Fire is possible because oxygen is so reactive, but the secret to its continued success is that there is the “right” amount of it relative to other atmospheric gases. Fire has a hard time sustaining itself when oxygen levels fall below an atmospheric concentration of 15 percent. Meanwhile, in concentrations above 35 percent, dinner by candlelight would be ill-advised. Currently, atmospheric oxygen sits in the Goldilocks zone, just shy of 21 percent: exactly what we need to live and prosper, and exactly what fire needs to burn in ways that have proven extraordinarily beneficial to us—most of
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