What does air changes per hour (ACH) mean, and how does it relate to air cleaners?

Key points

• ACH (air changes per hour) is a measure of how many times the air in a room is replaced (or effectively cleaned) in one hour.

• Buildings usually have a ventilation ACH (outdoor air brought in) and can also have an equivalent ACH from air cleaners (cleaned recirculated air).

• You can estimate an air cleaner’s equivalent ACH (eACH) from its CADR: eACH ≈ CADR ÷ room volume (with CADR in m³/h and volume in m³).

• ACH is a useful planning metric, but real performance depends on mixing, placement, operation speed, and maintenance.

• For indoor air quality projects, the core question is often: “How much clean air do we need for this space, and can we deliver it quietly and consistently?”

This article explains ACH: how ventilation ACH differs from air-cleaner equivalent ACH, and how to use both for real rooms such as homes, offices, classrooms, and clinics.

What is ACH?

ACH is the number of room-volumes of air moved (or effectively cleaned) per hour.

If a room has an ACH of 4, that means the system is moving an amount of air equal to four times the room’s volume every hour—either by bringing in outdoor air (ventilation) or by cleaning indoor air (air cleaning).

CIBSE uses air change rates as a way to describe ventilation provision in rooms.

How do you calculate ACH?

In metric units, if airflow is in m³/h and room volume is in m³:

ACH = airflow (m³/h) ÷ room volume (m³)

Example

A room volume of 50 m³ and ventilation airflow of 200 m³/h:

ACH = 200 ÷ 50 = 4 ACH

That is why ACH is a helpful “common language” between ventilation engineers and people choosing portable air cleaners: it connects devices to room size.

What is the difference between ventilation ACH and air-cleaner ACH?

Ventilation ACH (outdoor air)

Ventilation ACH refers to outdoor air entering the room (and an equal amount of indoor air leaving). This is what improves:

• removal of occupant-generated CO₂ (a key indicator of ventilation adequacy), and

• dilution of indoor-generated pollutants.

Ventilation is essential, but it can be constrained by:

• weather (opening windows is not always practical),

• energy use (heating/cooling outdoor air),

• outdoor pollution peaks (traffic, smoke, etc.), and

• building HVAC limits.

Equivalent ACH (eACH) from air cleaners

Portable air cleaners do not bring in outdoor air. Instead, they recirculate indoor air through a device and remove pollutants. For that reason, we often use equivalent ACH (eACH) or clean air ACH to express their effect.

The idea is:

“How much cleaned air per hour does the device deliver, compared with the room volume?”

How does CADR relate to ACH?

If you have a CADR value (Clean Air Delivery Rate), you can translate it into equivalent ACH:

eACH ≈ CADR ÷ room volume

• CADR in m³/h

• room volume in m³

• result in per hour

Example

Room volume = 50 m³
Air cleaner CADR = 250 m³/h

eACH ≈ 250 ÷ 50 = 5 eACH

This means the air cleaner provides the equivalent of about five room-volumes of particle-cleaned air per hour, assuming the room is reasonably mixed.

Why doesn’t “ACH” guarantee performance?

ACH is a planning metric. The actual benefit depends on whether the air cleaner (or ventilation supply) is cleaning the air people are breathing.

Common real-world issues include:

1) Mixing and dead zones

A room can have a high calculated ACH while still having corners or zones with weak circulation. In practice:

• people sit where they sit,

• furniture and partitions matter, and

• airflow paths matter.

This is why placement and airflow design can affect performance even when the “numbers” look good.

2) Operating speed (noise matters)

Many air cleaners achieve their published CADR only at higher fan speeds. In real settings, teams often lower fan speed for comfort and noise, which reduces eACH.

Choose a device that can reach your target eACH without requiring an intolerably loud setting.

3) Maintenance and loading

Filters load over time. Sorbents saturate. If maintenance slips, real clean-air delivery declines. ACH calculations assume the system remains in its tested condition.

4) What pollutant are you trying to control?

Particles and gases require different mechanisms.

• ACH/eACH derived from CADR usually describes particle cleaning.

• Gas and VOC control needs separate evidence (adsorbents or catalytic performance).

• CO₂ control depends on outdoor air ventilation.

How much ACH do you “need”?

There is no single universal number, because the right target depends on:

• room use (quiet office vs busy classroom),

• occupancy,

• pollution sources (outdoor PM, indoor VOCs, etc.), and

• objectives (comfort, odours, smoke events, resilience).

However, ACH/eACH is still useful because it lets you frame a design question:

“If we want cleaner air in this room, what clean-air flow rate do we need, and how do we deliver it reliably?”

Choose a target range for clean air delivery (using ventilation plus air cleaning), then check whether it is achievable at acceptable noise and energy levels.

A simple step-by-step method for using ACH with air cleaners

Step 1: calculate room volume

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

Step 2: estimate existing ventilation ACH

If you have ventilation airflow data, compute:
ACH = airflow ÷ volume

If you don’t have airflow data, CO₂ trends can still provide a useful qualitative indicator of ventilation effectiveness, but it is not a direct ACH calculation.

Step 3: decide what problem you are solving

• Particles (PM₂.₅/PM₁₀) → eACH from CADR is a useful planning tool.

• VOCs/odours → look beyond CADR and check gas/VOC evidence.

• CO₂/occupancy → focus on ventilation.

Step 4: translate device CADR into eACH

eACH ≈ CADR ÷ volume

Then sanity-check whether the required fan speed is realistic for continuous use.

Step 5: verify with measurements

Once deployed, confirm outcomes with:

• PM₂.₅/PM₁₀ or particle sensors for particle-focused goals

• TVOC trends where relevant

• CO₂ monitoring for occupancy and ventilation patterns

This keeps the process grounded in what is actually happening in the room.

Implications for buildings

For many indoor air projects, the strongest approach is to treat clean air as a combined resource:

• Ventilation provides outdoor air and is essential for CO₂ control.

• Air cleaning can provide high rates of particle removal without increasing heating/cooling loads associated with large outdoor air volumes.

• Together, they help building teams reach clean-air targets more robustly across seasons and occupancy changes.

For a technology overview of air cleaners’ abilities, see:
https://healthyairtech.com/indoor-air-knowledge-hub/how-indoor-air-purification-technologies-work/

Summary

ACH is a simple way to express how much air is replaced or cleaned in a room per hour. Ventilation ACH describes outdoor air exchange; air cleaners contribute an equivalent ACH based on how much cleaned air they deliver (often estimated from CADR). Used carefully, ACH helps translate air cleaning into building language. Used carelessly, it can create false certainty. The goal is to deliver enough clean air in the zone—quietly, consistently, and with maintenance that keeps performance stable.