The air in Mexico City was once so toxic that people watched as dead birds fell out of the sky. In 1992, the United Nations declared the city the most polluted in the world, with its unregulated diesel engines, factory production, fossil-fuel powered energy plants, and widespread use of internal-combustion engines, all trapped in a high-altitude, mountain-lined valley.

In 2002, toxicologist and neuropathologist Lilian Calderón-Garcidueñas, who grew up east of the metropolis and attended medical school in the city, decided to look at the brains of 40 dogs that had lived either in the city’s polluted valley or in the cleaner air of Tlaxcala, a state on its eastern edge. The results were striking: The rural dogs’ brains appeared healthy. In the brains of the city dogs, however, she saw the beginnings of neurodegeneration in puppies as young as eight months old.

The findings prompted Calderón-Garcidueñas, who has joint appointments...