Air purifiers come in a whole range of shapes, sizes and more importantly filter types. Choosing the right home air filter for you means you need to have the appropriate filtration technology for the allergens, chemicals or odours you are looking to filter out.To the uninitiated the array of complicated sounding air purification technologies can be frankly bewildering. The following sections have been produced to help you known your HEPA filters from your photo-catalytic oxidizers.
HEPA filter
HEPA stands for High Efficiency Particulate Air and has become the gold standard in allergy filtration technology over the years. It is most commonly seen in anti-allergy vacuum cleaners but these same super-fine filters are also found in air purifiers.
HEPA is actually an industry standard and not a trade name. To be certified as HEPA a filter must trap in excess of 99.7% of all airborne particles larger than 0.3 microns in size. To put that into context a single bacteria measures around 1 micron and humans can only see something larger than 100 microns across such as a human hair. This is great news for allergy sufferers as a true HEPA filter will remove most dust and mites, mold spores pollen and pet dander. In addition they will also cleanse the air of cigarette smoke, bacteria and even some viruses.
Obviously the air must pass through the filter to remove all these possible contaminants and this is where factors such as the air changes per hour (ACH) are important, but this is common to all air filtration units.
The HEPA filter is not constructed as you might imagine like a simple sieve or gauze. It is actually a dense mat of glass fiber folded into an accordion-like arrangement which uses three distinct mechanisms (impact, interception and diffusion) to capture particles that are both larger and smaller than the target size. HEPA was originally designed for the nuclear industry to filter out radioactive particles from the work environments.
Whilst HEPA air purifiers will filter out nearly all common allergens they do not remove odors and gases. For this reason many multipurpose air filters will contain a secondary filtering technology such as activated carbon to do this.
Another thing to look out for when buying a HEPA air filter is that the motor is sufficiently powerful; more power is required to pull the air through these dense filters and underpowered units will not function effectively.
A final note is you will have to regularly clean your HEPA filter and replace it between every 2 and 4 years.
Electrostatic filtration
Electrostatic filters (or electrostatic precipitators to give them their even more technical name) work on a fairly simple two stage process. First off the airborne particles entering the filter are given a positive charge of static electricity. These particles are then attracted to negatively charged metal collectors where they remain.
In terms of particle size electrostatic filters are similarly effective to HEPA filters. This makes them also suitable for all major airborne allergens, however they have several (at least theoretical) advantages over other filter types.
Firstly, the actual filter does not need replacing. Whilst this may be a slightly big claim it is certainly true they will far outlast a standard filter and as they are made of metal some brands can be washed in a dishwasher.
Another advantage of electrostatic filter systems is they require less power as the air is not being forced through a closely meshed filter. This means not only lower energy bills but less noise.
There are some questions over the efficiency of electrostatic filters. For example it is suggested that the air needs to pass more slowly over the collectors than in a HEPA filter meaning it will take longer to clean the air in a room.
If you aren’t happy with this trade off then you might be pleased to know some air purifiers have a combination of HEPA and electrostatic filters for maximum effectiveness.
Activated carbon filter
This form of filter is made from carbon similar to charcoal that has been “activated”. What this means is it has been treated with oxygen in such a way as to create millions of microscopic pits in the surface of the carbon. This has the effect of massively increasing the surface area of the carbon which already has the ability to absorb a wide variety of pollutants effectively trapping them.
Activated carbon is not new technology. In fact it dates back to the gas masks of World War I where it protected soldiers from the deadly effects of chlorine gas.
These days you are most likely to find activated carbon filters in air purifying devices aimed at removing odors and airborne chemicals from the air. These particles are too small to be trapped by other types of air filters so activated carbon is the most effective system for those who suffer from multiple chemical sensitivity (MCS).
As well as removing common household pollutants such as cigarette smoke carbon filters are able to absorb a range of volatile organic compounds (VOCs) such as formaldehyde which is considered a carcinogen.
Whilst it may sound ideal to have a filter system which removes all airborne particles from the largest allergens right down to noxious smells there are disadvantages to activated carbon filters. The main one is they are generally less efficient than other air purifiers. So, the bottom line is choose activated carbon only if you need to remove odours and chemicals; if you simply want to remove allergens, stick to one of the other filter types.
Ionizers
Whilst ionization of particles may sound like something from science fiction it is actually not that new. The basic principle is the ionizer uses a high voltage current to produces negatively charged electrons which are released from a series of needle-like structures. This process is known as a corona discharge and the resultant electrons attach to passing air molecules forming negatively charged ions.
Now as opposites attract and likes repel in the world of electromagnetic forces, the negatively charged ions repel each other and actually disperse themselves without the need of a fan. When they encounter a neutrally charged particle such as dust, pet dander, pollen or even cigarette smoke the ion passes on its negative charge.
Now bigger and heavier these negatively charged molecules seek an earthed surface such as a wall or floor, which whilst solving one problem, creates another. Modern ionic air purifiers are supposed to collect most of the contaminant particles in a specially charged filter.
One advantage of ionic filters is the range of pollutants they can remove from the air; not only can they reduce levels of allergens but hopital trials found they helped eliminate bacteria and some viruses from the ward.
Safety concerns have been raised about ionic air cleaners as the process of ionisation can create hazardous ozone gas as a byproduct. This can actually exacerbate the symptoms of asthma and potentially cause long term lung damage. Although many manufacturers have addressed these issues care should be taken to choose an appropriately certified device.
Incineration air purifiers
Incinerator air cleaners work on a simple principle, which as the name suggests, involves destroying allergens and other pollutants by incinerating them. This not only destroys the pollutants but sterilizes them too.
The heart of the system is a ceramic incineration chamber which heats the air to around 200°C (400°F) effectively sterilizing it by killing off any microorganisms instantly. The air is then cooled within the device before being released back into circulation.
There are a few advantages of incinerator air purifiers; firstly they don’t have a filter that will need replacing. Another positive is that the heat draws the air through the purifier by convection without the need for a motor or fan – this means they are practically silent. You do pay a price for this though, this passive convection current model does not have the same throughput as some conventional air cleaners such as a traditional HEPA filter. On the other hand incinerator air purifiers are unrivaled when it comes to removing airborne bacteria and viruses.
Conclusion
If you are feeling a little overwhelmed with all the information and options available for buying a home air filter that’s understandable. To effectively choose the particular air filter type that is going to help relieve your symptoms you will have to identify what particles and pollutants you need to remove and what trade offs you can make.
For most common household allergies some kind of HEPA based filter is probably the best all round solution. If you are more interested in removing chemical triggers an activated carbon filter might be the way to go. However, it doesn’t end there; many modern air purifiers offer combinations of multiple filtration systems. Some models combine HEPA and electrostatic filters whilst other more exotic combinations such as electrostatic and ionizing technologies are available. We even know of one domestic air filter that has a 5 stage filter system!