Information about air filters in buildings

Indoor air naturally contains fungal spores, bacteria and germs that begin to accumulate and profilate due to humidity and warmth. Furthermore, odours and pollution like smoke and lint are part of everyday life. When opening the windows and doors pollen and fine dust infiltrate the house as well. Particle filters condition supply air up to a hygienic standard and clean indoor air from existing pollution.
Reading time: 13 min 


  • Which devices improve air quality?
  • Application of the filters
  • Classification of the filters
  • Replacing and cleaning the filters
  • Summary

Which devices improve air quality?

There are different devices that can be used to improve the quality of the indoor air. Depending on their equipment they serve a different purpose. Dehumidifiers remove excess moisture from the air and are commonly used in private households as well as construction drying. Humidifiers on the other hand add vapour to the air that tends to be very dry during the time of the year when the radiator and fireplace are operating constantly. Dry air does not only feel unpleasant, in fact it is also damaging the mucosa. Air purifiers filter the breathing air, in some cases they also feature fragrance, air washing and humidification settings. Mere air washers take in the polluted indoor air with the help of an integral fan and transport it through a water bath that removes any undesirable particles. Afterwards the processed air is returned to the room. Flykillers and insect traps also play an important role. Even though they do not condition the air, they eliminate flying insects from the house, barn or shop counter.

Particle filters or air grilles are also used in permanently installed ventilation systems and air conditioning appliances. The benefit of portable devices is that they adapt to new rooms and handle relocation without any further problems. The filters in our range are ideal for application at home, in restaurants and other culinary establishments, in hotels, vacation homes and the likes. They operate in warm and cold environments. The only thing they are not designed for is application in industrial areas and environments with a high chemical pollution. We will explain in the following guide what the different filter classes mean and where they can be used.

Application of the filters

The pollution also depends on the season of the year (pollen), surroundings (fields, forest, streets), weather, ozone, your habits relating to cleaning and household chores as well as pets in the home. Due to airtight insulated building envelopes it is quite common for the indoor pollution to be higher than outdoors. Especially if ventilation is done manually and subject to personal variation, used-up, unhygienic breathing air will come about within just a few hours. Filter elements screen unwanted particles in the air and are a crucial factor for hygienic conditions. They remove coarse particles as well as odour from the fireplace and cooktop.

The filter works if ambient air is sucked in by a fan. Think of a smoker taking a drag of a cigarette. The higher the pressure of suction the device builds up, the higher the filter efficiency is. That means the faster your fan rotates the more contaminants are removed.
➥ Appliances with several speed levels usually come with a quiet night mode to ensure undisturbed sleep.

Intake filters are mounted to exterior air grilles on ventilation ducts. They keep off contamination and curious animals from the street or garden. Fine dust from high traffic roads are also prevented from infiltrating the inside of the building. Interior filters and air purifying devices at home remove pollution that occur in the household, e.g. hairs, skin cells, dust and smoke. Recirculated air filters that are usually applied in fume hoods absorb odours from the kitchen as well as greasy particles in the kitchen air.

Unfiltered air is an unthinkable occurence in many professional environments, nobody is allowed in to polluted factory buildings or germ-infested operating rooms. Still, preventive measures against allergens and pathogens are a sensible investment for domestic areas, too. Odour and smoke particles (soot) can cause headache, fatigue and problems with concentration, furthermore they soil wallpapers, curtains and fresh laundry. Particularly in homes where residents smoke indoors an air cleaner contributes to the conservation of value of the building and prevents disputes with landlords or other tenants. The purifiers also trap mould spores that occur naturally in the air.

Verifiably polluted, unhygienic indoor air leads to asthmatic diseases in children. Colourful models can be applied in children’s rooms or nurseries. Just like the “grown up” models they are protected from tilting over and overheating.

Where to use air filters

  • Laboratory, research facility
  • Medical practice, hospital
  • Workshop, production hall
  • Chemical industry, pharmacy
  • Private household, particularly with residents suffering from allergies
  • Smoking area
  • Commercial kitchen
  • Fans connected to a fireplace, fume exhaust hood
  • Changing room, storage room with unpleasant odour
  • Increased pollution, e.g. due to wet paint
  • Mould growth (additional measure)

Filters trap:

  • Virus, bacteria
  • Germs, microorganisms
  • Mould spores
  • Mites and their eggs
  • Odour, smoke
  • Pollen, pet hairs, lint, other coarse particles
  • Dissolved frying grease
  • To some extent: chemical evaporation from building materials, asbestos

Classification of the filters

Filters are divided into 17 classes (efficiency) and three groups:

Coarse filters (classes G1-G4) are suitable for visible contamination with a relatively big particle diameter of more than 10 micrometres. They are great for installation in front of air grilles that take in air from outdoors, to prevent insects from getting into technical instruments or as a prefilter in ventilation systems. The rating with the highest efficiency in this category is G4. In this case the term efficiency describes the ability to separate particles from the rest of the air.

Medium and fine dust filters (classes M5-M6, F7-F9) stop particles with a size of 10 to 0.3 µm and are used as external air filters, in air conditioning systems and as a prefilter in sophisticated ventilation systems.

Filter for suspended particles (classes E10-E12, H13-H14, U15-U17) for particles with a size of less than 1 µm are used in environments with a requirement for high cleanliness and hygiene, for example in nuclear facilities and medical clean rooms, laboratories etc. Interestingly enough, the biggest challenge are not the smallest particles. In fact, keeping particles with a diameter of 0.1 to 0.3 micrometres at bay is much harder.

