Tech Guide
Clean Air

Successfully filter viruses, pollen, fine particulates and odors

The COVID-19 pandemic has brought into focus a question that people in many regions around the world have been exploring for some time: How can you improve the air quality in your home? Air purifiers and other filtering devices have become suddenly popular in Europe, and not just in the smog-plagued cities of America and Asia.

Viruses are simply one more good argument in favor of such devices. Different types of airborne particles and aerosols in your home’s air have been long proven to be harmful to your health and wellbeing. Allergy sufferers know this full well. There are many good arguments in favor of home air purifiers. So you can breathe easy – this Tech Guide will help you choose the right product.

What does good air quality entail?

Healthy breathing

The average adult breathes in and out more than 10,000 liters of air every day. This air always contains some pollutants; clinically pure air simply doesn’t exist in our everyday lives. While humans add to air pollution with combustion engines, industrial plants and other technical achievements, nature itself also contributes its fair share – for example, through airborne pollen, or desert dust from the Sahara. As with everything, it is the concentration of particles that makes the poison. However, everyone reacts differently to potentially harmful substances. People with pre-existing health conditions and children are particularly sensitive to many substances that are in the air we breathe. According to international studies, the risk of developing asthma during adolescence increases as the particulate matter in air becomes more concentrated.

Pollution in the room

It goes without saying that pollutants are difficult to dilute and disperse indoors. Outside, the wind ensures the air circulates well to move pollutants on, but inside, these airborne particles accumulate. With exhaled carbon dioxide and vapors from wall paints, furniture, carpets, cleaning agents and cosmetics adding to the particle load, the level of pollution indoors is often many times higher than outside. Where ventilation does not improve the air quality – perhaps because of the traffic around the house or pollen in the wind – air purifiers are an effective measure. However, the technology you use must fit the goal you want to achieve. Not all filters work equally well to remove all of the various types of harmful substances.

Something in the air

Viruses (0.02-0.3 micrometers)

These infectious pathogens are among the smallest contaminants in indoor air. Coronaviruses such as the SARS-CoV-2 virus are only 0.08-0.16 thousandths of a millimeter in diameter. The viruses don’t move through the air by themselves, however. Instead, they are carried by larger droplets called aerosol particles.

Smoke particles (0.1-4 micrometers)

Tobacco smoke is unhealthy, for both the smokers themselves, as well as for the people around them. You will have noticed the characteristic odor and the yellow nicotine haze that covers the walls and furniture over time in a smoker’s household. Filters that absorb these smoke particles and odors are a good way to combat both.

Particulate Matter (0.1-10 micrometers)

Particulate Matter (PM) is defined as airborne particles that are smaller than ten thousandths of a millimeter (10 micrometers). Included in this category of fine particulates (PM10), experts define ultra-fine particulates to have a maximum diameter of 2.5 micrometers (PM2.5). This extremely fine dust – such as that produced by the exhaust fumes of traffic and fireplaces – penetrates particularly deeply into the lungs, and is more harmful than normal household dust.

Bacteria (0.5-5 micrometers)

Compared to viruses, most bacteria are quite large and can be easily filtered. They commonly enter the house from outside through people breathing, coughing and sneezing. Problems arise when they accumulate and multiply somewhere. This can pose a danger to those with a weakened immune system.

Coarse dust (10+ micrometers)

When household dust is mentioned, what is usually meant are the visible particles on furniture in the room. Among other things, they consist of fluff, animal hair, particles generated by walking on floors, and the waste that dust mites excrete. Humans also contribute to this dust as people constantly shed skin cells.

Mold spores (5-15 micrometers)

You’ll find mold spores in the air of almost all rooms. They become dangerous in larger quantities, or where someone is sensitive to them. Even small concentrations can trigger an allergic reaction. If you ever find you have a mold problem, don’t just eliminate the spores from the air, but also at their source, like the dampness in walls.

Pollen (5-200 micrometers)

Around 15 percent of all adults have been diagnosed with a pollen allergy at some time in their life. The number of unreported cases is probably higher still. People who are plagued with hay fever can react badly to many different types of plants. In severe cases, allergies can last for most of the year.

CO – Carbon monoxide (gas)

This odorless and invisible gas is produced by incomplete combustion, for example, from a fireplace, barbecue or an open flame. It is highly toxic and can only be removed from indoor air by opening a window for ventilation. However, specialized air quality sensors can detect it and warn you well before the gas reaches dangerous concentrations.

