in one 2016 study Danish scientists cranked up indoor carbon-dioxide levels to 3,000 parts per million—more than seven times outdoor levels today—and found that their 25 subjects suffered no cognitive impairment or health issues. Only when scientists infused that same air with other trace chemicals and organic compounds emitted by the human body did the subjects begin to struggle, reporting “headache, fatigue, sleepiness, and difficulty in thinking clearly.” The subjects also took longer to solve basic math problems. The same lab, in another study, found that indoor concentrations of pure CO₂ could get to 5,000 parts per million and still cause little difficulty, at least for college students.