That made sense, in that humans are bags of gas. As breathing people know, we tend to emit carbon dioxide. (Though each exhalation still contains about four times as much oxygen as carbon dioxide.) And there are many subtler ingredients in the concoctions we breathe out. So Williams began to wonder, are these gases “significant on a global scale”? Could they be, even, contributing to climate change? Especially as the number of humans on Earth rockets toward 8 billion?

The answer was no. Just a clear, simple no. By measuring gases in soccer stadiums, the Planck chemists found no consequence of human breath. There might be some effect at a global scale, but it’s just nothing compared to the air-ravaging effects of transportation and agriculture.

But Williams didn’t come away from the stadium empty handed. As he sat and watched the fluctuating readings on the air sensors, he got an idea. In the manner of a typical European soccer crowd, the people went through fits of elation and anger, joy and sorrow. So Williams began to wonder, as he later put it to me, “Do people emit gases as a function of their emotions?”

If we do, it wouldn’t be unprecedented. Tear some leaves off of a tree, for example, and it will emit chemical signals that may be part of a system of communication between trees. The behavior for bees and ants is clearly chemically dominated.

“We’re not like that—not like robots following chemicals,” Williams explained. “But it could be possible that we are influenced by chemicals emitted by other humans.”

The idea of airborne pheromones—chemicals that specifically influence mating behaviors— has been a source of much fascination, but the actual evidence is weak. Some small studies have suggested an effect when people put cotton balls under their armpits, and then other people smell the balls—but in minor, unreliable ways.

“I don't know why so many previous researchers have been so into armpits,” said Williams. “A much better way to communicate would be through your breath. Because you can direct your breath, and your breath is at roughly the same height as the person you’re trying to communicate to, silently. In the dark, maybe, in your cave.” And if these behavior-modifying volatile chemicals exist (volatile meaning anything that goes into the air), then why would they be limited to sex? Why shouldn’t we be able to signal fear or anxiety? It is true that birds seem to know that I'm afraid of them.

Williams was so intrigued by the idea of gases and emotion that he designed another experiment—something more predictable than a German soccer game. This time he used a movie theater. Unlike the open-air stadium, the theater presented fewer variables. “You’ve got this box, the cinema, and you spool through air from outside at a continuous rate, and you have 250 people sitting there, not moving. And you show them all, simultaneously, something that should make them frightened or anxious or sad, or whatever.”