What: Particulate filters capture the pieces of soot that diesel engines emit. Think of them as a form of catalytic converter.
Why: Diesel particulates cause the dense clouds of dark smoke that make older diesel vehicles the auto industry's most visible polluters. More ominously, the California Air Resources Board has declared diesel particulates to be a carcinogen.
How: A fine web of some material traps the soot in a canister. Then, to keep the filter from clogging, the soot is burned off periodically at temperatures ranging from 750 degrees F to 1,650 degrees F.
When: Some systems are available today, but widespread commercialization is still a few years away.
How much: Ah, there's the rub - traps cost about 10 times more than catalytic converters. For example, Mitsubishi Motors Corp. offers a particulate filter for buses in Japan that costs $4,500, including installation. Prices probably will come down as the technology improves, but traps won't be cheap anytime soon.