The Core Mechanism: Fan + Filter
Every air purifier — regardless of brand, price, or marketing language — operates on the same fundamental principle: a fan draws room air inward, passes it through a filter medium, and pushes filtered air back out. The fan creates pressure differential; the filter captures or neutralises pollutants in the airstream.
The effectiveness of any air purifier is determined by two factors: how good the filter is at capturing the particles you care about, and how much air the fan moves through that filter per minute. Both matter equally. An excellent filter with a weak fan cleans air slowly. A powerful fan with a mediocre filter moves a lot of air without cleaning it well. CADR — Clean Air Delivery Rate — measures both together as a single whole-system metric.
The Filtration Stages Explained
A well-engineered air purifier uses multiple sequential filtration stages, each targeting a different category of pollutant.
Pre-filter
A washable or replaceable coarse mesh that captures large particles: hair, lint, large dust, pet fur. Its purpose is to protect the HEPA filter from large debris that would clog it prematurely. The pre-filter is typically the only washable component of the filtration stack.
HEPA filter (True HEPA)
A dense mat of randomly arranged glass fibres. Captures 99.97% of particles at 0.3 microns (the Most Penetrating Particle Size) through four mechanisms: inertial impaction, interception, diffusion, and electrostatic attraction. This stage handles fine particulate matter — dust mite particles, pollen, mould spores, pet dander, smoke particles, PM2.5.
Activated carbon filter
Activated carbon is carbon that has been processed to create millions of microscopic pores, giving it enormous surface area. VOCs, cooking fumes, formaldehyde, odour molecules, and some gases adsorb (bond) onto this surface as air passes through. The carbon layer does nothing for particles. The HEPA layer does nothing for gases. Both stages are necessary for comprehensive air cleaning.
Optional: UV-C stage
Some purifiers add a UV-C light chamber after the HEPA stage. UV-C at 253.7nm wavelength can damage the DNA of microorganisms — bacteria, viruses, mould spores — that pass through it. The effectiveness depends on the intensity of the lamp and the dwell time (how long particles are exposed). In most consumer purifiers, UV-C dwell time is too short to achieve reliable sterilisation. It is a supplementary feature, not a primary cleaning mechanism.
Optional: Ioniser stage
Some purifiers add a negative ion generator. Ions charge airborne particles, causing them to clump (agglomerate) and settle on surfaces more quickly, or adhere to a charged collection plate inside the purifier. See the technology comparison below for why ionisers are controversial and often counterproductive.
Every Air Purifier Technology Compared
What Each Technology Actually Removes
| Pollutant | HEPA | Activated Carbon | Ioniser | UV-C |
|---|---|---|---|---|
| Dust, pollen, pet dander | Yes — highly effective | No | Partial (settles to surfaces) | No |
| Smoke particles (PM2.5) | Yes — highly effective | Partial (captures odour component) | No | No |
| Mould spores | Yes | No | Partial | Partial (inactivates, doesn't remove) |
| Bacteria | Mostly (≥0.2µm) | No | No | Partial (exposure-dependent) |
| Viruses | Indirect (on larger particles) | No | No | Partial (exposure-dependent) |
| VOCs / formaldehyde | No | Yes | No | No |
| Cooking odours | No | Yes | No | No |
| Carbon monoxide / ozone | No | Partial (specialised carbon) | No | No |
| Humidity | No | No | No | No |
Air purifier technology effectiveness by pollutant type. HEPA and activated carbon complement each other — combined, they address the majority of indoor air quality concerns.
How Placement Affects Performance
The same air purifier can perform very differently depending on where it is placed in a room. The AHAM CADR test assumes a well-mixed, sealed chamber. Real rooms are not well-mixed — air stagnates in corners, behind furniture, and away from the fan's draw zone.
- Place near the primary particle source: Near a window if smoke is entering from outside, near a pet's bed if dander is the concern, near cooking areas for kitchen odour management.
- Avoid corners and behind furniture: These positions restrict airflow into the intake and reduce effective CADR. The purifier needs a clear zone of 18–24 inches on all sides.
