Urban and Regional Air Pollution
Information sheet

Pollutants in our air

Pollutants are substances which, when present at high enough concentrations, produce harmful effects on people and/or the environment. A number of pollutants affecting urban and/or regional air quality are listed in Table 1.

Sulfur dioxide

Sulfur dioxide is produced when coal and oil are burnt or when minerals are "roasted" to remove the sulfur. In some countries, particularly in the northern hemisphere, coal and oil contain significant amounts of sulfur. Unless special steps are taken to remove sulfur dioxide, it is released into the atmosphere. Power stations and industrial plants, which are often sited close to cities, can produce large quantities of the gas.

As well as affecting human health, sulfur dioxide can be harmful to plants, turning leaves yellow and drying, bleaching, and even killing, foliage.

In the atmosphere, sulfur dioxide can form acidic particles, or react with cloud droplets, contributing to acid rain.

Particles

Particles in the air (also known as aerosol) come from a number of sources, including motor vehicles, industrial processes and wood burning. Secondary formation of particles (formation from gaseous emissions) can also contribute significantly to particle levels. Some atmospheric particles are from natural sources. These include wind-blown dust, pollen, sea salt, and material from volcanic eruptions.

Fine particles (particles with a diameter of 10 micrometres or less) can be inhaled deeply into the lungs and have been associated with a wide range of adverse respiratory symptoms. Long- and short-term exposure to such particles has been linked with increased deaths from heart and lung disease.

Lead compounds, which are emitted by motor vehicles fuelled with leaded petrol, are cumulative poisons. They slowly build up in the body.

Urban haze

Urban haze is mainly due to fine particles, which cause scattering or absorption of light. Haze is typically brown and limits visibility.

Studies by CSIRO scientists have found that there are several types of particles present in haze in Australian cities: organic carbon compounds, elemental carbon or soot, salt, sulfates, nitrates and dust.

The term ‘air toxics’ is often used when referring to atmospheric pollutants. Air toxics are gaseous, aerosol or particulate pollutants, which are present in the air in low concentrations with characteristics such as toxicity or persistence so as to be a hazard to human, plant or animal life.

Photochemical smog

Sometimes, under certain meteorological conditions, the combined effects of a number of air pollutants are worse than the individual effects. Photochemical smog, sometimes seen as a whitish haze present over cities during summer, is an example of this.

Photochemical smog is formed on still days when the sun shines on air containing volatile organic compounds (VOCs) and oxides of nitrogen. Volatile organic compounds include hydrocarbons, as well as alcohols, aldehydes and ethers. VOCs in the air arise mainly from automotive fuels and industrial solvents. Chemical reactions driven by sunlight and involving VOCs and oxides of nitrogen form ozone, a gas harmful to humans, animals and plants.

Table 1 Major pollutants in our air

In addition to ozone, photochemical smog contains a number of other harmful secondary pollutants such as peroxyacetyl nitrate and aldehydes, which are severe irritants, particularly to the eyes. (Ironically, ozone in the stratosphere is essential for life as we know it. This ozone layer prevents much of the sun's harmful ultraviolet light reaching us.)

Air quality indoors

Australians on average spend about ninety-five percent of their time indoors and many pollutants occur at higher concentrations indoors than outdoors because of the materials and appliances used in buildings.

Many people’s main exposure to air pollutants occurs when they are indoors, such as at home, in the workplace or in entertainment venues.

Researchers are working towards measuring ‘individual exposure’ to pollutants. That is, a measure of the actual exposure that people have to air pollutants during their daily routines, rather than measures of pollution at fixed locations.

CSIRO regularly uses personal air pollution detectors, which monitor concentrations of pollutants that people breathe. The inexpensive samplers offer scientists, environmentalists, engineers and others a simple but accurate way of measuring selected pollutants in air. The sampler, based on a Swedish design, is small and requires no electricity so is ideal for remote use. Nitrogen dioxide, sulfur dioxide, ammonia and other gases can be measured with the device.

Acid rain

Pure rainwater is slightly acidic, primarily because of dissolved carbon dioxide. Air also contains naturally occurring organic acids and acidic particles. The pH of unpolluted rainwater ranges from about 6 to just below 5.

Fossil fuel combustion and industrial processes release into the air compounds containing oxides of sulfur and nitrogen. These compounds may then dissolve in cloud droplets, making rainwater more acidic.

As well, sulfur- and nitrogen-containing particles may mix through the atmosphere, eventually coming into direct contact with the ground and vegetation. In other words, the pollutants can reach the ground in a wet or dry form. Both forms can harm soil, lakes, plants, buildings and people.

Emissions from power stations, motor vehicles, and industry do contribute to acid rain. In some parts of Australia, these activities are making rainwater slightly more acidic.

Acidic pollutants released by one country can travel hundreds, or even thousands, of kilometres before being deposited. Acid rain is a real problem in Scandinavian countries, a large fraction being due to pollution released by other European countries.

There is more industrial activity in the northern hemisphere than in the southern hemisphere. Industry also tends to be concentrated in particular regions. This is why acid rain problems are worse in the northern hemisphere. The main regions affected are north-western Europe and eastern United States and Canada. Japan and parts of China also have acidity problems.

Air pollution in Australia

Compared with cities such as Los Angeles, Mexico City and Athens, air pollution problems in Australia are minor. In part, this is due to the fact that we have fewer sources of pollution, and local winds tend to rapidly disperse pollution over our cities.

Australia has a relatively small population. We are surrounded by oceans and do not receive masses of polluted air from other countries. Our oil and coal contain less sulfur than much of the oil and coal produced in other countries.

Nevertheless, each year, Sydney, Melbourne and other large Australian cities experience days of high air pollution. Summer and autumn are usually the worst times of the year.

Percentage contribution to air pollution emissions by motor vehicles in Melbourne

Reducing air pollution

Government legislation and tighter emission controls by industry have produced a marked improvement in air quality in many parts of the world. Many methods of lowering emissions have been developed.

In Australia, the emphasis is on prevention and early identification of air quality problems.

Domestic burning off adds to air pollution. In Australian cities, many municipalities have banned the use of incinerators. Agricultural and forest management practices that do not involve burning can also reduce the release of visibility-reducing particles.

However, motor vehicles present a significant and growing air pollution threat and are Australia’s single greatest source of atmospheric pollutants.

Since 1986, new cars in Australia have had catalytic converters in their exhaust systems. These converters reduce the amounts of oxides of nitrogen, carbon monoxide and unburnt petrol escaping into the air. Use of unleaded petrol is lowering the amount of lead in the urban environment.

Industrial activity, power generation and vehicle numbers in Australia are still increasing. Action today will help prevent us suffering the major air pollution problems currently being experienced in many other parts of the world.

Paul Holper and Julie Noonan
July 2000

Modified: 11 April, 2006