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How to choose an air purifier

​Indoor Air Pollution Fact Checks


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​​Sanctuary Air SA-250 Series

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The effectiveness of an air-cleaning device in removing pollutants from an occupied space depends on three  primary factors: 1) the filter's particle removal efficiency, 2) the air flow rate passing through the filter, usually expressed as cubic meters per hour; and 3) the air flow pattern, i.e.  the path that the clean air follows after it leaves the filter.

The effectiveness of an air-cleaning device can also be affected by

1) whether the purifier can be operated economically   24/7, because re-contamination is continuous, as is your breathing. 2) The noise level should not prevent people from spending time in the purified zone, 3) whether pure air can be distributed to all frequently occupied spaces within the premises at the same time.

The following table explains some of Key Performance Indicators of three types of air-cleaning devices/systems using HEPA filters:

An air purifier competes against continuous re-contamination occurring within the space. 

The effectiveness of an air-cleaning device is a measure of its ability to reduce airborne particle or gaseous pollutant concentrations in an occupied space.​​​​ AHAM recommends an effectiveness of 80% as a meaningful indicator, which is consistent with Australian Standard AS 1668.2-2002 (Part 2, Ventilation design for indoor air contamination control).  For example, a purifier with a coverage specification of 20 sq.meter, indicates that the device can achieve 80% effective reduction in steady particle concentrations in an indoor space of 20 sq.meter.

On the other hand, a filter's particle removal efficiency is usually expressed as a percentage of  pollutants removed from the air that passes through it. For example, a filter may remove 99% of the particles from the air that passes through it (i.e., have 99% efficiency). However, if the airflow rate through the filter is only 15 cubic meters per hour (cmh) in a typical room of approximately 29 cubic meters (e.g., 4 x 3 x 2.4), the filter will be relatively ineffective at removing particles from the air (i.e., 10 times less effective than if the airflow rate were 150 cmh).

A filter rating of 99.97% efficiency DOES NOT translate directly to 99.97% effectiveness of a purifier.


CADR is a voluntary standard developed by the Association of Home Appliance Manufacturers (AHAM), and is recognized by the American National Standards Institute. The standard compares the effectiveness of a brand new portable air cleaners in a room size test chamber, by measuring the CADR for each of three types of particles in indoor air: dust, tobacco smoke, and pollen. For example, an air portable unit that has a CADR of 250 for tobacco smoke can reduce tobacco smoke particle levels to the same concentration as would be achieved by adding 250 cubic feet per minute, equivalent to 425 cubic meters per hour  of clean air. 

Because air flow resistance increases as the filter efficiency rating increases and or filter surface decreases, it is very challenging for a portable unit to achieve high CADR using a genuine HEPA filter of practical size without incurring high energy consumption and/or high noise levels. 

As this is a voluntary American standard, some Australian and European models may not have a CADR rating.

The CADR testing does not apply to whole-house air-cleaning devices due to a different air flow pattern.


Due to the close proximity between inlet and outlet of a portable unit, a portion of purified air is re-drawn or short-circuited back to the unit  before  mixing sufficiently with the rest of air in the room. In effect, the purifier uses a portion of energy circulating pure air along the air path without purifying air further away.

On the other hand, ducted purification systems have a much more efficient air flow pattern: 

  • Pure air is distributed to multiple outlets distant from the inlet, effectively diluting air pollutants concentration in the rooms you intend to purify.
  • By applying different air flow rates to different outlets, the rooms can be prioritised for pure air delivery.
  • A higher dilution index (DI) can also be achieved for a priority zoom by closing the door and creating a positive pressure within the closure, effectively preventing air pollution penetration from outside of the zoom.


Purifiers installed in the return duct of ducted heating/cooling (HVAC) systems that rely upon the air mover within the existing HVAC system argueably consume no additional power when the HVAC system is operating. However, your HVAC, either be a reverse cycle conditioner or gas ducted heater, operates less than 15% of the time throughout a year. When cooling or heating is not required, it can be very expensive to use the air mover of your HVAC system to operate your air purifier continuously, because these air movers are design to move large volume of air for a short period of time and can consume  500 to 1500 watts. 24/7 air purification is very expensive for HVAC return-duct purifiers.

