Almost all positive ("+") natural ions are produced by radioactivity and about 40% of these natural air ions come from radioactive minerals in the ground. Each time a radioactive atom decays near the air, it produces 50,000 - 500,000 air ion pairs.
Another 40% of the natural air ions are produced from radon in the air (which produces about 250,000 ion pairs for each radon atom), and 20% are produced from cosmic rays (high-energy protons from distant supernovas). Indoors, ions "live" an average of 30 seconds before touching a surface and shorting to ground.
Outdoor ions usually "live" several minutes more than indoor ions. Negative ions usually come from radioactivity and evaporating water. Lightning, thunderstorms, and forest fires can contribute "+" and "-" ions, though these ions are not produced via common everyday conditions.
Normal fair-weather ion concentrations are 200 to 800 negative and 250 to 1500 positive ions per cubic centimeter. Indoor levels are usually lower. Several hours before a storm, + ion concentration will increase dramatically, sometimes exceeding 5000 ions per cubic centimeter (cm3). During a storm, - ions increase to several thousand while + ions decrease, often to levels below 500.
Ions can also be produced by high-energy events, such as an open flame or a glowing hot object. Hot objects usually emit equal numbers of + and - ions. High DC voltage (over 1000 Volts), especially when connected to pointed metal edges or needles, will produce ions of the same polarity as the voltage source, which is the basis of home ionizers.
Evaporating water will produce - ions in the air and as a consequence leave + charges behind in the water that hasn't yet evaporated. If the excess + charges left behind are not conducted back to ground, the water will become + enough that - ion production will cease.
For example, a fountain that has a motor that plugs into the wall will continuously produce - ions (until the water runs out) but a battery operated fountain will stop producing - ions after a few minutes if the fountain is well insulated from ground.
The same is true of a battery-powered air ionizer. In general, for every 3 x 1013 water molecules that evaporate, one water molecule carries an excess - charge.
Since a large concentration of + ions can attract - ions, high concentrations of + and - ions are often found together.
Typically, a high concentration (1000 or more) of both may be found in one area outdoors while low concentration (300 or less) is typically found one city block away. A cloud of pure + ions (no - ions) with a concentration of 1000 ions/cm3 would be very unstable and would fall apart if its diameter were more than about 30m (100'). For this reason, high concentrations of exclusively + (or exclusively - ) ions tend to be compact, and don't extend more than about 30m.
The only exception is during storms, when strong atmospheric electric fields can maintain a high concentration of exclusively one ion polarity. While testing indoors, you may find high - in one area of a room and high + in another, since rooms are relatively small.
The life time of "fast" ions (the most common type of ions) is determined by how long they last before they collide with a solid (or dust) which usually neutralizes their charge. Indoors, electric fields are stronger than outdoors. Plastic surfaces charge to a typical potential of -1000 volts, which produces electric fields of 500-5000 volts per meter (V/m) near the plastic surface. The electric field repels negative ions (air molecules with an extra O- or OH-). The mobility of "fast" ions is about 1.2 x 10-4 m/s (meters per second) per V/m, so at an electric field of 2000 V/m, - ions are repelled at a speed of 2000 x 1.2 x 10-4 = 0.24 m/s. Positive ions (air molecules with an extra H+ or positive ammonia molecule) are attracted to the plastic by the same field and their mobility is slightly lower (about 1 x 10-4 m/s per V/m) so they have a slightly slower speed.
When the + ions touch the plastic, they give up their + charge, which partially neutralizes the - charge on the plastic.
Under typical conditions, complete neutralization of the - charge on the plastic would occur in a few weeks.
However, dust blowing by will rub against the plastic and acquire a + charge. This dust carries the + charge away (ultimately to Earth ground). As a result, the plastic always retains a negative charge. A good way to standardize (and lengthen) the lifetime of indoor ions is to put them in a large cardboard box. Lifetime in a one cubic foot box is about 50 seconds, regardless of humidity, so if, for example, 4 pCi/L of radon is in the box, it will produce a continuous 1600 + ions/cm3 in the box.
You can produce negative ions directly by combing your hair with a plastic comb. If you then blow air past the comb, the air will have between 1000 and 10,000 - ions/cm3 immediately next to the comb.
The number is lower in high humidity.
Also, your breath contains about 20,000 to 50,000 - ions/cm3 from the evaporating water, though you must be grounded to exhale a concentration this high. If you are insulated from ground, you will become more positively charged with each exhalation (by about five volts) since your breath is removing negative charge.
Eventually, you will become sufficiently positive (after exhaling about 20 times), that the negative ions will immediately return to you. This is the same effect that occurs in building cooling systems that use an evaporating water tower. If not properly grounded, the water pump and vents will become very positive. (If the inside vents are isolated from the evaporating water via a heat exchanger, the vents may become very positive and produce a large number of + ions, which can be corrected simply by grounding the vent).
Indoors, near ground level or in the basement, most + ions come from radon.
The number of ions is directly proportional to radon concentration multiplied by average ion lifetime. (Strong electric fields indoors will reduce the ion lifetime.)
Since it is unlikely that a high level of 1000 + ions/cm3, (or 1.00 on the Air Ion Counter) can come from anything else other than flame, smoke, or a hot electric heating element, it is probable that 1000 + ions/cm3 in a basement signifies the presence of at least 4 pCi/L (picocuries per liter) of radon (2000 ions/cm3 = 8 pCi/L, etc.) Four pCi/L is the maximum allowable amount / limit in the U.S.
If radon is the source of the ions, then the concentration of ions will be approximately equal throughout the basement.
If, instead, it is +1000 near a hot water heater but only 100 + ions/cm3 elsewhere, radon is not the source. A slightly higher concentration of + ions near cracks in the concrete foundation or near corners indicate the radon is entering into the room / area / basement. If the average + ion count is low (for example, less than 100), then there is essentially no radon present. It is not possible to "hide" the ions that radon produces. "No ions" means "no radon".
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