In recent years, the search for a new air conditioner is no longer just about cooling and heating. Manufacturers have turned "Indoor Air Quality" (IAQ) into the number-one sales argument. We read about built-in Plasma Ionisers, ultraviolet (UV-C) lamps, activated carbon filters and technologies that promise to exterminate viruses, bacteria and allergens.
If you have allergies, asthma, or simply worry about the air your family breathes, it is very easy to get carried away and pay hundreds of euros extra for a machine with these "magic" features.
As engineers, it is time to state a harsh but necessary truth: A wall-mounted air conditioner is not an Air Purifier. Let us put these technologies under the microscope of physics to see what they truly deliver and when the marketing exceeds reality.
Open the cover of any air conditioner and you will see two large plastic mesh screens. Many people believe these filter the room's air.
These basic, washable filters are extremely coarse. They were not designed to protect your lungs from pollen or fine particulates (PM2.5). They were designed exclusively to protect the machine itself (the fan and coil) from coarse dust, hair and lint. If all that debris got through, the air conditioner would clog within a month. They do not clean the room's air.
The Ioniser is the most popular "extra" in modern AC units. By pressing the tree icon on the remote, you activate a small device inside the machine that releases thousands of negative ions into the room air.
Dust, smoke and pollen floating in the room usually carry a positive electrical charge. The negative ions attach to these particles like magnets. As they combine, they become heavy and, due to gravity, "fall" to the floor or stick to walls and furniture.
It genuinely helps reduce odours (e.g. cigarette smoke, cooking) and gives a sense of "freshness" (like the air after a thunderstorm).
The ioniser does not make the dust disappear from the room. It merely forces it to settle on the floor. If you don't vacuum, the first gust of air will lift it back up. Moreover, older or cheap ionisers produce traces of ozone (O₃) as a by-product, which in high concentrations can seriously irritate the respiratory system.
Ultraviolet radiation (UV-C) is used in hospitals because it destroys the DNA of viruses and bacteria, killing them. Its incorporation into residential air conditioners became the ultimate trend after the pandemic.
For a UV-C lamp to kill a virus, the virus must be exposed to its light for several seconds. However, the air conditioner fan draws and expels room air at frightening speed (hundreds of cubic metres per hour). The air passes in front of the tiny UV LED in fractions of a second. In so little time, it is impossible to sterilise your living room's air.
The UV-C lamp in air conditioners does not protect you from those coughing in the living room. It protects the machine itself. Because the AC interior is dark with permanent moisture, it is the perfect breeding ground for mould. The UV-C light "bathes" the internal coil, preventing mould growth and the familiar smell (the "wet sock" odour when you first switch on).
True HEPA filters (High-Efficiency Particulate Air) are medical-grade. They capture 99.97% of particles (including viruses). So why aren't they fitted in all AC units?
A true HEPA filter is incredibly "dense" (like thick cardboard). To push air through it requires a massive, powerful fan motor (like those in stand-alone Air Purifiers). A residential wall-mounted air conditioner's fan is far too weak. If you fitted a full HEPA filter, it would "choke" the machine. Air wouldn't pass through, the coil would ice up and the unit would be destroyed.
When you see "Activated carbon filters" or "Bio-filters" on air conditioners, they are usually two tiny strips (about 20×5 cm) that "clip" onto the main mesh screen. Most of the air simply... bypasses these small strips. They offer a marginal improvement, but they do not turn the room into a "sterile chamber".
The built-in purification systems in modern air conditioners (Ionisers, UV-C, filters) are welcome additions. They help keep the machine itself clean internally and slightly reduce odours in the space.
Do not choose an air conditioner based solely on air quality claims, and certainly do not pay exorbitant sums for it. If you or your child have a serious problem with allergies, asthma, or you live in an area with polluted air (smog, wood-burning stoves), the only solution is to buy a separate, stand-alone Air Purifier with a large, genuine cylindrical HEPA filter. Let the air conditioner do what it does best: regulate temperature!
This article also has a video version with the same topic in a more direct walkthrough format.
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