Diesel motor emission is a complex mixture of hundreds of constituents in either gas or particle form. Diesel particulate matter (DPM) is composed of a center core of elemental carbon and adsorbed organic compounds including PAHs and nitro-PAHs, and small amounts of sulfate, nitrate, metals, and other trace elements. DPM consists of fine particles including a high number of ultrafine particles. These particles are highly respirable and have a large surface area where organics can adsorb easily. Exposure to DPM can cause acute irritation and neurophysiological, respiratory, and asthma-like symptoms and can exacerbate allergenic responses to known allergens. Consistently, lung cancer risk is elevated among workers in occupations where diesel engines have been used. However, quantification of the cancer risk with respect to DPM concentrations is not possible. Furthermore, ambient fine and ultrafine particles, of which DPM is an important component, contribute to cardiopulmonary morbidity and mortality and lung cancer. In conclusion, diesel exhaust poses a cancer risk greater than that of any other air pollutant, as well as causing other short- and long-term health problems. One effective way to effectively reduce emission of DPM is the use of particle traps.