➥ 1 micrometre (µm) equals 0.001 millimetre

You can see the arrangement of the different filters on the inside of the Vornado AC300 True HEPA air purifier.

Filters in sophisticated ventilation units are usually installed in the order prefilter, special filter (aerosols) and carbon filter (odour):

The prefilter is made of non-woven material that is converted into a tearproof, flexible material. Oftentimes glass fibre is used for ventilation systems to trap any contamination. The fabric removes coarse pollutants so that fine filters and electrics behind the filter are not strained.

HEPA filters (filter classes H13-H14) are highly efficient particulate filters. The filters are often constructed with a wavy or serrated design to increase the surface of the absorbing surface. Several factors play part in stopping infiltration of particles: The path is blocked by the fibres, the acceleration is slowed down due to friction or the particles are removed from the airstream by vibration.

Magnetic filters (ionisation) electrically charge small particulate matter such as bacteria and extract them from the airflow. The microorganisms are killed off and can be washed away with water. They do not pose any danger for humans and environment anymore. The ionic cubes in our range do not produce any harmful emission.

Activated carbon is combustible product permeable to air with a large inner surface. A gram of special carbon would take up a room with a size of 1000 square metres if the surface would be laid out. The porous structure is made up similarily to a sponge and absorbs odours. It is suitable cleaning liquids, gas and is also used in the medical field to detox the body.

➥ The CADR figures tell just how much pollen, dust and smoke a purifier is able to filter. The higher the figure, the fast the device is able to filter the particles. Determining the value is done by an independent institute and is a voluntary decision by each brand, meaning not all products have been tested for the CADR rating.

Replacing and cleaning the filters

Solid deposits on the filter are called filter cake. However, they are neither pleasant to look at nor do they smell nicely – they must be disposed of regularly. Filters of exhaust or redirected air must be checked and cleaned particularly often due to risk of fire, each manufacturer gives specific and obligatory instructions for this purpose. Replacing filters is easy and quick to do and is described in the manual. Try not to breathe in near the used filter and prevent unnecessary contact with your body. In order to avoid touching the new filter too often (germs on the human skin), hold it on the solid (plastic) frame. You can choose to wear protective gloves for this. Apart from cleaning the filters all connected ducts and surfaces should be cleaned too, otherwise grease and deposit in pipes can lead to blockage and interfere with the operation.

Prefilters are made of robust material and in most cases you can clean them with a hoover. If the item is finished in a light colour you can visibly see if the filter is full. Otherwise you can snap your finger on the filter or lightly pad on it with the back of your hand. If dust comes out, it is time for cleansing. Some prefilters can be washed in water and inserted again once they are fully dry. After about six months the filter should be disposed of and replaced by a new model (check for manufacturer’s instructions). If there is a high dust pollution, filters might need replacements more often. Insufficient removing of unpleasant odours is a good indicator for fully saturated activated carbon. These filters are not suitable for cleaning and washing, they must be replaced completely every single time. HPP magnetic filters can be cleaned with cold water.

Do not touch the filter material when installing the filter

Air washers with special disks revolving inside water require extreme hygienic standards. Our “Robert” model must be cleaned thoroughly once a week. Drain the water container fully and use germicidal ionic silver cubes. Otherwise microorganisms begin breeding and end up in the lungs through the discharged mist. The device should be descaled frequently and not be placed next to any source of heat.
Ionic silver cubes from the Stadler Form range are effective for about 1 year after contacting water.

The clean air that is dispensed by filter units is odourless. In common rooms, WCs and bathrooms the addition of essential oils and fragrances creates a stimulating, calming or pleasant atmosphere. Some air purifiers have an integrated container for scented oils. The essential components can cause a certain chemical reaction with plastics so look out for specific instructions and compatibility of your appliance.

The five most common mistakes made when inserting a filter:

Postponing the replacement → Put on a sticker with the due date of the replacement
Using cheap copycat products → Eliminates any right of warranty claims, use the official and appropriate replacements of the same brand
Inserting washable filters while they are still wet → Microorganisms begin to proliferate, adhere to the recommended drying time by all means
Using indoor units outdoors → Device will become damaged due to inability to deal with the conditions outdoors
Not cleaning the housing → Remove germs from the surface of the device by using disinfectant solution like vinegar. Vinegar also traps unpleasant odours and removes limescale.


Filters are divided into 17 classes based on their filtration efficiency. Basic, coarse prefilters are usually used infront of fine particle filters to prevent unnecessary contamination or damage of the sensitive fine dust models. As filters trap a variety of contamination they must be cleaned or replaced frequently. The result is clean breathing without any pollution which is ideal for office and commercial buildings, at home and event locations.

  • Prefilter: Holds off coarse particles from all technical parts behind the filter. Materials can be metal or plastic grilles or a wide-meshed fleece.
  • Activated carbon filter: Traps odours thanks to its porous surface structure.
  • Washable HPP filter: Magnetises and immobilises microscopically small particles in the air.
  • HEPA filter: Is a highly efficient woven filter medium and is made of mineral wool or similar. It is commonly used in combination with a prefilter.
  • Water filter / water bath: Removes fine particulate matter and pollen, humidifies the air.


Show all

Picture sources:
Doctor © Unsplash -, CCO Licence
Filter replacement © Andrey Popov -