CO2 – Carbon dioxide (gas)

Breathing out naturally produces carbon dioxide which escapes with the ‘stale air’. Outside, the concentration of carbon dioxide is about 410 ppm (parts per million). In indoor environments where several people are breathing, it can easily reach many times this level. It is strongly recommended that you air any space where the concentration reaches 2,000 ppm.

VOC – Volatile organic compounds (gas)

Many different substances are classified as volatile organic compounds, or VOCs, from alcohols, to hydrocarbons, and through to acids. Some are formed when solvents evaporate. Furniture, adhesives, paints, cleaning agents, and even people release VOCs into the air.

Radon (gas)

This radioactive gas escapes naturally from the earth, in some regions more strongly than in others. Radon becomes a problem when it enters a building through porous foundations and accumulates inside. The Federal Office for Radiation Protection recommends that you air your home frequently to combat this. Filters that capture gaseous substances may further reduce your exposure.

H2O – Steam (gas)

A room’s humidity has a large impact on its occupants’ well-being. When it is too high and thus too wet, mold can grow. A too dry climate favors colds. It is therefore worthwhile to keep an eye on the relative humidity indoors. This measurement takes temperature into account as cold air can store less water than warm air.

Practical tip

The relative humidity inside should not fall below 40 percent. If you don’t want to use a humidifier, leave the bathroom door open after you shower or bathe to distribute the remaining steam and water vapor throughout your home.

Air purifiers

How air purifiers work

Almost all of these devices work on the same principle: A fan sucks air from the room, pushes through various purification stages and releases it again. Only a few thermal purifiers have no fan, instead taking advantage of the way that heated air circulates naturally in a thermal cycle. To neutralize as many pollutants as possible, manufacturers combine several types of filters, including activated charcoal to combat unpleasant odors, and HEPA filters to tackle particulate matter.

Filtration types

As the term HEPA (High Efficiency Particulate Air) is not protected, it is therefore widely used. Many manufacturers apply this term broadly when talking about the filter at their vacuum cleaner’s air outlet. Technically, HEPA filters are graded by their efficiency. The EN-1822 EU standard separates filters into a number of classes, where the E10 to H14 classes are most relevant for households.

Filter category Filter class Efficiency
EPA E10 ≥ 85 %
EPA E11 ≥ 95 %
EPA E12 ≥ 99,5 %
HEPA E13 ≥ 99,95 %
HEPA E14 ≥ 99,995 %


The higher the number, the better the filter performs when separating particles from the air that flows throughout it. According to the EU standard, only the two highest classes, H13 and H14, belong to the HEPA category, i.e. they are highly efficient. They remove almost 100 percent of all airborne particles and are typically used in operating rooms and doctors’ offices. However, comparatively little air can pass through the filter and they do require powerful fans. Of course, this impacts energy consumption and noise levels.

Household devices therefore typically use filters that are less efficient, separating fewer particles, but adding an electrostatic charge. The higher air flow rate ensures there is a comparable purification effect, but requires less energy. Anyone who suffers from allergies should pay attention to the filter efficiency of the device, usually indicated by the manufacturer as a percentage. The European Centre for Allergy Research Foundation (ECARF) seal is applied to devices that can remove at least 95 percent of pollen, mold spores and bacteria from the air.

What helps to destroy viruses?

Viruses appear in indoor air primarily as aerosol particles. They are attached to small droplets that are produced when people talk, sneeze or cough. Because these droplets are larger than the virus itself, they can be easily removed by HEPA filters. To maintain a small virus load in a room, however, the device and its fan must be able to completely exchange all of the air in the room several times an hour. Experts recommend at least three, if not six to eight full air exchanges per hour. The purifier must therefore be large enough to accomplish this task.

Most manufacturers specify how many cubic meters a purifier can exchange in one hour (m3/h) in the device’s technical data. Unfortunately, it is often not so easy to compare. The most useful indicator you should refer to is the CADR value (Clean Air Delivery Rate). This is measured following a standardized process and should have a value of three to six times the room’s volume. Additional features, such as a UV light or plasma filtration technology, can further reduce the pathogen load. Note that these devices do not replace adequate ventilation through open windows, nor distance or hygiene rules, but they do supplement these measures.

