Biologically formed nanoparticles of the strongly magnetic mineral, magnetite, were first detected in the human brain over 20 y ago [Kirschvink JL, Kobayashi-Kirschvink A, Woodford BJ (1992) Proc Natl Acad Sci USA 89(16):7683–7687]. Magnetite can have potentially large impacts on the brain due to its unique combination of redox activity, surface charge, and strongly magnetic behavior. We used magnetic analyses and electron microscopy to identify the abundant presence in the brain of magnetite nanoparticles that are consistent with high-temperature formation, suggesting, therefore, an external, not internal, source. Comprising a separate nanoparticle population from the euhedral particles ascribed to endogenous sources, these brain magnetites are often found with other transition metal nanoparticles, and they display rounded crystal morphologies and fused surface textures, reflecting crystallization upon cooling from an initially heated, iron-bearing source material. Such high-temperature magnetite nanospheres are ubiquitous and abundant in airborne particulate matter pollution. They arise as combustion-derived, iron-rich particles, often associated with other transition metal particles, which condense and/or oxidize upon airborne release. Those magnetite pollutant particles which are <∼200 nm in diameter can enter the brain directly via the olfactory bulb. Their presence proves that externally sourced iron-bearing nanoparticles, rather than their soluble compounds, can be transported directly into the brain, where they may pose hazard to human health.

Significance: In the famous Chinese Loess Plateau (CLP), weakly magnetic, windblown dust layers alternate with variably magnetic fossil soils, recording monsoonal variations through the last ∼3 My. The soils contain strongly magnetic iron oxides, formed in situ, the mineralogy and paleoclimatic significance of which are controversial. Reduction of iron to form Fe ²⁺ -bearing magnetite has been linked to soil wetting/drying. Conversely, oxidation of iron to form Fe ³⁺ -bearing maghemite, which ages into Al-substituted hematite, has been linked to paleotemperature. This study uses structural fingerprinting and electron energy loss spectroscopy to resolve this debate, proving that magnetite is the dominant soil-formed magnet. The magnetic variations of the CLP paleosols thus record changes in monsoonal rainfall, providing a key time series for testing of general circulation climate models.