Iron is known to be toxic to brain cells, and tiny magnetic iron particles (magnetite) are thought to be involved in the development of neurological disorders. Now, for the first time, we have identified the abundant presence of these highly reactive particles in human brains.Previous studies have suggested that there are increased amounts of magnetite in Alzheimer’s-affected brains, and that these particles may be linked with the development of the disease. We wondered if this increased brain magnetite might come from inhaling polluted air.Very small, round particles made out of magnetite (called magnetite nanospheres) are abundant in city air pollution. They are formed at high temperatures and condense as iron-rich droplets as they cool. These particles range in diameter from less than 5nm (nanometres) to more than 100nm (for comparison an HIV is 120nm in diameter) and are often found together with pollution particles made out of other metals.Vehicles are a major source of these magnetite nanospheres. They are created by fuel combustion (especially diesel), iron wear from the engine block and frictional heating from brake pads. In addition to some occupational settings, high concentrations of magnetite pollution nanoparticles may be produced indoors by open fires or poorly-sealed stoves used for cooking or heating.