HEPA Filters Explained: What H13, CADR and Room Size Mean

The short answer: compare the complete purifier

Do not choose an air purifier from a filter label alone. A highly efficient filter does little for a room if very little air passes through the appliance, or if the setting you can tolerate is much slower than the one used for its headline claim. Look at the complete machine, the room you will use it in and the noise level you can live with.

HEPA describes particle filtration, H13 refers to a filter classification under a relevant testing standard, and CADR describes clean-air delivery. These terms answer different questions. A purifier can use a different documented filtration technology and still have useful performance, so compare verified whole-machine figures rather than assuming every product uses the same filter type.

What HEPA and H13 do, and do not, establish

HEPA filters are designed to capture airborne particles. Exact efficiency claims depend on the standard and test conditions used. A headline percentage should identify the particle size and whether the claim applies to the filter medium or the complete appliance. H13 classification is not a general promise that a home becomes sterile or medically safe.

Do not casually treat every “99.97%” label as an H13 certification. Different standards and test methods use different conditions. If certification matters to your purchase, ask for documentation of the specific filter or machine rather than extrapolating from marketing wording.

Particle filtration also differs from removing gases and odours. An appliance may include activated carbon or another treatment stage, but the amount, design and maintenance of that stage matter. A particle-efficiency figure cannot be used to promise removal of every gas. Read separate evidence for the pollutant you are concerned about.

CADR links filtration to airflow

Clean Air Delivery Rate, or CADR, is a measure used to describe the delivery of cleaned air for particular particle tests. AHAM's verified programme distinguishes smoke, dust and pollen ratings. Check the units and test basis before comparing two figures; cubic metres per hour and cubic feet per minute are not interchangeable numbers.

CADR is more useful than an unexplained fan airflow number because it relates airflow to cleaning performance. However, a maximum-speed rating does not tell you the exact performance in a quiet night mode. If the bedroom setting is the one you will run most often, investigate that setting rather than buying from the highest figure alone.

Room-size claims can use different assumed cleaning rates and ceiling heights. That is why one large square-metre label should not settle the comparison. Ask what the claim assumes and how often the room air is intended to be processed. An open-plan living area with doors to other rooms is a different load from a closed bedroom.

A transparent room-sizing example

Measure floor area and approximate ceiling height. A 4m by 4m bedroom with a 2.4m ceiling has a volume of 38.4 cubic metres. For an illustrative target of four equivalent clean-air changes per hour, the calculation is 38.4 multiplied by four, or about 154 cubic metres per hour.

This is a planning example, not a health recommendation or a guarantee of pollutant removal. Real conditions include incoming particles, room mixing, doors, furniture, the selected fan speed and where the appliance sits. Use it to understand why volume and clean-air delivery belong together, then compare documented performance at your intended setting.

A high ceiling increases room volume even when floor area is unchanged. Open doors effectively enlarge the space the cleaner is serving. If you want the best result in a particular room, decide whether it can reasonably operate as that room's cleaner rather than expecting a compact unit to service the whole house.

Air purifier CADR by room size: practical NZ examples

To estimate a starting CADR, multiply the room floor area in square metres by its ceiling height in metres, then by the chosen number of equivalent clean-air changes per hour (eACH). The result is in cubic metres per hour (m³/h). These are illustrative clean-air targets rather than a requirement set by a New Zealand regulator. They assume a single reasonably well-mixed room, a 2.4 m ceiling and the purifier operating at a speed that delivers the stated CADR.

For example, a 20 m² room with a 2.4 m ceiling contains 48 m³ of air. Multiplying by five gives a target CADR of 240 m³/h. With a 2.7 m ceiling, the same floor area contains 54 m³, so the equivalent target becomes 270 m³/h. This makes ceiling height important, particularly in older villas and homes with vaulted ceilings.

CADR may be published in cubic feet per minute (cfm) instead. To compare like with like, multiply cfm by approximately 1.699 to obtain m³/h. A claim expressed only as fan airflow is not necessarily an independently tested smoke, dust or pollen CADR. When a brand does not state a comparable CADR, request test information before relying on a room-size claim.

How to choose an air purifier for a bedroom or open-plan lounge

Bedrooms: balance usable speed against noise

For a small or medium bedroom, start with the relevant row in the table and then look at the sound output of the speed you expect to use overnight. A purifier's maximum CADR may be measured at its loudest setting, while a quiet or sleep setting may provide substantially less clean air. Where possible, seek documented performance at different fan settings. Check for switchable display lighting, a suitable physical footprint and a stable place away from curtains or bedding.

Open-plan lounges: do not treat floor area as the only factor

A combined lounge, dining room and kitchen may have a much larger effective air volume than a nominal 30 m² living area. Kitchen activities, people moving around and doors opening can add particles faster than a sealed-bedroom example suggests. If the combined space is large or irregularly shaped, two suitably placed units may circulate air more effectively than one unit hidden in a corner. Treat that as a layout decision, not a promise of a fixed percentage improvement.

