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Living Well in the cITy

Residential Air Purifiers

When you are researching something like home air purifiers you need to arm yourself with all the information you can to help you make an informed purchasing decision. Let's start with exactly what it is you want your air purifier to do for you.

Air Contamination

Contaminates in the air come in all different sizes. Some things in the air are large enough that fairly simple filtering technology can trap the particles and remove them from the air. These are approximate size ranges for a few common airborne particles.

  • Pollen: 2Ám to 200Ám
  • Pet Dander: 2Ám to 10Ám
  • Mold: 3Ám to 12Ám
  • Smoke: 0.01Ám to 4Ám
  • Bacteria: 0.3Ám to 60Ám

A micron or micrometer (Ám) is a unit of measure: 0.001mm or about 0.000039in. Generally, the more expensive the filter, the better it will be at removing the small stuff.

Some things such as vapours and fumes are chemical compounds made of molecules that require special filtration. VOCs (Volatile Organic Compounds) are chemical compounds typically found in indoor environments. Paint, glue and other man-made chemicals will 'off gas' over time, releasing VOCs into the air. And then there are airborne micro-organisms: germs and bacteria. Some air purifiers don't just filter them our of the air, but they actually destroy and completely neutralize them.

Filtering Technology

Listed here are the most commonly used methods employed by commercially available air purifiers for cleaning the air.

Charcoal Filters

Activated carbon filtering has been around for a long time. It is essentially charcoal that has been treated to give it a high level of micro-porosity. The porous carbon can trap extremely fine particles. An air purifier will force air through a screen or basket of small carbon pellets. Filters like this are relatively inexpensive. They are also very effective at removing fumes and odours form the air.

It is possible to clean charcoal filters although some manufacturers will tell you to just replace them. Let them soak for a while in warm water, shake and rinse. The filter will not be "as good as new" but you will remove some of the larger dust particles that get trapped up against the charcoal. At the very least, give the filter a good going over with a vacuum to remove larger dust accumulation that prevents air from circulating through it.

HEPA Filters

HEPA (High-efficiency particulate air) filters are made from mats containing randomly oriented fibres. They are not designed to block the flow of particles but to trap them. Particles become trapped in the filter after impacting and embedding on the fibres or bouncing around until they eventually come to rest in the filter. HEPA filters will not trap gases and odours. That's why you'll usually see HEPA filters used in conjunction with a charcoal filter.

In the United States, a filter cannot be classified as a True HEPA filter if it does not remove at least 99.97% of airborne particles greater than 0.3 micrometers (Ám) in diameter. HEPA-like filters don't filter to the same degree and are not really HEPA filters. And without a HEPA rating, you have no guarantee that they even filter out what they claim as there is no requirement for them to be independently tested.

The HEPA filters found in commercially available home air purifiers can not typically be washed and are usually replaced once they reach the end of their recommended life. But I have seen some advertised as "hospital grade" that can be cleaned. Air purifiers utilizing HEPA filters usually have some sort of inexpensive pre-filter to catch lots of larger air-borne particles before they can clog up the HEPA filter.

Ultraviolet (UV) Light

Ultraviolet germicidal irradiation (UVGI) is used to kill bacteria and viruses. Short-wavelength ultraviolet radiation (UV-C) can kill micro-organisms exposed to it. It can also damage human skin, causing burns or cancer (much like over-exposure to sunlight). These wavelengths of light are emitted by the sun and do the reach Earth but our atmosphere reflects most of this light. Some devices will use UV light in combination with Titanium Dioxide (TiO2) coated screens. The Titanium Dioxide is used as a photo-catalyst. As the UV light strikes it, ions are created that will bind to airborne pollutants and oxidise them, breaking them up and rendering them harmless.

The effectiveness of UV to purify air depends on how long the air remains in close proximity to the UV light source. Dust and other particles in the air can coat UV bulbs, reducing their effectiveness over time. The use of UV-C can also produce a small amount of ozone which can be harmless or beneficial depending on what you believe.


There is already too much information on the internet about the apparent health benefits of negative ions. My goal here is to summarize how these so called 'ionic' air purifiers work (in general terms).

An ion is a molecule that has a negative or positive charge. By adding or removing electrons from the atoms in the molecule, the net charge of the molecule can be changed. Any atom or molecule that does not have a neutral charge (same number of electrons and protons) is referred to as an ion. Like forces repel one another but opposite forces will attract. This means that the negative ions will be attracted to anything that has a positive charge and they will 'stick' to it. And positive ions are attracted to anything with a (relative) negative charge.

Air purifiers that make use of ions to clean the air have emitters inside of them that will add free electrons to the air as it passes over them. These free electrons will bind with larger neutral particles in the air to form negative ions. Exactly how the ions are used to clean the air depends on how the air purifier was designed. It might make use of positively charged collectors in the air purifier that will attract these negatively charged particles to remove them from the air. Or the negatively charged particles will be blown back into the room where they will be attracted to surfaces that have a positive charge (relative to the particle), like a wall for example. They may also attract other particles (with less negative charge) and clump together to form a bigger particle that either falls out of the air or gets trapped by a filter in the air purifier. Some purifiers will release both positive and negative ions that will clump together and be more easily trapped by the filter media in the purifier.

One side effect of ion generation is ozone. The electrical discharges that create the ions also create ozone gas.


Ozone is a molecule made up of 3 oxygen atoms. The layer of ozone found in the Earth's upper atmosphere protects us from harmful UV radiation. The process used to ionize particles also splits apart oxygen O2 molecules in the air. Sometimes the resulting free oxygen atoms will bind with two others to form ozone (O3). You will find lots of air cleaners that are advertised as ozone generators, claiming that not only will the ozone clean the air but it also has several health benefits. It is used in some water purification processes, but the chemistry is not the same and it does not have the same affect on humans. The effectiveness of ozone generating air purifiers is questionable and much of the literature promoting the benefits of ozone is produced by the manufacturers of such devices.

The problem with ozone is that it oxidises rapidly with other molecules. That extra oxygen atom can easily break away from the other two and combine with other molecules, oxidizing them and forming new compounds. And this is how ozone purifies the air: by oxidizing and destroying things in the air that we don't want. Unfortunately the ozone can also affect human tissues, particularly those in the lungs, leading to respiratory illnesses. Oxygen molecules with 2 oxygen atoms (O2) are good to breathe. Those with 3 oxygen atoms (O3) are bad.

My advise regarding air purifiers marketed as ozone producers is to stay away from them. Particularly those that claim they only produce "good" ozone. Read this.

Making a Choice

A great website with a searchable database of air purifiers and ratings is here. Everyone has different needs and you should make sure the filter you plan to purchase will actually remove whatever it is you don't want in the air you breathe: smoke, pollen, VOCs, bacteria. Room size is another important consideration. Purchase a system matched to your room size. If the filter is going into a bedroom, consider the noise it will produce. Even the brightness of the lights on the control panel can be annoying to some people.