You state that negative ions are found around a waterfall. Does rain or my shower produce negative ions?

Rain and your shower produce negative ions because they are generated by the Lenard effect where water splashes. Far more negative ions can be found in the basin of a waterfall because water is splashed more violently and humidity is lower. Negative ions can exist stably when humidity is 40-60%, while it is usually higher than this level when raining or in your shower room. Please also refer to "Why do many negative ions exist in a public bath or shower room?".