Ionisers

There are two main types of air purifier. Both can remove particles from the air, but a mechanical air purifier generally uses a filter, such as a High-Efficiency Particulate Air (HEPA) filter while an electronic air purifier uses electrostatic attraction to trap particles. An ionising air purifier is a type of electronic air purifier. The other type is known as an electrostatic precipitator.

An ioniser air purifier releases ions – i.e. charged particles - into the air. These collide with airborne particles such as dust or smoke particles and thereby give them a charge. Once the particle pollution has a charge, the particles are attracted to surfaces, such as walls, curtains, or a filter within the air purifier, which then takes the pollution out of the air. Depending on the size of the particle pollution, particulates can combine with non-charged particulate pollution in the air, which will then increase the weight of some of them so that they sink to the floor. Thus, in a mere ioniser air purifier (i.e. an ioniser that does not use filters or other forms of a collection surface in itself), all the actual air purification takes place outside the ioniser where the charged pollution sticks to the environment or other non-charged particles.

The electrostatic precipitator, on the other hand, draws air inside, where particles are given an electric charge. They then pass between plates which have a charge opposite to that of the particles, which causes attraction. The charged particles stick to the plates and the air leaves the purifier minus the particles. This, however, only works as long as the collection surface is not completely covered by charged pollution, which would function as insulation. Once it is, the electrically charged pollution passes through the air purifier and past the collection surface without being attracted and captured.

Ioniser air purifiers tend to be very quiet in operation. This is due to them using a weaker fan and either a low-quality filter or no filter at all, whereas a mechanical or HEPA air purifier may create more noise as it uses a more powerful fan to push the air through a denser, higher quality, filter material. The United States Environmental Protection Agency (EPA) has said that electronic air cleaners of both kinds are effective at removing fine particles from the air under certain circumstances. It is, however, recommended to only use ioniser air purifiers that combine the ionisation technology with a filter (and ideally a good one). An ioniser without a filter will blow electrically charged pollution into your environment. This pollution can attach itself to a nearby wall (which creates what is called a ‘black wall effect’) or might be inhaled, which can cause severe problems for people with chronic respiratory conditions.

If you are looking for an air purifier to remove the odour or chemical pollution, such as formaldehyde or other Volatile Organic Compounds, then an ioniser air purifier is mostly not an appropriate choice, because the ionisation process can only remove particulate pollution. Odours and chemicals need to be removed by an absorbent material such as activated charcoal.

As mentioned above, an ioniser air purifier removes particles from the air and deposits them on surfaces. Unless you clean the collection surface on a regular basis and before it is covered with charged particulates, a unit like this might release charged particulates back into the air. Sometimes this is visible as the so-called 'black wall' effect. Furthermore, some ionising air purifiers might create ozone, either as a by-product of the ionising process, or deliberately. Air purifiers that do the latter are sometimes called ozone generators. Ozone smells nice and ‘clean’, but can cause severe health problems as it will break down all natural compounds including your lungs. So if you do want to go for an electronic air purifier, check that it does not produce ozone and that it uses an appropriate filter.

The efficiency with which an air purifier collects the air should be as high as possible. A common problem with ionisers is that any air pollution that is not captured by the air purifier but is electrically charged, will be released back into the room and will attach itself to surfaces in your home.

Another issue with some air purifiers that use ionisation is that their filtration efficiency tends to decrease with use. Many ionisers might start out with a particle collection efficiency of 90% or even 99%, but that efficiency can often drop quickly to as low as 50% or less. The reason that the collection efficiency decreases is that if enough dust is accumulated on the air cleaner’s collection surface, then that dust functions as insulation. The charge will not pass through the layer of dust, letting the particles pass through the collection surface without being captured. Thus, to ensure that your ioniser is working most efficiently, you should ensure that its filters are replaced regularly.

An ozone generator relies on the chemical reactivity of ozone to destroy airborne pollutants. Ozone generators are not particularly effective as a general air cleaner, as many pollutants remain unaffected by ozone. The ozone may even react with pollutants already present to produce further pollution. But even more concerning is that an ozone generator, or an ionising air purifier that produces ozone as a by-product, can increase the overall ozone in your home.

Ozone is a form of oxygen that contains three oxygen atoms ('ordinary' oxygen, which makes up 21% of the atmosphere and is essential for life, contains two oxygen atoms). The stratospheric ozone layer, ten or so miles above the Earth's surface, protects us from harmful radiation coming from the sun. Ground-level ozone, which often builds up when vehicle exhaust fumes are exposed to sunlight, is a different matter (or, as the EPA puts it, ozone is 'good up high, bad nearby’). Ozone has long had a mistakenly 'healthy' image because its odour is reminiscent of sea breezes. In fact, we now know that this smell is dimethylsulphide, which is produced by phytoplankton in seawater and not ozone at all. But the myth that the smell of ozone is the smell of fresh clean air still lingers. The extra oxygen atom in ozone is reactive and will bond to molecules in the body if it is inhaled. The EPA lists the following health hazards of ozone exposure:

• Coughing, throat irritation and breathlessness
• Lung inflammation
• Asthma exacerbation
• Exacerbation of bronchitis, emphysema
• Impairment of the immune system

The Sharper Image Ionic Breeze, introduced in 1998, was the most popular air purifier in the United States. It was an electrostatic precipitator, collecting particulate pollution on plates inside the machine (so, despite the name, it is not an ionising air purifier as described above). The Ionic Breeze ran into trouble when the consumer organisation Consumer Reports carried out tests that showed the system had very limited ability to remove particles from the air. The company sued Consumer Reports, but lost the case, and were forced into bankruptcy in 2008. Along the way, it was also shown that the Ionic Breeze produces significant amounts of ozone which could be detrimental to health and certainly would not help air quality. The brand names and the company are back on the market, but this is not the original Ionic Breeze.