Choosing an air purifier can feel overwhelming. With specifications like CADR and ACH, it's difficult to know which machine is genuinely effective and which is simply too weak or powerful for your space. In the UK, where we spend a significant amount of time indoors, understanding these metrics is the critical first step to improving your air quality.

This guide removes the confusion, explaining how to measure your room, understand the key performance numbers, and calculate exactly what you need. By the end, you will be able to confidently select an air purifier that is perfectly sized to create a healthier environment in your home.

 

Quick Summary: Key Takeaways

For those short on time, here are the most important points from this guide:

• Calculate Room Volume, Not Area: The most accurate way to size an air purifier is by using your room's volume in cubic metres () — length × width × height. Floor area alone can be misleading, especially in rooms with high ceilings.
• ACH is Your Health Metric: Air Changes per Hour (ACH) tells you how many times a purifier cleans all the air in your room each hour. For allergies or asthma, aim for at least 4-5 ACH.
• Use the Formula: To find the power you need, use this calculation: Required CADR (m³/h) = Target ACH × Room Volume (m³)
• Placement Matters: For general coverage, place the unit centrally. To tackle specific issues, place it near the source of the pollution (e.g., a pet's bed). Always allow at least a metre of clear space around it.
• Prioritise Safe Technology: Insist on a True HEPA filter for particles and activated carbon for gases (VOCs). Avoid "ionisers" or other technologies that can produce harmful ozone as a byproduct.
• Check the True Cost: The initial price is only part of the story. Always check the cost and lifespan of replacement filters to calculate the long-term cost of ownership.

 

Understanding the Core Metrics: CADR vs. ACH

To compare air purifiers effectively, you need to understand the two foundational pillars of performance: the Clean Air Delivery Rate (CADR) and Air Changes per Hour (ACH). While they are connected, one measures the machine's raw power, while the other describes its real-world impact in your room.

 

What is Clean Air Delivery Rate (CADR)?

The Clean Air Delivery Rate (CADR) is an industry-standard metric that measures the volume of filtered air a purifier can deliver. Essentially, it's a measure of speed and efficiency. In the UK and Europe, this is expressed in cubic metres per hour (m³/h).

The standard, verified by the Association of Home Appliance Manufacturers (AHAM), provides three separate CADR ratings for three common pollutant sizes:

• Tobacco Smoke: Representing the smallest particles (0.1 to 1.0 microns).
• Dust: Representing medium-sized particles (0.5 to 3.0 microns).
• Pollen: Representing the largest particles (5.0 to 11.0 microns).

For general air quality, the smoke CADR is the most important as it reflects the unit's ability to capture the fine particles that pose the greatest health risks.

 

How is CADR Measured?

The CADR test is highly standardised. It's conducted in a small, sealed chamber (11.8 m²) where a specific pollutant is introduced. The air purifier is set to its highest fan speed, and a sensor measures how quickly the particle concentration decreases. The final score reflects how much faster the machine cleans the air compared to the rate at which particles would naturally settle on their own.

 

What are Air Changes per Hour (ACH)?

While CADR measures a machine's potential, Air Changes per Hour (ACH) measures its practical effect. ACH defines how many times an air purifier can filter the entire volume of air in a specific room in one hour. A purifier with a high CADR will still be ineffective if it's placed in a room that's too large, resulting in a low ACH. This metric answers the most important question: "How well will this machine actually clean my room?"

 

Why ACH is the Most Important Metric for Health

Indoor pollution isn't static; new contaminants are constantly introduced from cooking, cleaning products, pets, and even our own breath. A sufficient ACH is vital to remove these new pollutants before they build up. Public health organisations recommend different ACH targets based on need:

• 2 ACH: A baseline for general air quality improvement (the air is fully filtered every 30 minutes).
• 4-5 ACH: Strongly recommended for those with asthma, allergies, or for rooms with smokers or pets (air filtered every 12-15 minutes).
• 6-8+ ACH: Necessary for high-risk environments like classrooms or healthcare settings to reduce airborne transmission risks.

Your goal is to choose a purifier with a CADR rating high enough to achieve your target ACH in your specific room.

 

A 3-Step Guide to Sizing Your Air Purifier

Now we will translate the theory into a simple, practical process. Follow these three steps to determine the exact power level you need from an air purifier for your room.

 

Step 1: Calculate Your Room's Volume (m³)

First, you need to accurately measure the space. Because airborne pollutants fill the entire three-dimensional volume of a room, calculating the floor area isn't enough. This is especially true in the UK, where ceiling heights can vary from 2.4 metres in modern builds to over 3 metres in older properties. Using floor area alone could lead you to buy a purifier that is 25% undersized.

