What is Clean Air Delivery Rate (CADR) and how should you interpret it?

Key points

• CADR (Clean Air Delivery Rate) tells you how much clean air an air cleaner delivers for a specific pollutant type (usually particles).

• CADR is best thought of as: CADR = airflow × removal efficiency (for that test pollutant).

• A higher CADR generally means a device can reduce pollutant levels faster in a given room, but CADR does not tell you everything about real-world performance.

• CADR results depend on test conditions (room/chamber, mixing, pollutant type, and how “clean” is defined).

• For practical decisions, CADR should be interpreted alongside noise, energy use, maintenance, and—where relevant—gas/VOC performance and by-product checks.

Clean Air Delivery Rate (CADR) is one of the most widely used numbers in portable air cleaning.CADR is a useful indicator—but only if you understand what it does and does not represent.

This article explains CADR, how it relates to room air changes, what it misses, and how building teams and homeowners can use it without being misled.

What is CADR, in one sentence?

CADR is the volume of clean air a device delivers per unit time for a specified pollutant.

You will usually see it reported in m³/h or cfm.

CADR is most commonly used for particles (for example, “smoke”, “dust”, and “pollen” in some standards). It is not automatically a gas/VOC metric.

What does CADR actually measure?

A portable air cleaner does two basic things:

1. Moves air through the device (airflow)

2. Removes some fraction of a pollutant from that air (removal efficiency)

CADR combines these into one number.

A helpful approximation is:

CADR ≈ airflow × single-pass removal efficiency (for a test pollutant)

So:

• A device with high airflow but weak filtration can have a modest CADR.

• A device with strong filtration but low airflow can also have a modest CADR.

• High CADR usually requires both.

Why are there different CADR values (smoke, dust, pollen)?

Many CADR standards report different values because different pollutant sizes behave differently.

• “Smoke” typically refers to smaller particles.

• “Dust” and “pollen” refer to larger particles.

A device might perform differently across these sizes because of:

• filter characteristics,

• leakage/bypass around the filter,

• fan curve and pressure drop, and

• how the test method defines particle categories.

For that reason, CADR should be read as “performance for a defined test pollutant under a defined method,” not as a universal measure of “all pollution removal.”

How do you use CADR to choose a device for a room?

A simple method is to connect CADR to the idea of clean air changes.

Step 1: estimate your room volume

Room volume (m³) = floor area (m²) × ceiling height (m)

Example:
A 20 m² room with a 2.5 m ceiling has a volume of 50 m³.

Step 2: relate CADR to an “equivalent clean air change rate”

If CADR is in m³/h:

Equivalent clean air changes per hour (eACH) ≈ CADR ÷ room volume

Example:
If CADR = 250 m³/h in a 50 m³ room:

eACH ≈ 250 ÷ 50 = 5 air changes per hour of “clean air” (for the pollutant used in the CADR test).

This is a helpful way to compare devices and to understand how quickly air cleaning can reduce pollutant concentrations—assuming the room is reasonably mixed.

Step 3: check with noise and usage

Many devices can achieve a high CADR only at their highest fan speed. In real rooms, people often run devices on quieter modes. Therefore:

• Compare CADR at the fan speed you will actually use, if data are available.

• If only one CADR is advertised, consider whether it likely reflects a high setting.

What CADR does not tell you?

CADR is valuable, but it leaves out several issues that matter for real buildings.

1) CADR is usually about particles, not gases

If your main concern is VOCs, odours, NO₂, or ozone, CADR may not address it directly. Particles and gases require different mechanisms.

2) CADR assumes a certain test environment

CADR is determined under controlled conditions, typically in a chamber with defined mixing assumptions. Real rooms can have:

• dead zones,

• complex airflow patterns, and

• changing sources (cooking, cleaning, outdoor pollution peaks).

So CADR is best treated as a comparable baseline, not a guarantee of real-room outcomes.

3) CADR doesn’t include by-products or “secondary pollution”

CADR focuses on how fast a pollutant concentration falls. It does not, by itself, tell you whether a technology generates unintended by-products. For that lens, see:
https://healthyairtech.com/indoor-air-knowledge-hub/secondary-air-pollution/

CADR, ventilation, and “clean air”: how do they relate?

Ventilation brings in outdoor air and exhausts indoor air. Air cleaning recirculates indoor air through a device and removes pollutants.

From an engineering point of view, you can often think in terms of total “clean air”:

Total clean air ≈ outdoor air ventilation + filtered/cleaned recirculated air

This is why CADR is useful: it puts air cleaning into the same “flow rate” language used for ventilation.

That said, ventilation and air cleaning are not interchangeable for every pollutant. For example:

• Ventilation is essential for CO₂ control.

• Air cleaning can be effective for particles without needing to increase heating/cooling loads associated with outdoor air.

Common misunderstandings about CADR

“A high CADR means it removes everything”

No—CADR is pollutant-specific and method-specific.

“CADR guarantees performance in my room”

Whilst CADR is a strong comparison metric, real rooms differ in mixing, sources, and operation mode.

“CADR tells me about gas removal”

Usually it does not. Gas/VOC performance needs separate evidence (adsorbent capacity, catalytic performance, specific gas tests).

Practical checklist for interpreting CADR

When you see a CADR value, consider asking:

1. CADR for what pollutant?
   Smoke/dust/pollen (particles) are common.

2. At what fan speed was it measured?
   Will you realistically run the unit at that setting?

3. How does it translate to my room size?
   Use eACH ≈ CADR ÷ room volume as a first pass.

4. What about noise and energy?
   A device you can’t run constantly won’t deliver its CADR benefit.

5. Do I also need gas/VOC control?
   If yes, look for specific gas/VOC evidence, not CADR alone.

Summary

CADR is a useful, comparable metric for how much particle-cleaned air a device delivers. It is best interpreted as airflow × particle removal efficiency under defined test conditions. For real buildings and homes, CADR should be combined with room volume (to estimate equivalent clean air changes), and then tempered with practical realities: noise, energy use, maintenance, and whether your problem is particles, gases, or bioaerosols.