Because the dust is held electrostatically, and removing it does nothing about the charge that attracted it.

A charged surface exerts an electrostatic attraction on nearby particles. Blow with plain compressed air and the mechanical force of the stream may dislodge some of them - but the charge is untouched, so as soon as the air stops, the surface attracts fresh dust from the surrounding atmosphere.

In practice a part cleaned with plain air can be visibly contaminated again within seconds, which is a familiar and frustrating experience on plastics before painting or printing.

Ionized air removes the cause. The ions neutralise the surface charge, so the attraction disappears, and the air stream then clears the loose particles which no longer have anything holding them.

The test is simple: if dust returns quickly after blowing, the problem is static and an ionizer is the answer, not more air pressure.

By applying high voltage to sharp emitter points, which ionises the air immediately around them.

A high voltage applied to a fine, sharp point produces an extremely strong electric field at the tip. That field strips electrons from air molecules nearby, creating positive and negative ions in the surrounding air - a corona discharge.

Both polarities are produced, either because the supply alternates or because emitters of both polarities are provided. That matters, because a charged surface may be positive or negative and needs ions of the opposite sign.

The air stream then carries those ions to the target. When they reach a charged surface, the surface attracts the polarity it needs and repels the other, so it neutralises itself - the process is self-regulating, and an uncharged surface simply leaves the ions alone.

The emitters themselves are the wearing and fouling component, and their condition determines how many ions are actually produced.

A bar for a continuous line or web; a blower or gun for individual parts and localised work.

An ionizing bar is a linear array of emitters, made in lengths to suit the application, and it treats everything passing along its length. That suits a moving web of film or paper, a conveyor carrying product, the mouth of a bagging machine, or a sheet-feeding station. Air is usually supplied along the bar to carry the ions to the surface, either from compressed air or from a small blower.

An ionizing blower or air gun produces a directed stream for treating a specific part - cleaning a moulding before painting, preparing an enclosure before assembly, or blowing out a container.

Overhead and room ionizers exist for treating a whole work area, used in electronics assembly to protect sensitive components.

The choice follows the geometry: continuous and linear points to a bar, discrete and handled points to a blower or gun.

Within the effective range stated by the manufacturer - which is finite, because ions recombine as they travel.

Ions are short-lived. As they move through the air they encounter ions of opposite polarity and neutralise each other, so the concentration reaching a surface falls with distance. Beyond a certain range, too few arrive to neutralise the charge in the time available.

Air assistance extends the range considerably by carrying the ions rather than letting them drift, which is why bars for longer distances use compressed air or a blower.

Manufacturers publish an effective distance and a neutralisation time at that distance. Both matter: on a fast-moving web the surface is only in front of the bar briefly, so the ionizer must neutralise it in that time.

Mounting too far away is a common installation fault, and it produces an ionizer that appears to be working - it is powered and blowing - while achieving very little.

Regular cleaning, because contamination reduces output progressively and gives no indication.

Emitter points attract exactly what the ionizer is deployed to deal with: dust, fibres and airborne contamination are drawn to the charged tips and build up on them. As they do, the sharpness that concentrates the electric field is lost and ion output falls.

The failure is gradual and silent. The bar is still powered, still blowing air, and still appears to be working - while producing progressively fewer ions until the static problem returns and is blamed on something else.

So emitter cleaning belongs on a schedule, using the method the manufacturer specifies. Some bars have removable emitters or self-cleaning arrangements to reduce the burden.

More usefully, ion output can be measured with a static meter on the target surface - checking that residual charge is within limits is a direct test of whether the installation is doing its job, rather than an inspection of whether it appears to be.

Wherever insulating materials are handled at speed, and wherever a discharge would cause damage or danger.

Plastics processing is the classic case: film, sheet and moulded parts acquire charge readily, attract dust before painting or printing, and cling to tooling and to each other. Printing and converting suffer the same on paper and film, with sheets misfeeding and stacks failing to separate.

Packaging lines encounter it in bag opening, labelling and filling. Powder handling is affected both by clinging and by the ignition risk a discharge presents.

Electronics assembly is a different requirement: the concern is discharge damaging sensitive components, so ionization is part of a broader static control regime alongside grounding and dissipative materials.

And anywhere solvent vapours or combustible dusts are present, static discharge is an ignition source - which makes control a safety measure rather than a quality one.

Yes - they generate high voltage, and some types produce ozone, so both need attention.

The high voltage is at the emitters, and equipment is designed so it is not accessible in normal use - but the supply and the bar should be isolated before cleaning emitters, and damaged cables or cracked housings should take the unit out of service. Shockless designs limit the current available at the emitters specifically to reduce this risk.

Ozone is produced by corona discharge, more by some designs than others. In a small enclosed space with poor ventilation it can accumulate, and it is an irritant with occupational exposure limits. Manufacturers state ozone production, and it is worth checking where ionizers run continuously in a confined area.

In hazardous areas the equipment must be rated for the atmosphere - which is a genuine consideration, since static control is often wanted precisely where flammable vapours or dusts are present.