Many people assume that every air‑conditioner cleans the air as well as cools it. In reality, the answer is more nuanced. Modern air‑conditioning units include filters, but those filters are primarily designed to protect the equipment and maintain efficiency rather than to act as standalone air purifiers. Here we explain how air‑conditioning systems move air, what their filters do and how you can optimise indoor air quality.
How air conditioners handle air
A typical domestic air‑conditioner recirculates the air already inside a building. Although the condenser coil may sit outside, central and window units do not normally draw in fresh air from outdoors. Instead they pass room air over a cold coil to remove heat and humidity, then send the cooled air back into the room. Portable air conditioners work in a similar way and vent hot air outdoors via a hose.
Because the same indoor air keeps circulating, any airborne particles, odours or gases produced inside will stay in the home unless they are trapped by a filter or removed by ventilation. Opening windows, running bathroom fans and using kitchen range hoods helps exchange stale indoor air for fresh outdoor air. Upgrading the filter in your heating, ventilation and air‑conditioning (HVAC) system to the highest MERV rating your system can handle (ideally MERV 13) can also reduce unwanted particles when combined with proper ventilation.
What air‑con filters are designed to do
The primary purpose of an air‑conditioning filter is mechanical protection. Filters capture dust, hair and other debris so that these particles do not accumulate on the evaporator coil or clog the condensate drain. Removing particulate matter from the air circulating through the system prevents build‑up on interior components, maintains efficiency and extends equipment life. An additional benefit is that those particles then do not continue through the ductwork and into your living spaces. In some cases, good filtration can improve indoor air quality by as much as 60 %.
Air‑con filters achieve this by forcing the air through a fibrous medium. As air passes through, larger particles are captured. The degree of filtration depends on the filter material and its rating.
Filter types and their effectiveness
|
Filter type |
Key characteristics |
|
Fibreglass |
Basic, inexpensive filters found in many systems; allow good airflow but typically capture only around one‑fifth of airborne particles. They protect equipment but do little for air quality. |
|
Pleated (cotton or polyester) |
Greater surface area than fibreglass. Pleats trap more dust and pollen; their MERV ratings are typically in the mid‑range. |
|
High‑efficiency particulate air (HEPA) |
Capture nearly all particles 0.3 µm and larger. They can remove tiny pollutants including bacteria and viruses. Because they restrict airflow, most domestic HVAC units cannot accommodate them; they are better suited to dedicated air purification units. |
|
Activated carbon |
Often combined with pleated media; carbon adsorbs odours and volatile organic compounds (VOCs). Adds airflow resistance but can be acceptable in many systems. |
|
UV & electronic filters |
UV filters use ultraviolet light to kill microorganisms; electronic precipitators use electrostatic charges to capture particles. They typically work alongside conventional filters. |
Understanding MERV ratings
Filters are rated by their minimum efficiency reporting value (MERV), which measures their ability to capture particles between 0.3 µm and 10 µm. Higher MERV numbers indicate better filtration but also greater airflow resistance. Most homes use filters between MERV 8 and MERV 12 to balance airflow and filtration. Upgrading to MERV 13 or the highest rating your system can handle captures smaller particles, including viruses. However, upgrading is not always possible; more robust filters make the fan work harder and may reduce cooling performance.
When are air‑con filters effective?
Filters only work when air is passing through them. In many homes air conditioners run less than a quarter of the day; therefore the filter removes pollutants only during those periods. To increase filtration time you could set the fan to ‘On’ rather than ‘Auto’ (which runs the fan continuously), but this increases electricity use and may lead to over‑cooling. For continuous filtration without overworking your air‑conditioner, consider a standalone air purifier that can operate 24/7.
Do air‑conditioners improve indoor air quality?
In summary, yes — to a degree. Air‑con filters capture larger particles such as dust, pet dander and pollen, which keeps those contaminants from circulating in your home. Using a higher‑quality filter can even trap fine particles, bacteria and some viruses, and with the right filter you could improve indoor air quality significantly. Upgrading to a MERV 13 filter, when your system can handle it, provides better protection against viruses.
However, air‑conditioning filters are not designed to remove everything. They mainly deal with particulates, not gases or odours. Activated carbon filters can absorb some chemicals and smells, but standard filters will not remove VOCs from cleaning products or cooking. Furthermore, filters are only effective when air is moving through them, and many systems recirculate air rather than bringing fresh air inside. For better indoor air quality you should:
- Ventilate – open windows and use fans to bring outdoor air in when conditions are safe.
- Run your HVAC fan longer – continuous fan operation increases filtration time, though it may increase energy use.
- Use a portable air cleaner – HEPA purifiers can capture particles that an air‑con filter may miss.
- Choose the right filter – consider pleated, carbon or HEPA filters appropriate for your system; consult a professional if unsure.
Maintain the system – inspect and replace filters every 30–90 days depending on usage and conditions. Dirty filters restrict airflow, reduce efficiency and can contribute to problems such as frozen coils and water leaks.