The most continuous and intimate contact the average person has with nanoparticles is almost surely through the air, which is replete with them. Nanoparticles are being generated continuously and in large numbers by vehicles and industries in urban areas and by vegetation and sea spray in rural areas. Volcanoes are sporadic sources of huge numbers. Nanoparticles have large surface area to volume ratios and react rapidly in the atmosphere, commonly growing into particles large enough to interact with radiation and to have serious consequences for visibility and local, regional, and global climate. They also have potentially significant health effects.