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The science is, at first glance, surprisingly fundamental. Researchers have long believed that carbon dioxide harms the brain at very high concentrations. Anyone who’s seen the film Apollo 13 (or knows the real-life story behind it) may remember a moment when the mission’s three astronauts watch a gauge monitoring their cabin start to report dangerous levels of a gas. That gauge was measuring carbon dioxide. As one of the film’s NASA engineers remarks, if CO₂ levels rise too high, “you get impaired judgement, blackouts, the beginning of brain asphyxia.”

“They knew they’re going to go nuts and not do 2+2 if that gauge gets too high,” Karnauskas said. The same general principle, he argues, could soon affect people here on Earth. Two centuries of rampant fossil-fuel use have already spiked the amount of CO₂ in the atmosphere from about 280 parts per million before the Industrial Revolution to about 410 parts per million today. For Earth as a whole, that pollution traps heat in the atmosphere and causes climate change. But more locally, it also sets a baseline for indoor levels of carbon dioxide: You cannot ventilate a room’s carbon-dioxide levels below the global average.

In fact, many rooms have a much higher CO₂ level than the atmosphere, since ventilation systems don’t work perfectly. On top of that, some rooms—in places such as offices, hospitals, and schools—are filled with many breathing people, that is, many people who are themselves exhaling carbon dioxide. As Karnauskas said: “We’re little CO₂-producing machines ourselves.”

“Imagine a conference room,” he said. “You have middle-aged people—20 of them—sitting in a small room, breathing. That CO₂ easily exceeds 1,000 parts per million.”

And that leads to the final part of his and his colleagues’ argument: As the amount of atmospheric CO₂ keeps rising, indoor CO₂ will climb as well. They project that, in a worst-case emissions scenario, it may be impossible to ventilate a crowded room below about 1,300 parts per million. That could induce some real cognitive damage. In 2016, researchers at Harvard and Syracuse University found that human cognitive function declined by about 15 percent when indoor CO₂ reached 945 parts per million, and crashed by 50 percent when indoor CO₂ reached 1,400 parts per million.

Under a very high carbon-emissions scenario, “our complex decision-making functions could be reduced by as much as half by the end of the century,” Karnauskas said.

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He and his colleagues admit that their calculations are back of the envelope. “There’s got to be a lot more work on this,” he told me. And I had to wonder: Is this for real? Why hadn’t I heard about it before? Is carbon pollution not only heating the planet but actually making us more sluggish thinkers?