The National Environment Protection (Ambient Air Quality) Measure (AAQ NEPM) sets standards for up to seven primary air pollutants (described below).
The AAQ NEPM requires jurisdictions to monitor these air pollutants and report annually on ambient air quality and actions taken to achieve the ambient air quality goals.
Ozone (O3) is a colourless, odorous, gaseous secondary pollutant.
In the troposphere, it is formed by chemical reactions between reactive organic gases and oxides of nitrogen in the presence of sunlight.
Ozone is a major component of photochemical smog and is often used as a measure of it.
The formation of ozone in the upper levels of the atmosphere or 'stratosphere' is by a different process. Ozone there is not regarded as a pollutant because it is produced naturally.
It is important in absorbing harmful ultraviolet radiation and preventing it from reaching the earth.
In the lower troposphere, ozone is more readily formed during the summer months and reaches its highest concentrations in the afternoon or early evening.
Ozone is strongly oxidising and can irritate the eyes and the respiratory tract.
It affects vegetation growth and also damages materials such as rubber, fabric and paint.
Breathing ozone can affect lung function and worsen asthma.
People exercising outdoors when ozone levels are high may notice difficulty in:
- coughing and
- throat irritation.
Nitrogen dioxide (NO2) is a reddish-brown, pungent and highly corrosive gas.
It is one of the main oxides of nitrogen present in the atmosphere, others are nitric oxide (NO) and nitrous oxide (N2O).
Nitrous oxide occurs in much smaller quantities than the other two, but is of interest as it is a powerful greenhouse gas and thus contributes to global warming.
The major human activity which generates oxides of nitrogen is fuel combustion, especially in motor vehicles.
Oxides of nitrogen form in the air when fuel is burnt at high temperatures. This is mostly in the form of nitric oxide (a colourless gas) with usually less than 10% as nitrogen dioxide.
Once emitted, nitric oxide combines with oxygen ('oxidises') to form nitrogen dioxide, especially in warm sunny conditions.
These oxides of nitrogen may remain in the atmosphere for several days and during this time chemical processes may generate nitric acid, and nitrates and nitrites as particles.
Oxides of nitrogen also play a major role in the chemical reactions which generate photochemical smog.
Nitrogen dioxide is found at highest concentrations near busy roads and can also be high indoors when unflued gas appliances are used.
Nitrogen dioxide irritates the lungs and makes people with asthma more susceptible to lung infections and to asthma-triggers like pollen and exercise.
Carbon monoxide (CO) is an odourless, colourless gas produced by incomplete oxidation (burning).
As well as wildfires, carbon monoxide is produced naturally by oxidation in the oceans and air of methane produced from organic decomposition.
In cities, the motor vehicle is by far the largest human source, although any combustion process may produce it.
Carbon monoxide usually remains in the atmosphere for a month or two. It is removed by oxidation to form carbon dioxide, absorption by some plants and micro-organisms, and rain.
When inhaled, carbon monoxide enters the bloodstream through the lungs and binds to the oxygen-carrying site on the blood's haemoglobin, which reduces oxygen transport in the body.
This can reduce the amount of oxygen reaching the body's organs and tissues, especially the heart.
People suffering from heart disease are most at risk.
They may experience chest pain if they are exposed to carbon monoxide, particularly while exercising.
At high concentrations carbon monoxide is very toxic, causing:
- reduced ability to think and
Sulfur dioxide (SO2) is a colourless gas with a sharp, irritating odour, and occurs in the atmosphere from both natural and human activities.
Natural processes which release sulfur compounds include decomposition and combustion of organic matter; spray from the sea and volcanic eruptions.
The main human activities producing sulfur dioxide are the smelting of mineral ores containing sulfur and the combustion of fossil fuels.
Sulfur dioxide dissolves in water to form sulfuric acid. This is a corrosive substance that damages materials and the tissue of plants and animals.
Sulfur dioxide irritates the airways of the lungs.
People with asthma who are physically active outdoors are most likely to experience the health effects of sulfur dioxide.
This may include wheezing, chest tightness and shortness of breath.
Not only are there gaseous pollutants, there are also aerosols (solid or liquid particles) that are suspended in the air and may reduce visual amenity and adversely impact health.
Examples of particles in the air include:
- plant spores
- bacteria and
Particulate matter may be a primary pollutant, such as smoke particles – emitted directly into the air, or a secondary pollutant formed from the chemical reaction of gaseous pollutants.
Human activities resulting in particulate matter in the air include:
- burning of fossil fuels
- agricultural and hazard reduction burning
- the use of incinerators and
- the use of solid fuel for cooking and heating.
Larger particles usually settle out of the air quickly while smaller particles may remain suspended for days or months. Rainfall is an important mechanism for removing particles from the air.
Particles occur in a wide range of shapes and are rarely spherical. The size is classified in terms of how they behave in air relative to spherical particles and this is termed the aerodynamic equivalent diameter.
Particles in air are measured as PM10 (particles with aerodynamic equivalent diameters of 10 micrometres or less) and PM2.5 (particles with aerodynamic equivalent diameters of 2.5 micrometres or less) – the latter are about 3% the diameter of a human hair.
The size of a particle also 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 there
- fine particles can be carried deep into the lungs and irritate the airways.
When exposed to particle pollution, people suffering from heart disease may experience symptoms such as chest pain, and shortness of breath.
Particle pollution can aggravate existing respiratory diseases such as asthma and chronic bronchitis.
Lead (Pb) is a soft heavy metal and can be found in particulate matter suspended in air.
Lead and its compounds and are normally emitted from mining, smelting and processing of mineral ores.
Many houses built before the 1970s used paints containing lead, so renovation of old houses can disturb the old paint.
Lead used to be a constituent of petrol until it was eventually phased out in 2001, this has resulted in a fall to insignificant levels in air.
Lead in air is normally monitored by analysing total suspended particles (TSP) collected on filter samples. TSP are particles with sizes up to 50 micrometres and include PM10 and PM2.5.
Due to low levels in air and absence of smelters, lead it is not routinely measured at Darwin air quality monitoring stations.
Lead is a highly toxic metal that is poisonous in all forms.
It is absorbed into the human body through both ingestion and inhalation.
Inhaling or consuming lead and its compounds can affect the human body, particularly the nervous system, and may result in growth and developmental problems in children.