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What is an Air Ion?

 

When air molecules capture one or more electrons, they become negative ions, called NAIs. When they lose one electron, they become positive ions, named PAIs.

 

Oxygen (O2) molecules tends to bind electrons and consequently to build NAIs.
N2, CO2, and H2O molecules tend to lose one electron and consequently to build PAIs.

 

Superoxide (O2.-) is the main NAI produced by air and slowly crosses cell membranes, except the erythrocyte membrane, which has an “anion channel”.

 

Two superoxide ions can slowly undergo a dismutation reaction, building hydrogen peroxide (H2O2), a more stable radical that can easily cross cell membranes (Halliwell, 1984).

 

2 O2.- + 2H+  =>  H2O2 + O2

 

This dismutation reaction is accelerated in the human body thanks to an enzyme, the superoxide dismutase (Kondrashova, 2000a).

 

Superoxide has a pKa of 4.7. In neutral pH solutions, like human blood (pH7.4), it reacts with water molecules to produce perhydroxyl (HO2.-) and hydroxyl (HO.-) radicals. In presence of ozone, it forms hydroxyl radicals:

 

2 O2.- + H2O  => O2 + HO2.- + HO.-

 

O2.- + O3 + H2O => 2O2+ 2HO.-

 

Superoxide (SO) and hydrogen peroxide (H2O2), in presence of metallic ions (Cu2+, Fe2+) and/or sunlight, can produce highly reactive hydroxyl radical (HO.-).

 

During a thunderstorm or when high tensions are applied in an air ioniser, the high electrical discharge can split oxygen molecules (O*), that will build ozone if they react with an oxygen molecules.

 

Oxygen, superoxide, ozone, peroxide and hydroxyl species belong to the family of “Reactive Oxygen Species” (ROS), that can participate in a potpourri of oxidation-reduction reactions (Daniels, 2000b). ROS, except ozone, are naturally produced molecules in the human body and are by-product of 1 to 5% of all aerobic reactions (Pryor, 1995). At high concentrations, there are cytotoxic whereas at low concentrations, they are involved in the regulation of several key physiological processes (Oberley, 2001) and play an important role in cell signalling processes, micro-organism killing, neuromediator and metabolism regulation (Forman, 2002).

 

The following table reports the redox power of various ROS; the more positive the potential, the better the species as an oxidizing agent. Hydroxyl radicals are the most oxidant and can initiate lipid peroxidation, what SO and HP cannot. Lipid hydroperoxides are stable, unless they come into contact with metallic ions (Cu2+, Fe2+) (Halliwell, 1984).

 

 

 

      Redox potential

Superoxide radical

O2.-

-0,33

Oxygen

O2

1,23

Hydrogen peroxide

H2O2

1,78

Ozone

O3

2,07

Atomic oxigen

O*

2,42

Hydroxyl

OH.

2,80

 

Both PAIs and NAIs will, within a fraction of seconds, bind a number of molecules (water, NOx,…) round itself, mostly water, forming molecular clusters, called small ions or cluster ions. PAIs bind around 12-14 molecules and NAIs bind around 8-10 molecules. Only these small NAIs are biologically active since they can enter the bloodstream through the lungs. Consequently, only small NAIS generating air ionisers can be used for therapeutical applications.

 

Since small NAIs are smaller and consequently more mobile, they are quicker neutralized. This explains that, in non polluted conditions, the PAIs/NAIs balance is around 1.2. In polluted conditions, since most of the liquid and solid contaminants are positively charged, NAIs are quickly neutralized and the PAIs/NAIs balance dramatically increases. Krueger & al (1985) observed that it can increase up to 50 in a non aerated room in a city.

 

During their brief lifetime, around 1 min, small ions may collide with as many as 1012 larger neutral molecules and form medium or large ions (Kotaka, 1978; Gefter, 2001). The larger they become, the slower they are. Consequently, they have less chance to be neutralized and have a longer lifetime (Daniels, 2000b). The following table illustrates this.

 

 

Diameter

(nm)

Mobility (cm2/Vs)

Lifetime

in fresh air

Lifetime

in polluted air

Small ions

- NAI:

- PAI:

0.36 – 1.6

3.2 – 0.5

1.9

1.4

< 1 min

4-5 min

Medium ions

2 - 20

0.4 – 0.035

 

 

Large ions

30 - 80

0.03 – 0.004

15 – 20 min

1 hr

 

 

When air ions start adsorbing on solid or liquid particles, like micro-organisms, viruses & allergens, they start building aggregates that easily sediment and get neutralized on grounded surfaces.  

 

© 2008 Air Quality Concept