Nanoparticles are loosely defined as particles with diameters in the range of 1 nm to 50 or 100 nm (nanometers). Nanoparticles are bigger than air molecules (0.3 nm), but are smaller than the upper limits regulated by ambient air quality standards. (U.S. National Ambient Air Quality Standards regulate the mass of particles with diameters less than 2500 and 10,000 nm, PM2.5 and PM10, respectively.) Although nanoparticles do not contributelarge quantities to PM2.5 or PM10 mass, they dominate the number concentration and most of its surface area. Nanoparticles are produced by condensation of hot vapors in fresh combustion emissions. They also form from natural and manmade gases as secondary aerosol by photochemical oxidation of gaseous compounds. Nanoparticles may contain transition metals, organic material, sulfuric acid, and free radicals. Owing to their small size and high mobility, they diffuse rapidly and may combine with each other, with larger particles, and with nearby deposition surfaces. Owing to their short lifetimes and low mass concentrations, nanoparticles are not conveniently measured in source emissions and ambient air. For similar reasons, they are not easily generated for exposure, inhalation, and toxicological studies to determine their potential adverse effects on human health.