Guide written by:
John, DIYer & IT developer, Brighton
In domestic and professional spaces, poor air quality has a direct impact on the health of the occupants. Exposure to pollutants, fine particles and pathogens can cause serious illness. Follow our advice to choose the right purifier for your situation.
- Mode of operation
- Type of filtering
- Noise level
- Size and power
How does an air purifier work?
A turbine propels the air through some filters which target certain pathogens, either through physical or chemical processes of capture or transformation.
The pre-filter, first to receive the incoming air, captures lint and other larger particles that might otherwise interfere with the subsequent filtering system. Then, with solid materials removed, the air passes through a series of different filters that make up the purifier. It's the specific combination of filters which determines the level of purification, which can be as fine as 0.3µm (microns), removing bacteria and viruses as well as volatile organic compounds (VOCs).
The filters in an air purifier can work on the basis of particles, ionization, combustion, photocatalysis or plasma. Each filter has its specific use, based on the presence of a certain type of pollutant.
Why filter the air in your living space?
The air we breathe can be polluted from various sources: paint on the walls; living room furniture; traffic if you live in a busy area; nearby industrial units; and even synthetic materials in your sofa!
A purifier lets you improve the quality of the air in the room it's placed in, reducing exposure to pollutants which can in some cases be carcinogenic.
Production of ozone: health risks
Often people associate the phrase 'air purifier' with the dangers of ozone. Breathing air with an ozone content of over 120µg/m³ for 8 hours a day irritates the mucous membrans and respiratory system and can cause asthma. Aggressive ionization produces ozone in large quantities, unsafe for spaces occupied by people. For domestic use, ensure that your purifier doesn't produce ozone.
VOCs: another health hazard
VOCs are given off by manufactured products such as glues and solvents as well as when certain materials combust. Chipboard and melamine furniture, laminate flooring, synthetic leather, sofa stuffing and mattresses are all sources of formaldehydes which pose a respiratory health risk. These substances are known to be carcinogenic by health authorities. Their concentration in the air should therefore be limited.
What type of filter to choose?
Large public air purifiers will have a combination of different filters.
Rated by the minimum size of particles they can capture, expressed in microns (µm) or millionths of a metre. A HEPA (High Efficiency Particulate Air) filter is a good bet. These are capable of removing 99.97% of particles greater than or equak to 0.3µm in diameter. These fine particles can be ashes from combustion, pollen, or dust from tyres and brake pads. They can cause respiratory illnesses such as asthma. It is therefore clear that your priority is to have a good quality filter, all the more so if you're allergic to pollen (hayfever), asthmatic or living near a major road.
Ionization is the process of adding or removing electrons from the atoms of molecules in order to modify their physical properties. This type of filter removes electrons from particles and substances in the air. If they're negatively charged, they're attracted to positively charged surfaces, a bit like the north and south poles of a magnet attracting one another. Once neutralized in this way, the particles are less harmful.
Bacteria, fungi and particles are burned on contact with a surface heated to 200°C. The air is then cooled before being released so as to avoid unintended heating of the room.
The air circulates in nanotubes covered in an oxyde or sulphide. Titanium dioxide is often used to speed up the oxidation of pollutants by ultra-violet (UV) radiation. At the other end you get water, carbon dioxide (CO2) and healthier air to breathe.
When ionized air contains as many free electrons as neutral atoms, it's a plasma. A considerable amount of energy is required to excited the molecules, which are then expelled out into the room where they oxidize and destroy the various pollutants. The aggressive nature of this type of filter makes it suited to industrial settings but not for domestic or office use.
Will it be a noise issue?
The airflow and turbine action can together produce a considerable amount of noise which can be annoying in a living space or office where you need to concentrate. Check the sound pressure, expressed in decibels (dB). A level of 50dB corresponds to normal conversation, already fairly loud for a bedroom for instance. Below 20dB you've got a much more discreet piece of kit – this can be achieved either with a very sophisticated purifier or one which has a range of speed settings. The volume of air treated per hour will of course be less as a result. A rapid filtering mode is advantageous for particularly high-risk spaces that are not always occupied.
How to maintain your air purifier?
Each type of filter plays its part, and the functioning of the device as a whole depends on their correct operation. First of all it's important to remove dust, lint and debris from the pre-filter by hand or with a brush, and then remove this to access the subsequent filters.
Each type of filter requires specific maintenance. Refer to the manufacturer's technical documentation to find out whether and how to clean certain filters. Handle them with care and follow instructions precisely to avoid damaging them. The cleaning procedure often involves running them under lukewarm water, using a damp cloth or a soft brush and letting them dry well before reassembling.
Watch out for hazardous components which shouldn't be touched with bare hands, such as electrostatic filter traps.
Disposable HEPA filters should be replaced periodically. Check the recommended lifespan and the price of spares in advance to avoid nasty surprises. Maintenance-free filters, like the combustion ones, should be replaced when they stop working. Availability of spare parts is the guarantee of a wise purchase.
Size and power
The nominal power of an appliance determines its power consumption under normal conditions. For a simple HEPA filter, the power required to run the turbine will only be a few watts (W).
UV, ionization and combustion filters use more power and a multi-stage purifier can consume around 100W. This value will go up the more air the device is capable of filtering per hour.
Size: air treatment cycle
For good quality air, the rule of thumb is that a purifier should be able to treat 3-4x the total volume of air in the room per hour. Multiply the volume in m³ by 3 or 4 and you'll have the value you need to compare against the models on offer. If it's for a living space, you might want to consider the aesthetics of the device. If you want to be able to move it around, a compact model weighing less than 10kg is highly advisable.
How to control humidity with your air purifier?
The humidity level or hydrometric level of the air in a living space should be in the range of 30–70%. Lower than that, dry air can cause certain respiratory complaints and facilitate the propagation of viruses like flu. Higher than 70%, the excess moisture can condense on cool walls. This promotes the growth of mould on walls, clothing and leather items. Apart from the unpleasant smell, mould spores released into the air cause severe attacks in asthma sufferers.
Air purifiers: humidifiers / dehumidifiers
A purifier-humidifier is essential for dry air. If, on the other hand, condensation is appearing on your windows and exterior walls and mould is growing, a purifier-dehumidifier is what you need. Think about humidity when purchasing a purifier, and if necessary opt for a humidifier or dehumidifier as required.
Learn more about interior maintenance...
To find out more about interior maintenance, follow our editors' advice and check out their other guides:
Guide written by:
John, DIYer & IT developer, Brighton, 70 guides
Since I was a child, I was always interested in manual and technical works. Always fascinated by woodworking, I took advantage of my first flat as a playground. On the cards: electricity (of course, safety first!) and some partition walls; but also decorating with the help of the missus, made-to-measure furniture and little tricks to optimise the space, all the while remaining as original as possible. When the little one arrived, I started building bits and pieces for him! Lacking space, I have not got a permanent workshop and certain tools I dream about but are not part of my collection. Not to worry, I already know a lot about DIY and I have a high-tech profile that I hope will guide you in your decisions!