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Wednesday, 16 February 2011 01:25

Indoor Air: Ionization

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Ionization is one of the techniques used to eliminate particulate matter from air. Ions act as condensation nuclei for small particles which, as they stick together, grow and precipitate.

The concentration of ions in closed indoor spaces is, as a general rule and if there are no additional sources of ions, inferior to that of open spaces. Hence the belief that increasing the concentration of negative ions in indoor air improves air quality.

Some studies based on epidemiological data and on planned experimental research assert that increasing the concentration of negative ions in work environments leads to improved worker efficiency and enhances the mood of employees, while positive ions have an adverse affect. However, parallel studies show that existing data on the effects of negative ionization on workers’ productivity are inconsistent and contradictory. Therefore, it seems that it is still not possible to assert unequivocally that the generation of negative ions is really beneficial.

Natural Ionization

Individual gas molecules in the atmosphere can ionize negatively by gaining, or positively by losing, an electron. For this to occur a given molecule must first gain enough energy—usually called the ionization energy of that particular molecule. Many sources of energy, both of cosmic and terrestrial origin, occur in nature that are capable of producing this phenomenon: background radiation in the atmosphere; electromagnetic solar waves (especially ultraviolet ones), cosmic rays, atomization of liquids such as the spray caused by waterfalls, the movement of great masses of air over the earth’s surface, electrical phenomena such as lightning and storms, the process of combustion and radioactive substances.

The electrical configurations of the ions that are formed this way, while not completely known yet, seems to include the ions of carbonation and H+, H3O+, O+, N+, OH, H2O and O2. These ionized molecules can aggregate through adsorption on suspended particles (fog, silica and other contaminants). Ions are classified according to their size and their mobility. The latter is defined as a velocity in an electrical field expressed as a unit such as centimetres per second by voltage per centimetre (cm/s/V/cm), or, more compactly,

Atmospheric ions tend to disappear by recombination. Their half-life depends on their size and is inversely proportional to their mobility. Negative ions are statistically smaller and their half-life is of several minutes, while positive ions are larger and their half-life is about one half hour. The spatial charge is the quotient of the concentration of positive ions and the concentration of negative ions. The value of this relation is greater than one and depends on factors such as climate, location and season of the year. In living spaces this coefficient can have values that are lower than one. Characteristics are given in table 1.

Table 1. Characteristics of ions of given mobilities and diameter

Mobility (cm2/Vs)

Diameter (mm)




Small, high mobility, short life



Intermediate, slower than small ions



Slow ions, aggregates on particulate matter
(ions of Langevin)


Artificial Ionization

Human activity modifies the natural ionization of air. Artificial ionization can be caused by industrial and nuclear processes and fires. Particulate matter suspended in air favours the formation of Langevin ions (ions aggregated on particulate matter). Electrical radiators increase the concentration of positive ions considerably. Air-conditioners also increase the spatial charge of indoor air.

Workplaces have machinery that produces positive and negative ions simultaneously, as in the case of machines that are important local sources of mechanical energy (presses, spinning and weaving machines), electrical energy (motors, electronic printers, copiers, high-voltage lines and installations), electromagnetic energy (cathode-ray screens, televisions, computer monitors) or radioactive energy (cobalt-42 therapy). These kinds of equipment create environments with higher concentrations of positive ions due to the latter’s higher half-life as compared to negative ions.

Environmental Concentrations of Ions

Concentrations of ions vary with environmental and meteorological conditions. In areas with little pollution, such as in forests and mountains, or at great altitudes, the concentration of small ions grows; in areas close to radioactive sources, waterfalls, or river rapids the concentrations can reach thousands of small ions per cubic centimetre. In the proximity of the sea and when the levels of humidity are high, on the other hand, there is an excess of large ions. In general, the average concentration of negative and positive ions in clean air is 500 and 600 ions per cubic centimetre respectively.

Some winds can carry great concentrations of positive ions—the Föhn in Switzerland, the Santa Ana in the United States, the Sirocco in North Africa, the Chinook in the Rocky Mountains and the Sharav in the Middle East.

In workplaces where there are no significant ionizing factors there is often an accumulation of large ions. This is especially true, for example, in places that are hermetically sealed and in mines. The concentration of negative ions decreases significantly in indoor spaces and in contaminated areas or areas that are dusty. There are many reasons why the concentration of negative ions also decreases in indoor spaces that have air-conditioning systems. One reason is that negative ions remain trapped in air ducts and air filters or are attracted to surfaces that are positively charged. Cathode-ray screens and computer monitors, for example, are positively charged, creating in their immediate vicinity a microclimate deficient in negative ions. Air filtration systems designed for “clean rooms” that require that levels of contamination with particulate matter be kept at a very low minimum seem also to eliminate negative ions.

On the other hand, an excess of humidity condenses ions, while a lack of it creates dry environments with large amounts of electrostatic charges. These electrostatic charges accumulate in plastic and synthetic fibres, both in the room and on people.

Ion Generators

Generators ionize air by delivering a large amount of energy. This energy may come from a source of alpha radiation (such as tritium) or from a source of electricity by the application of a high voltage to a sharply pointed electrode. Radioactive sources are forbidden in most countries because of the secondary problems of radioactivity.

Electric generators are made of a pointed electrode surrounded by a crown; the electrode is supplied with a negative voltage of thousands of volts, and the crown is grounded. Negative ions are expelled while positive ions are attracted to the generator. The amount of negative ions generated increases in proportion to the voltage applied and to the number of electrodes that it contains. Generators that have a greater number of electrodes and use a lower voltage are safer, because when voltage exceeds 8,000 to 10,000 volts the generator will produce not only ions, but also ozone and some nitrous oxides. The dissemination of ions is achieved by electrostatic repulsion.

The migration of ions will depend on the alignment of the magnetic field generated between the emission point and the objects that surround it. The concentration of ions surrounding the generators is not homogeneous and diminishes significantly as the distance from them increases. Fans installed in this equipment will increase the ionic dispersion zone. It is important to remember that the active elements of the generators need to be cleaned periodically to insure proper functioning.

The generators may also be based on atomizing water, on thermoelectric effects or on ultraviolet rays. There are many different types and sizes of generators. They may be installed on ceilings and walls or may be placed anywhere if they are the small, portable type.

Measuring Ions

Ion measuring devices are made by placing two conductive plates 0.75 cm apart and applying a variable voltage. Collected ions are measured by a picoamperemeter and the intensity of the current is registered. Variable voltages permit the measurement of concentrations of ions with different mobilities. The concentration of ions (N) is calculated from the intensity of the electrical current generated using the following formula:

where I is the current in amperes, V is the speed of the air flow, q is the charge of a univalent ion (1.6x10–19) in Coulombs and A is the effective area of the collector plates. It is assumed that all ions have a single charge and that they are all retained in the collector. It should be kept in mind that this method has its limitations due to background current and the influence of other factors such as humidity and fields of static electricity.

The Effects of Ions on the Body

Small negative ions are the ones which are supposed to have the greatest biological effect because of their greater mobility. High concentrations of negative ions can kill or block the growth of microscopic pathogens, but no adverse effects on humans have been described.

Some studies suggest that exposure to high concentrations of negative ions produces biochemical and physiological changes in some people that have a relaxing effect, reduce tension and headaches, improve alertness and cut reaction time. These effects could be due to the suppression of the neural hormone serotonin (5-HT) and of histamine in environments loaded with negative ions; these factors could affect a hypersensitive segment of the population. However, other studies reach different conclusions on the effects of negative ions on the body. Therefore, the benefits of negative ionization are still open to debate and further study is needed before the matter is decided.



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