负离子是在一定的条件下产生的。例如，由于强烈的紫外线和宇宙射线照射，外层电子吸收能量成为自由电子；大气中闪电、强电场，岩石、土壤中的射线也能使空气发生电离；瀑布、喷泉、海浪、雨水等会使水与空气摩擦而产生自由电子。这些自由电子，如果被氧分子捕捉后就会产生大量的负氧离子。
Negative ions are produced under certain conditions. For example, due to intense ultraviolet and cosmic radiation, outer electrons absorb energy into free electrons; lightning in the atmosphere, strong electric field, radiation in rocks and soils can also ionize the air; waterfalls, fountains, sea waves, rainwater and so on can make water and air friction and produce free electrons. These free electrons, if captured by oxygen molecules, will produce a large number of negative oxygen ions.