Magnetite is an iron-oxide mineral that occurs naturally on Earth. Because it is also an important component of many anthropogenic materials (e.g., coal fly ash) and synthetic products (e.g., black toner powders), magnetite can be released to the environment through human activities (1). In PNAS,Maher et al. (2) describe the abundant presence in the human brain of magnetite nanoparticles, some of which they attribute to air pollution. This finding could have major implications.