On the other hand, an independent ducted purification system will have a dedicated air mover that is specifically designed to operate continuously. A well designed independent ducted purification system can achieve the same power consumption of a typical portable unit for the reasons that:

  • A ducted system can have a very large HEPA filter without floor space restriction. Air resistance decreases exponentially as filter surface area increases;
  • A ducted system has a more efficient air flow pattern than a portable unit.

An independent ducted purification system can operate 24/7 economically.

Portable Units

In-duct purifier installed in the return duct of a HVAC system

Independent ducted air purification system
Particle Removal Efficiency

Equivalent to efficiency of the HEPA filter of choice. Look for a genuine HEPA filter of sizable area. The larger the filter area is, the lower its air resistance and the greater its dust holding capacity.

Air flow rate 

NOTE [1]

CADR is a measure of a portable air cleaner’s delivery of contaminant-free air, expressed in cubic feet per minute.  It is essentially the air flow rate multiplied by filter efficiency. CADR is  limited by the filter size and the noise level of a portable unit.

CADR rating does not apply to ducted whole-house purifiers due to a different airflow pattern.
Air flow rate of an in-duct purifier depends on the air mover of existing HVAC. e.g. a R/C ducted system.
Air flow rate of an independent ducted purifier can be optimisedwithout restrictions of an existing HVAC.

Air flow pattern

NOTE [2]

Some Short-circuiting of the air path is unavoidable due to close proximity of air inlet and outlet.

Air is distributed more  evenly via a multiple outlet ducted system. Positive air pressure can be created for priority zones.

Space coverage

Space coverage is the floor area (assuming ceiling height of 2.4 m), in which a purifiercan achieve an 80% reduction in steady pollutant concentrations.

Follow AHAM 2/3 rule:

e.g. 120 sq.ft (11.15 sq.meters) requires a portable unit with smoke and tobacco CADR at least 80.

Under the guidance of AS 1668.2-2002, a typical dwelling of  150 sq. meters (1,615 sq. ft) will require a ducted system with an air flow rate about 750 cubic m/h or 210 L/sec to achieve 80% reduction.

Energy efficiency

NOTE [3]

Portable units typically consume 80-120 Watts at maximum air speed, depending on CADR and models.

It can be very expensive to run the air mover of an existing HVAC system 24/7, costing anything from $1,200 to $2,400 per year.

High quality independent ducted systems consume no more power than typical portable  purifiers.  24/7 operation is economical.

Noise level

The noise threshold specified by WHO for undisturbed sleep is 30 dBA. It is important to check a unit's noise level at all speed settings.
The noise level is that of the existing HVAC system.
Noise can be isolated to an inaudible level. You can have undisturbed sleep with the full benefit of pure air.
Space efficiency
Requires floor space
Requires no floor space, not visible
Filter life
For a given air flow or CADR, filter life increases exponentially as area increases. In general, larger filters save money in replacement over a given period.

Filter accessibility

Filters of a portable unit are easily accessible and changeable by user
It is important to consider ease of access for filter change. Some ducted systems require entry to the loft or underfloor and may require a serviceman. A well designed system provides room-side access avoiding health and safety hazards associated with entering the loft or basement.

Other performances

A portable unit serves a single room. It achieves the best result when the room is closed.
A in-duct purifier can improve the cleanliness of the existing HVAC network.
Independent ducted systems  can utilise room-located heating or cooling devices distributing warm or cool air evenly to ducted zones.

What are the Key Performance Indicators of a purifier?

To choose air-cleaning devices,  it is important to understand the difference between efficiency and effectiveness.

How to choose an air purifier

Indoor Air Pollution Fact Checks

Quality Australian Made Ducted Air Purifiers


  Purely for health

 03 8560 2111

​​Sanctuary Air SA-250 Series