Enlarge (credit: Getty | Zhang Peng)

Our noggins naturally catch and collect the toxic, magnetic nanoparticles that we inadvertently inhale from polluted air, according to a study published Monday in the Proceedings of the National Academy of Sciences. Those wee particles, made of the strongly magnetic iron oxide compound, magnetite, have been found in human brains before and were thought to be normal and harmless byproducts of biological processes. But according to the new study, a closer examination of minuscule metal balls in 37 human brains revealed that they’re actually from smog, formed during combustion or friction-derived heating, such as slamming on the brakes of a car.

Whether the particles are harmful is hazy, but the authors note that the nanomagnets have two troubling features: they can interact with misfolded proteins associated with Alzheimer’s disease to produce reactive oxygen species, which can severely damage cells; and large amounts of them in the brain correlated with Alzheimer’s disease in earlier studies. Given these potential risks, the authors—a team of researchers from the UK and Mexico—suggest that exposure to them “might need to be examined as a possible hazard to human health.”