The gold-standard calculation is your room's volume in cubic metres (m3):

Length (m) × Width (m) × Height (m) = Volume (m3)

This figure represents the total amount of air the purifier needs to clean.

 

Step 2: The Essential Formula (From CADR to ACH)

Once you know your room's volume, you can work out how any air purifier will perform in it. To find the ACH a specific purifier will deliver in your room, use this formula:

ACH = CADR (m3/h) ÷ Room Volume (m3)

• Example: Your living room has a volume of 50 m3. You're looking at a purifier with a smoke CADR of 200 m3/h.
• Calculation: 200 m3/h ÷ 50 m3 = 4 ACH. This is a good choice for managing moderate allergies.

 

More usefully, you can rearrange the formula to find the minimum CADR you need to achieve your health goal:

Required CADR (m3/h) = Target ACH × Room Volume (m3)

• Example: In the same 50 m3 room, you have asthma and want to achieve the recommended 5 ACH.
• Calculation: 5 ACH × 50 m3 = 250 m3/h. You should only look for models with a smoke CADR of at least 250 m3/h.

 

Step 3: A Practical Sizing Shortcut (UK Room Size Table)

To simplify the process, this table provides an at-a-glance reference for common UK room sizes and the minimum CADR required to hit different ACH targets.

Note: This table assumes a standard ceiling height of 2.4 metres. If your ceilings are significantly higher, please calculate your room volume manually and use the formula above for an accurate result.

 

Beyond the Numbers: Real-World Performance Factors

Sizing a purifier correctly is the most important step, but it's not the only one. A high CADR is necessary, but it doesn't guarantee effective air cleaning. You must also consider the limitations of the test, the unit's placement, and the quality of its filtration system.

 

The Great Debate: Is CADR a Flawed Metric?

While CADR is the best tool we have for an apples-to-apples comparison of raw power, it has valid criticisms.

• The Case Against CADR: Critics argue the test is unrealistic. It's done in a small, sealed chamber with external fans mixing the air. This isn't like a real home and may unfairly benefit purifiers with poor air projection. The test also focuses on speed, not quality, and doesn't measure effectiveness against ultrafine particles or gaseous pollutants (VOCs).
• The Case For CADR: Despite its flaws, CADR is the only independently verified, internationally recognised standard for comparing particle removal speed. The AHAM Verifide seal confirms a manufacturer's claims are accurate. Without it, consumers would have to rely on unverified marketing claims.
• The Verdict: A balanced approach is best. Use CADR as your first filter to create a shortlist of models powerful enough for your room. From there, you must investigate other crucial factors to determine true real-world effectiveness, such as the quality of its air filters, its noise levels in different modes, and its total running costs over several years.

 

Strategic Placement for Maximum Effectiveness

Where you put your air purifier has a dramatic impact on its performance. Evidence from airflow modelling studies reveals five key rules for placement.

1. Give it Breathing Room: The unit needs to draw in and expel air without obstruction. Maintain at least 1 to 1.5 metres of clear space around it. Never place it behind a sofa, under a table, or tight against a wall.
2. Place it Near the Source: To capture pollutants before they spread, position the purifier near the primary pollution source. This could be next to a pet's bed, near where a smoker sits, or in an area prone to damp.
3. Central is Best for General Coverage: If there's no single dominant source, a central location gives the machine the best chance to create an effective, whole-room air circulation pattern.
4. Consider Existing Airflow: If you have an HVAC or mechanical ventilation system, place the purifier near the air supply outlet (where air comes in) to help distribute the clean air more widely.
5. Bedroom Placement: Position the unit 2 to 3 metres from the head of the bed. This is close enough to clean your breathing zone effectively but far enough to avoid being disturbed by the airflow.

 

The Critical Role of Filtration: HEPA & Sealed Systems

Powerful airflow is useless if the filtration system is poor. The filter is the heart of the machine.

• Insist on "True HEPA": The relevant UK and European standard is EN 1822. This legally requires a filter to remove at least 99.95% of airborne particles at the 0.3-micron size. This specific size is known as the Most Penetrating Particle Size (MPPS) because it's the hardest for filters to catch, making it the toughest test. Be wary of marketing terms like "HEPA-type" or "HEPA-like," which have no legal definition and are used on inferior filters.
• Demand a Sealed System: A certified HEPA filter means little if the purifier's casing has gaps that allow dirty air to leak past it. A fully sealed system ensures that all the air drawn into the machine is forced through the filter, maximising its effectiveness.

