A new study suggests that near-death experiences are caused by high levels of carbon dioxide in the bloodstream. A caveat:

How carbon dioxide might actually interact with the brain to produce near-death sensations was beyond the scope of the study, so for now "the exact pathophysiological mechanism for this is not known," Klemenc-Ketis said. However, people who have inhaled excess carbon dioxide or have been at high altitudes, which can raise the blood's CO2 concentrations, have been known to have sensations similar to near-death experiences, she said.

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