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 its countertops were crammed with instruments for monitoring airborne particles: a condensation-nucleus counter, a differential-mobility analyzer, and so on. Wires threaded all around the room, and stainless-steel hoses led to four trailers outside, which contained equipment too big to fit in the kitchen.

Andrew Abeleira, a postdoctoral researcher, cracked eight eggs on the edge of the countertop and whisked them; Vance chopped tomatoes while heating oil to fry sausage patties. The banality of the activities was belied by the precision with which the team carried them out: a rigid protocol dictated when each gas burner could be lit, how hot the frying pan should be, and at what setting to toast the bread. The aim was to turn Thanksgiving into a reproducible, scientifically valid experiment.

Tapping a pair of tongs on the cooktop, Vance wondered aloud whether it was nine-twenty yet, the appointed time for switching on the coffeemaker. “Oh, shoot, toast!” she exclaimed, popping two slices of honey-wheat in the toaster. A minute later, a student volunteer named Caleb Arata, looking at data on his laptop, announced a spike in the presence of so-called volatile organic compounds. The term describes any carbon-based chemical that evaporates at room temperature, and it encompasses a huge variety of molecules, emitted both by plants and by human activities. VOCs are responsible for much of what we smell—toast, flowers, gasoline—although some have no odor at all. And, while certain of them, such as benzene and toluene, are known to be harmful when inhaled, for the most part their health effects have not been studied.

“The scariest thing in this house is probably the toaster,” Erin Katz, another student volunteer, said. “I just had no idea that toasters emitted so many particles.”

After breakfast, the serious work began: peeling sweet potatoes, trimming Brussels sprouts, simmering turkey parts to make a stock for gravy. Culinary ambition had not been sacrificed to scientific rigor: Arata had spatchcocked the turkey and dry-brined it for two days; Abeleira tossed the sprouts in balsamic dressing; Katz downloaded a recipe for sweet-potato casserole from a foodie Web site. The oven stayed on for five hours straight, the burners in constant rotation. Vivaldi’s “Four Seasons” played from a Bluetooth speaker, and the four cooks began to sweat, the air-conditioning system unequal to all the activity.

While stirring, scrubbing, and basting, the cooks darted back and forth between the kitchen and their laptops, in the dining area. Every action, however seemingly inconsequential, had to be logged: opening the oven door, changing the trash bag, even a bout of sneezing. At 1:37 P.M., the team briefly debated whether to set fire to an oven mitt; one had accidentally caught light at that time during the previous Thanksgiving, and, as responsible scientists, they were keen to insure that the data sets from the two days matched. Eventually, they decided that the integrity of their experiment wouldn’t be fatally compromised if they failed to sacrifice a second mitt.

The conversation turned into a kind of play-by-play pollution commentary. When Vance peeled an orange for the cranberry sauce, Arata noted that its fragrance—that is, its monoterpene VOCs—had made the readings on his instrument soar. Abeleira, checking levels of nitric oxide and carbon dioxide during a brief lull before the turkey went in, observed, “They’re orders of magnitude higher than outdoors.” It was the same for fine particulate matter—particles small enough to reach deep inside our lungs. By around eleven o’clock, the fine-particulate concentration had risen to such a level that, if the house were a city, it would have been officially labelled polluted. Concentrations peaked when the stuffing, and, later, the pies, came out of the oven. And, for nearly an hour, fine particulate matter was within the range that the Environmental Protection Agency’s Air Quality Index defines as “very unhealthy.” If outdoor air reaches these levels, a public alert is triggered, warning that even healthy individuals are at risk of serious damage to the heart and lungs.

These days, a “very unhealthy” designation for outdoor air is rare. After the passage of the Clean Air Act, in 1963, and the creation of the Environmental Protection Agency, in 1970, the chemical composition of outdoor air became federally regulated, with penalties for polluters. Since the seventies, emissions of many harmful gases, such as carbon monoxide and sulfur dioxide, have fallen by half, and particulate counts by eighty per cent. But this victory may be less significant than we assume, because, in America, we spend, on average, ninety per cent of our lives indoors. (By way of comparison, this means that humans spend more time inside buildings than sperm whales spend fully submerged in the ocean.) The statistic, from an E.P.A.-funded study conducted in 2001, might seem implausible, but it probably understates the case. More recent data, from the U.K., show that, on average, Britons are outside for just five per cent of the day—an hour and twelve minutes.

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Unlike outdoor air, the air inside our homes is largely unregulated and has been all but ignored by researchers. We know barely the first thing about the atmospheres in which we spend the vast majority of our time. Homechem—House Observations of Microbial and Environmental Chemistry—was the world’s first large-scale collaborative investigation into the chemistry of indoor air. Thoroughly dissecting the data accumulated will take a couple of years, at least, and, even when the findings are published, no one will be able to state their public-health implications with certainty; Homechem was designed to explore what the chemistry of indoor air is, not what it’s doing to us. But the experiment’s early results are just now emerging, and they seem to show that the combined emissions of humans and their daily activities—cooking, cleaning, metabolizing—are more interesting, and potentially more lethal, than anyone had imagined.

In September, 1776, Congress sent Benjamin Franklin and John Adams on an ultimately fruitless mission to Staten Island to negotiate peace with the British. One night, the two shared a room at a country inn, an adventure recorded in Adams’s diary. Adams, “who was an invalid and afraid of the Air in the night,” shut the window. To which Franklin responded, briskly, “The Air within this Chamber will soon be, and indeed is now worse than that without Doors: come! open the Window and come to bed, and I will convince you.”