California scientific panel has gone too far out on a limb by estimating the number of cancers among people in the general population--who are exposed just from breathing ambient air while driving on freeways, walking near intersections, or sitting at bus stops.

Industry representatives say that if the state board adopts the cancer estimate, it makes the science sound conclusive when it remains highly uncertain because of holes in the research.

"Something with this huge of an implication for the economy of the state of California should not be a back-of-the-envelope calculation," said William Bunn, medical director of Navistar International, the largest diesel engine manufacturer. "Before you say ambient air causes cancer, you should be very certain, and we don't think that evidence is there."