Devices have been developed to reduce particle emissions from vehicles with diesel engines, such as a trap that filters the particles from the exhaust. Periodically, the trap is cleaned (regenerated) by electric heating, thereby burning the particles before they can clog the trap. There is concern that potentially harmful chemicals associated with the particles may be emitted from the trap during normal use and regeneration. Dr. Susan Bagley and colleagues at Michigan Technical University determined whether a ceramic trap, during normal use and during regeneration, alters diesel emissions in any way that might pose a threat to human health. The investigators developed procedures (1) to collect and measure the number and size of diesel exhaust particles, (2) to determine the concentrations and types of chemicals bound to the particles and present in the exhaust gas, and (3) to measure the ability of the chemicals to present in the gas or extracted from the particles (called particle extract) to cause mutations in bacteria. They used these procedures to characterize exhaust from a heavy-duty 1988 Cummins engine that was equipped with a Donaldson ceramic particle trap system designed to meet the 1994 standards for particle emissions.