 

Hidden Risks and True Costs

An effective air purifier should improve your health, not introduce new risks. It's vital to assess the safety of its technology and understand the full long-term cost before you buy.

 

Hidden Dangers: Ozone, VOCs, and Secondary Pollutants

While most modern purifiers are safe, certain technologies can release harmful byproducts into your home.

• The Risk of Ozone: Some purifiers, especially those using "ionisation" or "plasma," can generate ozone (O₃) as a byproduct. Ozone is a known lung irritant that can trigger asthma and cause respiratory damage. UK government advisory bodies like SAGE recommend against using air cleaning devices that produce ozone. The safest approach is to prioritise physical filtration over chemical reaction.
• The Challenge of VOCs: Volatile Organic Compounds (VOCs) are gases emitted from paints, furniture, and cleaning products. Standard HEPA filters cannot capture them. To remove VOCs, a purifier needs an activated carbon filter. A thick, granular carbon filter is far more effective than a thin, carbon-dusted sheet.
• The "Solution as a Source" Paradox: Research has shown that some technologies claiming to destroy VOCs can actually create new, more harmful pollutants like formaldehyde. This turns the purifier into a source of pollution. The strong scientific recommendation is to choose purifiers that rely on proven, safe mechanical filtration (HEPA and carbon) and avoid unverified chemical-based technologies.

 

The True Price of Clean Air: Total Cost of Ownership

The initial purchase price of an air purifier is only part of the story. To find the best long-term value, you must consider the Total Cost of Ownership (TCO), which includes two main ongoing expenses:

• Filter Replacements: This is the most significant long-term cost. Check the price and the recommended replacement schedule for a model's filters before you buy. A cheap unit with expensive, short-lived filters can be a poor investment.
• Electricity Use: Air purifiers are designed to run continuously. While most modern units are energy-efficient, their power consumption (wattage) will contribute to your electricity bill over time.

 

Final Verdict: A Checklist Before You Buy

Choosing the right air purifier requires a systematic approach. Before you purchase a unit, run through this final checklist to ensure it’s effective, safe, and economical for your specific needs.

1. Calculate Your Needs First: Measure your room's volume (m3) and determine your target ACH. Use this to calculate the minimum CADR (in m3/h) you require. Don't consider any model that falls below this threshold.
2. Prioritise Filtration Technology: Insist on a True HEPA filter (certified to EN 1822) for particles. If gases or odours are a concern, ensure it has a substantial activated carbon filter.
3. Avoid Risky Technologies: Be extremely cautious of purifiers using ionisers, PCO, or plasma due to the risk of ozone and other harmful by products. Safe, mechanical filtration is the proven approach.
4. Check Long-Term Costs: Look beyond the sticker price. Investigate the cost and replacement frequency of its filters to understand the true long-term cost of ownership.
5. Plan for Placement: Envision where the unit will go. Does it have enough clear space to be effective? Can it be placed centrally or near the main source of pollution?
6. Consider Noise Levels: Check the decibel (dB) ratings for different fan speeds. For bedrooms or offices, a low-speed setting below 35 dB is essential for quiet operation.

 

Frequently Asked Questions (FAQs)

 

What is the most important spec on an air purifier?

For performance in your specific room, the most important metric is Air Changes per Hour (ACH). This tells you how effectively the machine will clean your air. To calculate this, you first need to check the purifier's Clean Air Delivery Rate (CADR) and your room's volume.

Can I use a smaller air purifier in a larger room?

You can, but it will be less effective. A smaller unit in a large room will result in a low ACH, meaning it won't be able to remove pollutants as fast as they are introduced. For health benefits, it's crucial to choose a purifier sized correctly for your room's volume.

Should I run my air purifier all the time?

Yes, for the best results, it's recommended to run your air purifier 24/7. Indoor air pollution is continuous, and pollutant levels can quickly return to their previous state once the device is turned off. Most modern units are energy-efficient enough for continuous operation.

Do air purifiers help with dust?

Yes, very effectively. Air purifiers with True HEPA filters are excellent at capturing airborne dust particles. For best results, look for a model with a high Dust CADR rating. Remember that they only capture airborne dust, so regular surface cleaning is still necessary.

 

Internal Links (Placeholders)

• Anchor text: "True HEPA filter" | Link to: Guide to Air Filters
• Anchor text: "activated carbon" | Link to: Guide to Air Filters
• Anchor text: "noise levels" | Link to: Guide to Noise Levels
• Anchor text: "total running costs" | Link to: Guide to Running Costs
• Anchor text: "best air purifiers for allergies" | Link to: Product Roundup: Best Allergy Air Purifiers

 

References & Notes

[To be populated with specific sources before publishing.]