Glossary of air quality terms

Glossary of words, phrases and acronyms used in air quality monitoring and reporting.

Air pollutant

The air pollutants monitored by the Department of Planning, Industry and Environment and used in the Air Quality Index (AQI) are ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide and air particles. Visibility (NEPH) is also measured and reported. These measurements are used in reporting the AQI which is a simple but effective way to communicate how air quality compares across regions and pollutants.

Air quality alerts

An air quality alert may be raised when pollutant concentrations reach levels which exceed national air quality standards for gaseous pollutants (ozone, carbon monoxide, nitrogen dioxide and sulfur dioxide), fine particles (rolling 24-hour average PM10 and PM2.5) and/or the NSW Department of Planning, Industry and Environment standard for visibility.

There are 2 types of air quality alerts:

  1. Hourly:  An hourly air quality alert may be raised by any site within the NSW air quality monitoring network whenever national air quality standards and/or NSW Department of Planning, Industry and Environment standard for visibility are exceeded. 
  2. Forecast: At 4.00pm every day, an air quality forecast is made for the next day the three regions within Sydney. If it is predicted that the AQI will likely exceed 100 the next day, an air quality alert is raised. In the case of an alert due to ozone, the predicted elevated levels of ozone may not occur until the afternoon as production of ozone is related to temperature among other variables (as temperature goes up, ozone concentrations increase).

Air quality alerts of POOR and VERY POOR categories are likely to impact people particularly sensitive to the effects of air pollution. Extreme episodes of air pollution such as during prolonged bushfires (HAZARDOUS category) can affect everyone's health.

Air Quality Index (AQI)

A value derived from air quality data readings which allows for more meaningful comparison of pollutants affecting air quality. The index is derived using the following formula:

 AQI pollutant = 
pollutant data reading   X 100

Computed AQI values correspond to various categories of air quality (either VERY GOOD, GOOD, FAIR, POOR, VERY POOR OR HAZARDOUS) and are used to raise air quality alerts.

More about the AQI and how it is calculated

Beta Attenuation Monitor (BAM)

This instrument works by collecting particles on a filter tape and measuring the reduction in beta rays travelling through the particles. From this, the concentration of airborne particles is calculated.

Carbon monoxide (CO)

Carbon monoxide is an odourless, colourless gas produced by incomplete oxidation (burning). Although any combustion process will contribute CO, in cities, motor vehicles are by far the largest human source. Other sources include wildfires, and natural processes such as the oxidation, in the oceans and air, of methane produced from organic decomposition.

Carbon monoxide enters the bloodstream through the lungs and inhibits transport of oxygen by blood, thereby reducing oxygen reaching the body's organs and tissues, especially the heart. People suffering from heart disease are most at risk and may experience chest pain from CO exposure particularly while exercising.

Data readings

Data readings are the actual scientific measurements for each air pollutant. The data readings are recorded in different units of measure, depending on the type of pollutant.


Standards refer to maximum concentrations of the pollutants. Goals refer to allowable exceedances of these maximum concentrations during a year. These are set by the National Environment Protection Council or NEPC.

See standard

Indicative monitoring

Indicative monitoring is not formally defined in Australia. It is defined in the UK and this text uses terminologies based on the UK definitions.

Air Quality instruments can be grouped into four classes: certified reference, certified equivalent, certified indicative, and indicative. The main difference is accuracy and repeatability of measurements and the cost of purchase and operation.

Instruments used in the NSW rural air quality network are a mix of different indicative instruments (TSI DustTrak™ 8520, TSI DustTrak™ 8534 and TSI EDT™).

The TSI DustTrak™ 8520 and 8534 instruments power up every 15 minutes to sample for 1 minute. They only continue measurements if concentrations exceed 25 µg/m3. This preserves battery power and reduces instrument wear and tear. The newer TSI EDT™ multy channel monitors run continuously and can potentially become certified indicative instruments.


Suspended fine particles as measured by a nephelometer or NEPH, and a measure of visibility reduction or the coefficient of light scattering due to atmospheric gases and particulate matter. It indicates how visual range is affected by these components in the air. The higher the NEPH value, the lower the visibility, and lower the visual range. For example, during smoky conditions, high NEPH values would be recorded at affected stations.


This instrument measures light scattering due to gases and particles in the air using very sensitive, light-scattering sensors (in a similar way to the portable air monitoring instruments) and calculates a visibility reduction index.


National Environment Protection (Ambient Air Quality) Measure, a national framework (federal legislation) to measure and monitor Australia's ambient (outdoor) air quality, as prescribed by the National Environment Protection Council (NEPC).

