Each time a radioactive atom decays near the air, it produces 50,000 - 500,000 air ion pairs. Negative ions usually come from radioactivity and evaporating water. Normal fair-weather ion concentrations are 200 to 800 negative and 250 to 1500 positive ions per cubic centimeter.
Indoor levels are usually lower.
Ions can also be produced by high-energy events, such as an open flame or a glowing hot object. This is the basis of home ionizers. The same is true of a battery-powered air ionizer. The only exception is during storms, when strong atmospheric electric fields can maintain a high concentration of exclusively one ion polarity. Indoors, electric fields are stronger than outdoors.