Pollutant size matters ...

There are two categories of indoor air pollutants: particulate matter (PM) composed of microscopic solids, liquid droplets, or a mixture of solids and liquid droplets suspended in air, and gaseous pollutants, e.g. Volatile organic compounds (VOC) such as formaldehyde. ​ 

The Full Spectrum of Airborne Particle Sizes

from molecules to pollens

Whether a source of air pollutants causes an indoor air quality problem or not depends on:

  1.  the type of air pollutant;
  2.  the amount and rate at which it is released from its source;
  3.  and the degree of ventilation and or purification  available in the home to remove it from indoors.

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Buildings that are especially likely to have IAQ problems include:

  • Homes located in busy transport corridors where outdoor air quality is poor with high level of PAH, Benzene, SO2, NO2 PM10, PM 2.5, tyre dust, soot,
  • Homes in industrial zones
  • Homes or neighbourhoods with wood heaters installed and/or located near forestry burning operations or bushfires. Wood smoke contains pollutants such as PAHs, Benzene and PM.
  • New or newly renovated homes. Carpets, fresh paints and lacquers, flooring adhesives etc can be major sources of strong odours and VOCs 
  • Inner suburban old homes. Dust deposited within the loft can be up to 1 cm thick and can contain commercially mineable concentrations of lead and other heavy metals due to decades of exposure to  leaded petrol exhaust. PM contamination may also arise from  pre-regulated materials such as lead paint, asbestos and residuals of organo-chloride pesticide.
  • Homes with pets, smokers, dampness problems etc.

In a 2013 Australian Parliamentary inquiry into air pollution,  it was argued that there is a need to explore what standards or regulations may need to be put in place in order to balance building energy efficiency gains against potential health costs arising from reduced ventilation. It is pointed out that a number of indoor pollutants and emission sources that may be harmful to human health are, in many cases, not regulated. [6]


Size (microns)​​

Penetration Depth
Coarse Particles



Nose & throat
PM10 Particles

d <10µm 


PM2.5 Fine Particles




PM0.1 Ultrafine and

Nano Particle​s



cell membranes

and blood steam

Gases & VOCs



cell membranes

and blood stream

as does the pollutant concentration and amount (dose)


It is important to note that particulates are not one particular chemical substance but a classification of particles by size rather than chemical properties. The chemical properties vary depending on sources of particles.

The size of the particles or PM is the principal determinant of how deeply it is inhaled into the human respiratory system, with smaller particles able to penetrate further into the lungs. As most particles with a diameter >10µm are generally filtered by the nose and throat, PM10 is typically used as the threshold value for studies on the effects of PM on human health.​ 

Understanding Air Pollutants

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Under the National Environment Protection Measure (NEPM), the Australian government has set a national ambient air quality standard for PM10 that is 50 µg/m3 in outdoor air averaged over a 24-hour period with maximum allowable exceedances of 5 days per year; The Measures for PM2.5 are: 25 µg/m3 averaged over 24 hours; and 8 µg/m3 averaged over one year[2]. The State of the Air Report SoE 2006 by the Australian Department of the Environment shows that that all capital cities but Hobart exceed the NEPM standard for PM10 concentrations and most had experienced an increase in concentrations over the reporting period; Of the four cities (Sydney, Melbourne, Perth and Brisbane) that monitor PM2.5, all exceed the NEPM 24-hour advisory reporting standard [3].​​

​​In a typical building, indoor pollutants mix with outdoor pollutants that penetrate inside and become trapped due to inadequate ventilation. According to the EPA, indoor air is typically 2 to 5 times more polluted than outdoor air and can be much worse...up to 100 times in some cases[4]. WHO estimated that more than 30 percent of buildings have significant IAQ (Indoor Air Quality) problems. The composition and concentration of pollutants within a building depends on construction materials,  household products, various human indoor activities and outdoor air quality depending on its location.

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Particles are associated with increased respiratory symptoms, aggravation of asthma, increased hospitalisation for heart and lung diseases, and premature death in humans and animals. Generally, the greater the amount of pollutant (exposure), the greater the health response. The duration of exposure is also important. If low-level exposure occurs over a long period of time perhaps many years), the total dose may be large.

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​Respiration of particles  challenges the body’s natural defense mechanisms and overexposure may strain these mechanisms, causing an adverse reaction. There has been increasing concern for fine particles that have a diameter of 2.5 micrometers (µm) or less since they bypass natural defense mechanisms more readily and make their way deep into the lungs. Recent research has identified a strong link between PM2.5 and life expectancy. PM2.5 is believed to be the most health-hazardous air pollutant, responsible for 10 to 20 times as many premature deaths as the next worst pollutant, ozone [6]. 

Nanoparticles are the smallest PMs that can pass through cell membranes and migrate into other organs, including the brain. Particles emitted from modern diesel engines (commonly referred to as Diesel Particulate Matter, or DPM) are typically in the size range of 100 nanometers (0.1 micrometer). These particles also carry carcinogens like benzopyrenes adsorbed on their surface.

Particle surface area or the particle count (numerical quantity) instead of particulate mass is arguably a more accurate measure of health hazard, because one particle of 10 µm diameter has approximately the same mass as 1 million particles of 100 nm diameter, but is much less hazardous, as it unlikely to enter the alveoli.

Whilst there is increasing epidemiological evidence of the association between short-term exposures to ultrafine particles and health, there are no routine monitoring data in Australia that could be used to set standards for such particles.[7]

How to choose an air purifier