Nitrogen dioxide (NO2)

Nitrogen dioxide is found at its highest concentrations near busy roads and can also be high indoors when un-flued gas appliances are used. It is a respiratory irritant which may worsen the symptoms of existing respiratory illness. Nitrogen dioxide makes people with asthma more susceptible to lung infections and asthma triggers like pollen and exercise.

Nitrogen oxide (NO)

NO is formed by oxidation of nitrogen or ammonia present in the atmosphere. Measured at certain monitoring sites, but not used in reporting the air quality index (AQI).

Oxides of nitrogen (NOx)

NOX formed during combustion is composed predominantly of NO (90-95%) and a lesser amount (5-10%) of NO2. NOx is measured at certain monitoring sites but not used in reporting the air quality index (AQI).

Ozone (O3)

Ground level ozone is a colourless, gaseous secondary pollutant. It is formed by chemical reactions between reactive organic gases and oxides of nitrogen in the presence of sunlight. Ozone is one of the irritant secondary pollutants in photochemical smog and is often used as a measure of the latter.

Ozone is more readily formed during the summer months and reaches its highest concentrations in the afternoon or early evening. If we breathe in too much ozone, it can irritate the lungs, affecting lung function and worsening asthma. You may notice difficulty in breathing, coughing, and throat irritation if you are exercising outdoors when ozone levels are high.


Solid or liquid particles may be suspended in the air and reduce visual amenity and adversely impact health. The size of a particle determines its potential impact on human health. Larger particles are usually trapped in the nose and throat and swallowed. Smaller particles may reach the lungs and cause irritation. Particles are measured in various size fractions (PM2.5, PM10 and TSP).

Particles as PM2.5, and PM10

PM10 - particles less than 10 micrometers in diameter. Sources include sea salt, crushing or grinding operations and dust stirred up by vehicles on roads. Particles less than 10 micrometres in diameter are measured using a Tapered Element Oscillating Microbalance (TEOM).

PM2.5 – fine particles less than 2.5 micrometers in diameter. Sources include all types of combustion, including motor vehicles, power plants, residential wood burning, forest fires, agricultural burning, and some industrial processes. May also include sea salt. Particles less than 2.5 micrometres in diameter can be measured using a Tapered Element Oscillating Microbalance (TEOM) or a Beta Attenuation Monitor (BAM).

At stations within the NSW rural network, low cost monitors such as DustTraks are also used for indicative monitoring of fine particles as PM10 and PM2.5.

See particles


See air pollutant

Regional AQI

See site AQI

Rolling average

Is an average of the previous N hours for each hour of the day. For example, for the hour ending 05:00, the 4-hour rolling average is calculated from the values for hours 02:00 to 05:00, for the hour ending 06:00 the next 4-hour rolling average is calculated as the average of values for hours 03:00 to 06:00 and so on. Rolling 4, 8 and 24-hour averages can include values from the previous day.

SD1, Sigma theta

Standard deviation in wind direction. The variability of the wind direction can be used as an indicator of the amount of turbulence and therefore mixing of air.  The term used is ‘sigma theta’, where sigma is a standard measure of variability and theta is a Greek letter commonly used to represent an angle in mathematics.

Site AQI

For each site, the highest AQI is used as the Site AQI. For each region, the highest Site AQI is used as the Regional AQI for that region.

More about the AQI and how it is calculated

Sulfur dioxide (SO2)

The main human activities producing sulfur dioxide are the smelting of mineral ores containing sulfur and the combustion of fossil fuels. Sulfur dioxide is a respiratory irritant and may worsen existing respiratory illness.


The standards for ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide and air particles are set by the National Environment Protection Council. Standards which apply for the Australian (and NSW) context are described at Standards and goals.

Tapered Element Oscillating Microbalance (TEOM)

Continually measures the concentration of airborne particles. It does this by collecting and weighing the particles using a very sensitive balance.

Total Suspended Particles (TSP)

Total suspended particulates are solid particles and liquid droplets 100 micrometres or less in diameter. They come from natural and human-made sources (eg pollen, bushfires, motor vehicle emissions). PM10 and PM2.5, the smaller components of TSP, are associated with adverse health effects ranging from respiratory problems to premature death of people with heart and lung disease.


Represents visibility reduction as measured by the Nephelometer (NEPH). The higher the NEPH measurement, lower the visual range is and poorer the visibility conditions. In New South Wales we adopt a visibility standard of 2.1x10-4 m-1 (210 Mm-1) to represent a visual range of 9 kilometres.