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 Office of Environment and Heritage and used in the Air Quality Index (AQI) are ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide and air particles. Visibility 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 OEH standard for visibility.

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.

There are two 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 OEH 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 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:

 AQIpollutant = 
pollutant data reading   X 100

More about the AQI and how it is calculated


Particles less than 2.5 micrometres in diameter measured using a Beta Attenuation Monitor (BAM).
See also particles (PM10, PM2.5)

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.


See standard


Suspended fine particles as measured by a nephelometer


This instrument measures the amount of 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 Measure

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.

Particles (PM10, PM2.5)

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 less than 10 micrometres in diameter are measured using a Tapered Element Oscillating Microbalance (TEOM). See TEOM-PM10. Particles less than 2.5 micrometres in diameter can be measured using a Tapered Element Oscillating Microbalance (TEOM) - see TEOM-PM2.5 or a Beta Attenuation Monitor (BAM) - see BAM-PM2.5.

PM2.5, PM10

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 were set in 1998 by the National Environment Protection Council. The standard for visibility is set by NSW Office of Environment and Heritage.

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.


Particles less than 10 micrometres in diameter measured using a Tapered Element Oscillating Microbalance (TEOM).
See also particles (PM10, PM2.5)


Particles less than 2.5 micrometres in diameter measured using a Tapered Element Oscillating Microbalance (TEOM).
See also particles (PM10, PM2.5)


Total suspended particulates