Fan noise

There is another reason for choosing a powerful purifier. These models don’t need to spin their fans as fast when operating normally, where smaller devices must run at full speed to achieve the same filtration. As a result, less noise is produced. This is particularly positive in bedrooms. Quiet models run at less than 28 decibels on their lowest settings – quieter than a whisper. Light sleepers who are sensitive to sound, can find 35 dB (A) and above to be disturbing.

Other useful features

When you can set a timer, or use a remote control, noise levels are less of a problem – you can easily run the purifier when it won’t disturb anyone. Voice control using Alexa, Siri or the Google Assistant is a standard feature for many manufacturers, as are built-in air quality sensors. A display on the device itself or in its smartphone app will show you when your air is “in the green”.

Air filtration technologies

Mechanical pre-filters

The first filter in an air purifier is usually a fine mesh or fabric. It works similarly to a sieve and fishes out the coarser impurities from the air that flows through it. Subsequent filters, including HEPA filters, don’t get clogged so quickly. Pre-filters typically don’t need to be replaced, but only vacuumed or washed clean.

Activated carbon filters

Just like in the range hood above your stove, activated charcoal traps lingering odors in the purifier, removing them from the air. In addition to its fine-pored structure, the activated charcoal absorbs volatile pollutants such as formaldehyde, VOCs, and gases. Carbon filters are less suited to filter solids, such as asbestos or fine particles, which is why they are usually combined with other filtration technologies.


Airborne particles can be “washed” or “scrubbed” out of the air. The air flow is guided over a wet surface where the water traps the pollutants. However, this only works to capture larger particles, such as household dust and pollen. For especially fine particles and viruses, additional filtration stages are needed.

High Efficiency Particulate Air (HEPA) filters

Fine particles are handed by a HEPA filter. Using interlocking glass fibers, it catches microscopic particles, including viruses. There are various classes of HEPA filters, where the best devices are so efficient that they remove almost 100 percent of particles.

Ionization and plasma filters

Some manufacturers use special technology to electrically charge the particles in the air. They ionize the air or use plasma generators to separate and dissolve pollutants. Important: This process should never produce ozone, because ozone is an aggressive form of atmospheric oxygen that attacks the respiratory tract.

Thermal sterilization

Heat disinfects. Air purifiers that use a thermal core use this fact to their advantage. They heat the air to 200 degrees for a short time, burning pathogens without heating their surrounding environment. This does not affect inorganic substances, like dust – it sterilizes rather than filters.

UVC light

Short-wave ultraviolet light (UVC) destroys the genetic material that makes up viruses, bacteria and mold spores, and has long been used for disinfection. Many air purifiers have this technology built-in: A UVC lamp shines on the HEPA filter, disinfecting it and the air that passes through it to neutralize pathogens.

Practical tip

Most of the filters used in air purifiers are consumables. They lose effectiveness over time and must be regularly replaced. Depending on the model, they may last only a few months. Keep in mind the cost of replacement filters when comparing the price of various purifiers.

Combination devices

More than just clean air

Some air purifiers combine additional functionality in the one device. They may move the air like a typical fan or heater as well as purifying it. Others can function as humidifiers, providing additional protection against cold viruses in winter where overly dry and heated air makes the sinuses susceptible to infection. An air purifier that also humidifies supports the body’s natural defenses.

A coastal climate

Asthmatics and allergy sufferers have long known that the coastal climate works wonders on their respiratory tract. They spend time at the sea to be able to breathe deeply. There, the number of electrically charged particles in the air, also known as ions, is significantly higher. Not to mention the finely atomized salt water that is everywhere in the coastal environment. Many people find breathing in these maritime aerosols both soothing and vitalizing. You can achieve both effects in your home climate. Air purifiers with a built-in ionizer charge the air, creating that feeling of freshness. If you add finely atomized salt water, you can recreate a healthy coastal climate.

Self-cleaning devices

Just like with humidifiers, combination devices have a water tank that must be kept clean. This typically means frequent water changes, regular descaling, and deep cleaning to prevent bacteria and mold from multiplying in the tank. Modern devices use technology to minimize this maintenance. A UV light may shine on the water to disinfect it, and/or antibacterial substances such as silver, may be used in the evaporation filter. Almost 100 percent of all viruses and bacteria in the water can be neutralized with these technologies.

Practical tip

Even if it is appropriate, never put an air purifier directly against the wall or in a corner of the room. The fan usually needs free space to suck in air at sufficient quantities. A distance of at least 30 cm works best.

