Vehicle Emissions: Gases, fumes, vapors, and odors escaping from the cylinders of a gasoline or diesel internal-combustion engine. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed & Random House Unabridged Dictionary, 2d ed)Motor Vehicles: AUTOMOBILES, trucks, buses, or similar engine-driven conveyances. (From Random House Unabridged Dictionary, 2d ed)Air Pollutants: Any substance in the air which could, if present in high enough concentration, harm humans, animals, vegetation or material. Substances include GASES; PARTICULATE MATTER; and volatile ORGANIC CHEMICALS.Air Pollution: The presence of contaminants or pollutant substances in the air (AIR POLLUTANTS) that interfere with human health or welfare, or produce other harmful environmental effects. The substances may include GASES; PARTICULATE MATTER; or volatile ORGANIC CHEMICALS.Pharmaceutical Vehicles: A carrier or inert medium used as a solvent (or diluent) in which the medicinally active agent is formulated and or administered. (Dictionary of Pharmacy, 1986)Positron-Emission Tomography: An imaging technique using compounds labelled with short-lived positron-emitting radionuclides (such as carbon-11, nitrogen-13, oxygen-15 and fluorine-18) to measure cell metabolism. It has been useful in study of soft tissues such as CANCER; CARDIOVASCULAR SYSTEM; and brain. SINGLE-PHOTON EMISSION-COMPUTED TOMOGRAPHY is closely related to positron emission tomography, but uses isotopes with longer half-lives and resolution is lower.Otoacoustic Emissions, Spontaneous: Self-generated faint acoustic signals from the inner ear (COCHLEA) without external stimulation. These faint signals can be recorded in the EAR CANAL and are indications of active OUTER AUDITORY HAIR CELLS. Spontaneous otoacoustic emissions are found in all classes of land vertebrates.