In a town shadowed by highway exhaust, a quiet experiment asked whether the air inside our homes shapes the mind within them. Researchers in Somerville, Massachusetts found that adults over 40 who breathed air filtered by HEPA purifiers for a month performed measurably better on tests of mental flexibility than when breathing unfiltered indoor air — a 12 percent gain, modest in scale but significant in implication. As particulate pollution grows increasingly linked to Alzheimer's, Parkinson's, and cognitive decline, this study places air quality alongside sleep, diet, and exercise as a modifia