Specialized devices

Portable purification

Many air purifiers are mobile. They are mounted on rollers and can be moved to wherever they are needed. But there are devices that are even more portable: They can be taken anywhere to provide a feeling of clean and fresh air – to the office desk, a guest toilet or even your car. Depending on the manufacturer and model, they may use short-wave UV (UVC) light, activated carbon filtration or electrostatic technology. Their filters will determine the effect they have on airborne particles, viruses and odors. In general, these portable devices only have a limited effective radius, where they purify the surrounding air within one to two meters, and not the entire room.

Surface disinfectant

UV light is frequently used in many professions, such as to disinfect medical instruments. A UV lamp irradiates the surfaces, emitting ultraviolet light with a wavelength of between 200 and 280 nanometers (0.20-0.28 micrometers.) This particularly short-wave band in the UV spectrum is called UVC, and has a germicidal effect. Disinfection lamps for use in households work in the same way – they expose the room to ultraviolet light.

Since UV radiation can be harmful to skin and eyes, the lamp should be fitted with a safety shut-off system based on motion detection. The UV source would automatically turn off the moment a human or animal walks into the irradiated area. There is one essential difference to a UV filter in a purifier with a fan: The lamp alone does not circulate the air, it only neutralizes pathogens that are directly exposed to the UV light. Viruses or bacteria in the shade, for example, under furniture, are not destroyed.

Practical tip

Filters require regular cleaning to do their job properly. Dust particles and lint accumulate on the pre-filters and should be removed with a vacuum cleaner or by washing. Weekly dusting of the intake openings is also a good idea.

Air quality sensors

Smart sensors

The problem posed by air pollution is that you typically can’t see or even smell it. But that doesn’t make particulates, formaldehyde, carbon monoxide, and the like any less dangerous. If you wait until symptoms like dizziness or headaches develop, you may already be too late. With sensors, that doesn’t happen. Sensors measure the air quality in the room and warn you if it deviates from the expected values. Both stand-alone devices with a display or an app on your smartphone provide you with sufficient warning when paired with such sensors. Smart home solutions can do more, for example, by displaying a warning activating colored lights when air quality limits are exceeded. Alternatively, they can automatically activate an air purifier via a wirelessly controlled power socket.


In addition to measuring room temperature and humidity, VOC sensors are some of the most common additional features. They can derive supplementary information using the concentration of organic compounds. For example, a higher level of VOCs can indicate “stale air” and the sensor can notify you when it’s time to open a window. Alternatively, CO2 sensors will directly measure the carbon dioxide content in the air. This should not be confused with CO sensors, which monitor levels of the toxic carbon monoxide gas. Expensive and high-quality sensors monitor more than a dozen different things – from fine particulate concentrations and ozone, to the radioactive radon gas and nitrogen oxides from combustion engines.

Practical tip

Don’t operate an air purifier while you have a window open, especially in dirtier environments or when the pollen count is high, because new particles will constantly stream in from outside. The purifier must then work harder, requiring more energy.

Proper ventilation

The traffic light principle

Filters and purifiers remove pollutants and odors, but they can’t supply fresh oxygen to the room from outside, or send overly humid air outside. That’s why regular ventilation is so important. The pandemic is yet another good reason to open windows and doors to give a room a good airing. Some smart home systems will remind you to air. When set up, they indicate air quality using the traffic light system of colors: Green means everything is fine, yellow indicates you need to improve the air quality, and red is a prompt to act quickly. This not only helps you to avoid viruses, it also protects your wellbeing in other ways: Oxygen-poor, stale air makes you tired.

Save energy

Winter poses a problem: A lot of heat escapes outside through open windows. Overly frequent intermittent ventilation therefore costs a lot in heating energy. But the smart home offers a solution for this too. Modern heating controllers with built-in ventilation detectors will turn the heater off when they notice an open window by using contact sensors installed on the window frame, or a sensor that detects a sudden drop in temperature. And if you have a good purifier in your room, you need to air less regularly. Thus, air purifiers also help you save on heating costs.

Practical tip

Wherever possible, ensure there is a path for a draft when you air. An open window alone exchanges only a little air. If several windows or doors are open at the same time, water vapor, gases and pollutants are drawn out faster. Air that moves across the room or through the entire home is best.

Pictures: Airthings, Beurer, Blueair, Bosch, Dyson, Eve Systems, iStock, LG, Netatmo, Osram, Philips, Signify, Soehnle