Ionic Air Purifiers

How do they work?

Ionic air purifiers charge airborne particles that pass through the purifier and electrostatically precipitate the particles on a metal electrode.

This is also known as air ionization, which is, the emission of ions into the air. This has been shown to be effective in the removal of airborne particles and microorganisms. The ionic air purifier is used to remove airborne microorganisms, dust particles, and aeroallergens. It can reduce exposure to dust mites, passive smoking, cockroaches, cat and dog dander, rodent debris, and mold. The particle removal efficiency is an important consideration.
There are several advantages to the ionic air purifiers.

1. They can collect particles from fairly large quantities of air
2. they are quiet and
3. the plates can be removed and cleaned and are cheaper than filter replacement.

There are two basic ionic processes: bipolar and unipolar

Bipolar ionic air purifiers emit bipolar ions. Bipolar ions encourage the agglomeration of smaller particles into larger ones which then fall to the ground by the force of gravity—their removal thereby purifying the air. The particles are then deposited on indoor surfaces and require periodic cleaning through vacuuming with HEPA filters and dusting. Room ventilation encourages the ion propagation and can enhance the ion-particle interactions.  It increases the particle charging by the air ions and this then makes their removal more efficient.

For unipolar ionic air purifiers, one study has concluded that they are “particularly efficient in reducing aerosol exposure in the breathing zone when they are used inside confined spaces with a relatively high surface-to-volume ratio.” (p. 235 Grinshpun, S.A. et al. “Evaluation of ionic air purifiers for reducing aerosol exposure in confined indoor spaces,” Indoor air 2005: 15)  Another recent study (N.J. Custis et al. 2003) have shown that cat and dog allergens bind to the ionic purifier plates. A recent study (Lee et al., 2004) has shown that the high-density emission of unipolar ions has potential for cleaning the air even in room-size areas which are contaminated with fine and ultrafine aeosol particles.

The Ginshpun study also concluded that  “all five tested unipolar ionic air purifiers were shown capable to significantly reduce the aerosol concentration in the breathing zone of a human manikin, especially in a confined space. Air mixing, especially vigorous one (900 CFM), established in a non-ventilated chamber enhanced the air cleaning effect.”  (p. 243)

What is also important is how long does it take for the unit to achieve its removal efficiency. Some models are able to achieve almost 90% efficiency within 5–6 minutes and can reach nearly 100% within 10–12 min for all particle sizes (0.3–3 μm)

Concerns:

One of the recent concerns, and which has received much attention, has been that ionic air purifiers produce small amounts of ozone as a by product. Ozone is a highly unstable compound and reverts to oxygen in a very short amount of time. It can also be pointed out the other common household devices also produce small amounts of ozone. Also, the amount of ozone decreases with distance and at three feet from the units the amount of ozone falls well within the acceptable ranges determined by the various agencies and the Consumer Reports article on the ozone generation of the ionic air purifiers also verified this.

Studies also shows that unipolar ionic purifiers that have particularly high ion production rates, while achieving very high particle removal efficiency, also can lead to electrostatic discharges—shocks. However, this effect can be mitigated simply by limiting the time length of the ionic air purifier is in operation. In small rooms, it is not necessary to operate a purifier for more than an hour and doing this with the intermittent mode.  

“we propose that ionic air purifiers with a very high ion production rate should be operated in an intermittent mode.” Grinshpun 243

“Additionally, the use of ionic air purifiers in an intermittent mode may reduce the ozone level that may be of concern when a powerful ionizer operates during a prolonged period of time (Niu et al., 2001).”  Grinshpun 243

A good rule of thumb is to purchase the appropriate sized ionic air purifier for the room you intend to use it in. If you move the air purifier to a smaller room then you know you need to run it for shorter lengths of time. You can also set set up your ionic air purifiers to run primarily when you do not expect to occupy the room ensuring that the air is purified when you choose to use the space.

It has been shown that ozone does interact with terpenes, a chemical found in many home cleaning products, notably the lemon and pine scented products, to produce formaldehyde. One recommendation would be to not use these products when the air purifier is on. (Hubbard HF. Et al, “Effects of an ozone-generating air purifier on indoor secondary particles in three residential dwellings”, Indoor Air 15(6):432-44, 2005)