Dr. Witten and colleagues investigated the inflammatory effects of diesel exhaust exposure on rat airways. The investigators focused on the role of neurogenic inflammation, an inflammatory response defined by the release of neuropeptides, such as substance P (SP), from sensory nerve fibers known as C fibers located within the lung tissue. Neurogenic inflammation has been implicated in responses to inhaled irritants such as ozone and cigarette smoke and has been implied to play a role in asthma. The investigators exposed female rats (8 weeks old) to two concentrations of whole diesel exhaust emissions (35 and 630 µg/m3 particulate matter) from a heavy-duty 1990s Cummins research engine. Exposures were conducted over 3 weeks (4 hr/day, 5 days/week); neurogenic and other inflammatory markers were measured immediately after the end of exposure. Half of the rats in each exposure group were pretreated with capsaicin, a neurotoxin that depletes sensory C fibers of neuropeptides and thereby inhibits the neurogenic inflammatory pathway.