Table 1.

UFPs/NPs (< 100 nm), natural and anthropogenic sources.

	Anthropogenic
Natural	Unintentional	Intentional (NPs)
Gas-to-particle conversions	Internal combustion engines	Controlled size and shape, designed for functionality
Forest fires	Power plants
Volcanoes (hot lava)	Incinerators	Metals, semiconductors, metal oxides, carbon, polymers
Viruses	Jet engines
Biogenic magnetite: magnetotactic	Metal fumes (smelting, welding, etc.)	Nanospheres, -wires, -needles, -tubes, -shells, -rings, -platelets
bacteria protoctists, mollusks, arthropods, fish, birds	Polymer fumes
human brain, meteorite (?)	Other fumes	Untreated, coated (nanotechnology applied to many products: cosmetics, medical, fabrics, electronics, optics, displays, etc.)
Ferritin (12.5 nm)	Heated surfaces
Microparticles (< 100 nm; activated cells)	Frying, broiling, grilling Electric motors