Contaminants in the atmosphere appear to have harmful effects on neurodevelopment and cognitive function. When Lilian Calderón-Garcidueñas discovered abundant hallmarks of Alzheimer’s disease in a batch of human brain samples a few years ago, she initially wasn’t sure what to make of it. The University of Montana neuropathologist had been studying the brains as part of her research on environmental effects on neural development, and this particular set of samples came from autopsy examinations carried out on people who had died suddenly in Mexico City, where she used to work as a researcher and physician. Although Calderón-Garcidueñas had collected much of the tissue herself while attending the autopsies in Mexico, the light-microscope slides she was analyzing had been prepared by her colleagues, so she was in the dark about what patient each sample came from. By the end of the project, she’d identified accumulations of the Alzheimer’s disease–associated proteins amyloid-ß and ...