Listen carefully, though, and he often addresses debates that transcend his day-to-day work, escaping the realm of science altogether and drifting toward the metaphysical: Is the human aversion to putrid smells nature, or nurture, or both? How can one measure a perception? And how do you give people the confidence in their noses that they have in their eyes and ears?

What the Nose Can Tell Us

A smell is, quite simply, a result of chemicals in the air, and the human nose is far better at detecting them than it often gets credit for. Some of the most recognizable and potent odors, like hydrogen sulfide (think rotten egg) can be sensed at even the tiniest concentrations, like 1 part per billion.

“If you were to map out the distance from New York to Los Angeles, 1 part per billion would account for only a few inches along that route,” Dr. Koziel of Iowa State said.

That fact also captures the difficulty of regulating odors. At such vanishingly small concentrations, hydrogen sulfide is unlikely to pose a health risk. Nevertheless “it’s very disruptive to people,” said Susan Schiffman, a clinical psychologist who has studied odor and taste for half a century.

Despite having the power to sicken, there are few laws in the United States to regulate odor. It makes up a significant portion of complaints to public agencies, including a quarter of the complaints to the federal Agency for Toxic Substances and Disease Registry. Yet there is debate over whether a smell can be inherently dangerous.

“It’s one thing to measure emissions, but odor is a sensation. Because it can be experienced so differently by so many people, it puts us in a bind about how we regulate,” said Pamela Dalton, a psychologist who studies odor perception at the Monell Chemical Senses Center in Philadelphia. “Any industry has the potential for off-site emissions, even a cookie factory,” she added.