Food magazines typically celebrate Thanksgiving in mid-July, bronzing turkeys and crimping piecrust four months in advance. By that time last year, Marina Vance, an environmental engineer at the University of Colorado Boulder, had already prepared two full Thanksgiving dinners for more than a dozen people. Vance studies air quality, and, last June, she was one of two scientists in charge of Homechem, a four-week orgy of cooking, cleaning, and emissions measurement, which brought sixty scientists and four and a half million dollars’ worth of high-tech instrumentation to a ranch house on the engineering campus of the University of Texas at Austin. The two Thanksgiving dinners were the climax of the project and represented what Vance called a “worst-case scenario.” She suspected that the Pilgrims’ harvest celebration, as it is observed in twenty-first-century America, qualified as an airborne toxic event. The morning of the second simulated Thanksgiving began simply enough, with the researchers making themselves breakfast. Vance and three helpers arrived at the house at half past eight. The kitchen was open plan and modest, with peeling laminate surfaces and flimsy cabinets, but it...