Air Purifiers

An air purifier is a device that removes particulate and/or gaseous pollution from the air. They range widely in size and design, from large installations used on an industrial scale to small portable machines that could be used in a bedroom. Residential air purifiers can either stand-alone or be used to clean the air in the whole house. There has always been a perceived need to clean the air we breathe. For example, Leonardo Da Vinci had the notion that finely woven cloth dipped in water would protect the military from toxic powders. At the end of the 18th century, the German explorer and naturalist Alexander von Humboldt devised a primitive respirator for use in mining, while Lewis P. Haslett’s ‘lung protector’, which received its patent in 1848, used moistened wool with a one-way valve to filter dust out of the air. The United States Army Chemical Corps and the United States Atomic Energy Commission developed theHEPA air purifiers. The filter was devised to protect against the spread of radioactive dust during the Manhattan Project, during the development of the Atom bomb. The first HEPA filters were rigid and unwieldy machines, rather contrasting to their modern counterparts.

There are many air purifying systems and models available, which fall into four basic categories, described below:

HEPA Air Purifiers
The US Environmental Protection Agency recommends High-Efficiency Particulate Air filters as the most effective of the mechanical air purifiers. The filter media in a HEPA air purifier is a specialised filter consisting of a folded mat of fibres. It has three different modes of action, each of which acts upon a different size of a particle. Interception deals with particles above 0.4 microns in diameter and involves the particle sticking to a fibre in the filter when it comes close to it. During impaction, the particle is blown towards the fibres by the airstream on which it is travelling, collides with them and sticks to the mat. Then there is diffusion, which mainly affects smaller particles, of a diameter below 0.1 microns. The particle collides with air molecules, which throw it off course. Diffusion keeps the particle within the filter until it is captured by impaction or interception. What about particles between these size ranges? They are captured, but with a very small decrease in efficiency.

It is a little known fact that an air purifier, such as the IQAir HealthPro 250, can take up to 99.95% of particle pollution out of the air. The movement of particles at 0.3 microns is erratic in nature, making it difficult for the air purifier to capture it. There is no other domestic air purifier that comes close to this kind of filtration efficiency.

Electronic Air Purifiers
Electronic air purifiers work by giving particles that pass through the unit an electric charge and then uses electrostatic attraction to trap the particles. There are different kinds of electronic air purifiers; two popular versions are electrostatic precipitators and ionisers. The electrostatic precipitator sucks air into the unit, charges up the air particles in the first section, and then passes the air particles to a pair of plates bearing an opposite charge, thereby sticking the particles onto the plates. An ioniser acts externally rather than internally in that it releases ions into the environment. This charges up the particles in the air and they become attracted to their surroundings by electrostatic attraction. It is very similar to rubbing a balloon against your jumper and making it stick to the wall. The problem here is that electrically charged pollution can be an inconvenience when it sticks to walls, furnishings and other objects in your house. For that reason, we do not recommend air purifiers that use ionisation alone and do not combine it with a HEPA-like filter.

The efficiency with which an air purifier collects the air should be as high as possible. A common problem with ionising air purifiers 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 something else within your home. So if the air cleaners collection efficiency is only 80%, then you are releasing 20% of charged air pollution particles into your environment.

Another problem with air purifiers that use ionisation is that their filtration efficiency tends to become less effective 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 particle pass through the collection surface without being captured.

To ensure that your ioniser is working most efficiently, you should make sure that its filters are replaced regularly.

UV Air Purifier
Ultra-Violet (UV) light is a very short wavelength of radiation, which kills microorganisms, such as viruses, bacteria and moulds if they are exposed to the light for a significant amount of time. The performance of UV light killing microorganisms depends upon the dose of UV light air pollution is exposed to, the length of time the microorganisms are exposed to the light, and the nature of the microorganism involved. Not all bacteria and mould spores can be killed by UV light. In general, microorganisms have to be exposed to UV light for several hours to be killed. Given that the air passes through an air purifier within a fraction of a second, UV light is not an effective way to eliminate microorganisms from the air.

Gas / Chemical Air Purifier
What if you’re concerned about Volatile Organic Compounds (VOCs) rather than house dust mite allergens or other particles? VOCs are gaseous molecules, rather than particles. Unfortunately, none of the air purifier technologies discussed above, working by themselves, will remove them. Only air purifiers containing activated granular carbon, or equivalent filters can remove VOCs. What is important here is the quality of the carbon that is being used. Carbon dust, which is relatively cheap, will not be very effective. Some air purifiers, like the IQAir HealthPro 250, combine a HEPA filter with a powerful gas filter, thereby removing the widest range of pollutants. Alternatively, you might want to consider an IQAir GC VOC or IQAir GC MultiGas for environments with greater chemical contamination.

The HEPA and electrostatic air purifiers remove particle pollution. The filtration efficiency of the HEPA air purifier, however, will be much more consistent than that of the electrostatic air purifier.

An air purifier fitted with high quality activated carbon filters will remove smoke and chemical gaseous pollution. For instance, the IQAir HealthPro 250 contains both a HEPA filter (a HyperHEPA filter which filters out particles down to 0.003 microns) and activated carbon and impregnated alumina filter to remove a wide range of gases and odours.

There have been a number of studies providing us with evidence, such as:

• Children with asthma exposed to second-hand smoke had fewer emergency hospital visits when their homes were fitted with a HEPA filter. 
• HEPA filters in hospital rooms decrease the rate of invasive aspergillosis. 
• Fitting a HEPA filter in the home was found to reduce cardiac disease markers while also reducing levels of particulate pollution. 
• In Chronic Obstructive Pulmonary Disease, many doctors do recommend air purifiers as part of a medical management strategy, which also includes inhalers, antibiotics and steroids.

Air washing air purifiers can be effective for removing larger particle pollution from the air. A problem, however, tends to arise whenever one combines water and bacteria from dirty air, which naturally results in growing more contamination. To avoid hygiene issues such as mould growing in your air cleaner, it is advisable to use chemicals to fight bacterial growth. It is important to keep in mind, however, that some of these chemicals will be released into the air when the water in the air washer air purifier evaporates. Inhaling these chemicals can result in severe reactions in people with respiratory conditions.

Ozone can clean the air by affecting the cells that are exposed to it. This makes ozone very effective in cleaning bacteria and mould infestations in hard to reach places. Ozone, however, does not discriminate one cell against another. It will affect a person’s lungs in the same way that it attacks unwanted bacteria. For this reason, ozone has absolutely no place in everyday air purification in a domestic setting. Ozone should only be used to clean the air if no humans or animals are around. Some ionising air purifiers are producing small amounts of ozone. The ionising process breaks down oxygen molecules in the air, producing ozone in the process. Some ionising air cleaners have been banned for these reasons.