NZ buyer checklist: costs, filters and claims

• Check the test basis: Look for a CADR unit, pollutant category and independent verification where available.
• Confirm the model: Match the actual purifier model and filter type to the manufacturer's documentation. Do not assume electrostatic filtration is H13 HEPA.
• Compare real operating noise: Check sound levels at the speed you will actually use, not just at the lowest setting.
• Budget for replacement filters: Confirm local availability, the correct part number, replacement interval and estimated ongoing cost.
• Consider running cost: Electrical energy in kWh equals rated power in kW multiplied by hours of operation. Multiply by your electricity rate to estimate cost.
• Check your space: Note ceilings above 2.4 m, adjoining rooms, obstructions and manufacturer clearance requirements.
• Separate functions: Heat, fan circulation and dehumidification are not substitutes for independently supported particle-cleaning performance.

If comparing a Dyson combination heater/purifier with a compact purification-focused model, record the verified CADR (if supplied), filtration type, price of approved replacement media and functions you genuinely need. Do not rank either model on an unsupported H13 label or invented CADR specification. You can compare the current TSS range on the linked product pages above, then use the room-size estimate to shortlist models with suitable documented performance.

Compare the claims that matter

Read exact product claims carefully

Compare these relevant TSS products using the current specifications on their pages: Dyson Pure Hot+Cool Purifying Fan Heater White/Silver, Dyson HushJet Purifier Compact (HJ10). Availability and fulfilment details can change; confirm them before ordering.

The Dyson Pure Hot+Cool and Dyson HushJet Compact are relevant TSS examples, but their filtration descriptions differ. The HushJet listing describes an electrostatic filter, so it should not be presented as an H13 model simply because it is sold beside HEPA products. Check the manufacturer's exact model documentation and separate particle and odour claims.

Replacement-filter availability is a practical part of the purchase. Check the model code, filter code and recommended maintenance. A filter that physically fits is not necessarily the approved replacement. Include likely replacement costs in the ownership budget without assuming an advertised service interval will match your household conditions.

For a purifier that also heats or circulates air, separate those functions from purification. A heating wattage does not describe clean-air delivery. A fan's cooling sensation is also different from refrigeration. Choose the additional functions because you will use them, rather than letting them obscure the purifier comparison.

Placement, noise and maintenance

Give the appliance the intake and outlet clearance stated in its manual. Placing it behind a sofa or tightly against a curtain can restrict circulation. Think about the room's normal layout, including pets and children, so the cleaner can run without becoming a trip hazard or blocked object.

Check noise and light settings for a bedroom. A more powerful purifier that you switch off because it is intrusive may be less useful than one you can run consistently. Do not assume a silent display means the fan itself is silent. Assess the setting that fits your actual routine.

Maintain prefilters and other washable components only as directed. Do not wash a HEPA or electrostatic filter unless the manufacturer specifically allows it. Replace consumables when required and keep vents clear. A clogged filter can reduce airflow and undermine the assumptions behind your original room sizing.

Purification is one part of a healthier indoor environment

Reduce sources of pollution where practical and ventilate appropriately for conditions. A purifier is not a substitute for addressing leaks, dampness, smoking indoors or unsafe combustion appliances. It also does not dehumidify a room unless that function is explicitly included.

For New Zealand homes, seasonal pollen, pets and cooking may influence which room you prioritise. Choose a realistic use case and compare current manufacturer information with the guidance from the EPA and AHAM linked below. If you need advice for a medical condition, discuss the requirements with an appropriate health professional rather than treating an appliance label as treatment advice.

Frequently asked questions

Is H13 always better than a higher-CADR purifier?

The specifications describe different things. Compare documented filter performance with clean-air delivery at the setting you will use, rather than choosing from one label alone.

Can a HEPA filter remove odours?

HEPA addresses particles. Odour or gas treatment needs separately documented technology, often a carbon stage, whose performance and replacement requirements also matter.

Will one purifier clean every room?

That depends on airflow, room layout and use. A closed bedroom is easier to define than an open home. Do not interpret one room-size claim as whole-house coverage.

Can I wash the filter to save money?

Only if the manual explicitly permits it. Washing an unsuitable filter can damage it. Check which prefilters or components are washable and use approved replacements.

Compare air-cleaning equipment for your room

Browse TSS air purifiers and compare the exact model details before you choose. For help matching equipment to your setup, contact Tech Supply Shed with your current model and requirements.

Sources and further reading

Technical references checked on 2 October 2026: EPA air-cleaner guidance; AHAM verified CADR guidance; Dyson NZ purifier specifications