Dr. Mauderly and coworkers exposed F344/N rats to clean air or to one of two levels (2.5 or 6.5 mg of particles/m3 of diesel exhaust or air) of either emissions from a light-duty diesel engine or carbon black particles. The exposures lasted for 16 hours/day, 5 days/week, for 24 months. The carbon black particles were similar to the soot particles in the diesel engine exhaust; however, they contained markedly lower amounts of adsorbed organic compounds. The investigators determined the number and types of tumors that formed in the rats, and assessed the possible contributions of a number of factors (such as tissue injury, or clearance or translocation of inhaled particles) known to be linked with the development and progression of lung cancer. They also ascertained whether the exposures resulted in the formation of DNA adducts (the products of chemicals or their metabolites reacting with the DNA) in lung tissue or in cells isolated from lungs.