- Central placement maximises room-wide coverage: In a bedroom, a central position or near the bed (not directly blowing on the sleeper) allows the fan to draw from all sides and distribute clean air across the room.
- Elevation matters less than clearance: Most purifiers draw from the sides or base and push air upward or forward. Raising the unit slightly can improve mixing in rooms with high ceilings but the effect is modest.
CADR and ACH: Measuring Whether It's Working
CADR (Clean Air Delivery Rate) measures how much filtered air the purifier produces per minute. ACH (Air Changes per Hour) translates that into a meaningful room-specific metric: how many times the entire room's air volume passes through the filter per hour.
A purifier with 250 CFM in a 300 sq ft × 8 ft room (2,400 cu ft): (250 × 60) ÷ 2,400 = 6.25 ACH
At 6 ACH, room air is filtered approximately every 10 minutes — appropriate for allergy management or wildfire smoke events.
AHAM recommends 4–5 ACH for general air cleaning. Five ACH or above is appropriate for allergy, asthma, or smoke contexts. At less than 4 ACH, the purifier may not meaningfully reduce particle concentrations in large or high-traffic rooms.
Common Misconceptions
An air purifier ventilates the room
Air purifiers recirculate and clean existing room air. They do not bring in fresh outdoor air or add oxygen. For ventilation, you need an HRV (Heat Recovery Ventilator) or simply open a window.
Ionisers are an upgrade over HEPA
Ionisers do not filter air — they redistribute particles. They cannot replicate HEPA's 99.97% capture rate. Some produce ozone. In independent testing, ionisers consistently underperform mechanical HEPA filtration for particle reduction.
Running at low speed all the time is fine
CADR decreases substantially at low fan speed. A purifier rated at 300 CFM at maximum might deliver only 80 CFM on its lowest setting — insufficient for a 200 sq ft room. Match fan speed to the ACH required for your room and use case.
Air purifiers remove CO2
No air purifier removes carbon dioxide. CO2 is a gas that requires ventilation — fresh outdoor air — to dilute. A sealed room with people and a running air purifier will still accumulate CO2. For CO2 management, open windows or use mechanical ventilation.
How air purifiers work — in summary
A fan draws room air through a pre-filter (large debris), a True HEPA layer (fine particles at 99.97% efficiency), and an activated carbon layer (gases, VOCs, odours). Clean air is returned to the room. The process repeats continuously, reducing particle and gas concentrations over time. The speed of cleaning depends on CADR relative to room volume. No air purifier removes CO2, humidity, or radon. No ioniser replicates HEPA performance. The combination of True HEPA + activated carbon addresses the majority of indoor air quality concerns.
Frequently Asked Questions
How does an air purifier work?
An air purifier draws room air through a fan, passes it through one or more filter stages (typically a pre-filter, HEPA filter, and activated carbon layer), and returns cleaned air to the room. The process repeats continuously, gradually reducing particle and VOC concentrations. The speed of improvement depends on CADR relative to room volume.
What does an air purifier actually remove?
A True HEPA air purifier removes dust, pollen, pet dander, mould spores, smoke particles, and PM2.5 fine particulate at 99.97% efficiency. The activated carbon layer removes VOCs, odours, cooking fumes, and some gases. Neither stage removes CO2, humidity, radon, or very large particles that settle quickly before reaching the purifier's intake.
Do ionisers work as well as HEPA air purifiers?
No. Ionisers charge particles so they settle on surfaces or clump together — they do not capture particles in a contained filter. They redistribute rather than remove. Some ionisers produce ozone, which is a respiratory irritant at concentrations needed for air cleaning. HEPA filtration is significantly more effective and safer.
What is the difference between an air purifier and an air filter?
An air purifier is an active device with a fan that draws air through filter media on its own. An air filter is a passive component — it only filters air that naturally flows through it, such as HVAC filters. A standalone air purifier creates its own airflow and cleans room air independently of any HVAC system.
Should I run an air purifier continuously?
Yes, for best results. Air purifiers work by continuously cycling room air. Turning a purifier off allows particle concentrations to rebuild. Running on low or medium continuously is more effective than running at maximum speed intermittently. Most modern purifiers use less electricity on low speed than a 40W light bulb.