How Air Purifiers Work | Filters, HEPA, CADR Explained Simply

How Do Air Purifiers Work?

At a basic level, an air purifier works by:

1. Pulling air from the room
2. Passing it through one or more filters
3. Releasing cleaner air back into the room

No magic is involved.
Air purifiers are mechanical filtration devices driven by a fan.

Understanding the components helps you judge what really matters — and what doesn’t.

The three main parts of an air purifier

Most household air purifiers consist of:

1. A fan
2. One or more filters
3. A housing that guides airflow

Everything else is secondary.

1. The fan – moving air through the system

The fan:

• Draws polluted air into the purifier
• Pushes air through the filters
• Determines how much air is cleaned per hour

A purifier that filters well but moves very little air is ineffective.

This is why airflow matters as much as filter quality.

2. Filters – where cleaning actually happens

Pre-filter

The first layer is usually a pre-filter.

It captures:

• Large dust particles
• Hair and lint
• Pet fur

This protects the main filter and extends its life.

HEPA filter – the most important part

A HEPA (High Efficiency Particulate Air) filter is designed to capture very fine particles.

A true HEPA filter:

• Captures at least 99.97% of particles of size 0.3 microns
• Is highly effective against PM2.5

Despite marketing claims, HEPA filtration is the single most important feature of an air purifier.

Activated carbon filter (optional)

Some purifiers include an activated carbon filter.

This helps reduce:

• Odours
• Some gases and chemical fumes

Carbon filters do not remove PM2.5.
They address different types of pollutants.

What is CADR and why it matters

CADR stands for Clean Air Delivery Rate.

It tells you:

• How much clean air a purifier delivers per unit time
• Usually expressed in cubic metres per hour (m³/h)

Higher CADR means:

• Faster reduction of indoor pollution
• Better performance in larger rooms

CADR is often more important than fancy features.

Matching purifier capacity to room size

Air purifiers are designed for specific room sizes.

A common mistake is buying:

• A purifier too small for the room

This leads to:

• Poor performance
• False sense of safety

As a general guideline:

• The purifier should be able to clean the room air at least 4–5 times per hour

This depends on both CADR and room volume.

Air changes per hour (ACH) – a useful concept

ACH refers to how many times the air in a room is cleaned in one hour.

Higher ACH:

• Reduces pollution faster
• Is helpful during high outdoor pollution events

Low ACH:

• May not keep up with incoming pollution

What about ionisers, UV, and “advanced” features?

Some purifiers include:

• Ionisers
• UV lamps
• Plasma or electrostatic features

These may have limited benefit in normal home settings and can introduce new concerns if poorly designed.

For most homes:

• Good airflow + true HEPA filtration matter far more

What air purifiers do not do

Air purifiers:

• Do not add oxygen
• Do not remove carbon dioxide (CO₂)
• Do not replace ventilation
• Do not eliminate pollution sources

They are support systems, not complete solutions.

Key takeaway

• Air purifiers clean air by mechanical filtration
• HEPA filters are the most important component
• CADR and room size matching matter greatly
• Simple designs often work better than feature-heavy ones
• Understanding basics prevents poor purchasing decisions

Compare & buy Air Purifiers

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