Air pollution is the transfer of harmful amounts of natural and synthetic materials into the atmosphere as a direct or indirect action of human activity. Air pollution can be divided into mobile and stationary sources. Mobile sources include various modes of transportation such as automobiles, busses, trains and aeroplanes whereas stationary sources include factories, incinerators and electric power plants (Fuggle  & Rabie, 1992 ). Air pollution in Greater Johannesburg constitutes a number of driving forces from human activities namely:

BURNING OF SOLID WASTE

The smoke from burning refuse is very dangerous, as it is a combination of many materials made up of different chemicals. For example the burning of plastics releases toxic chemicals (dioxins) harmful to human health.

TRANSPORTATION

Transportation systems such as private automobiles are the biggest cause of air pollution especially in Greater Johannesburg, due to the incomplete combustion of fossil fuels. Exhaust fumes from engines release different kinds of harmful pollutants, namely carbon monoxide, hydrocarbons, oxides of nitrogen, lead and smaller particulates (diesel vehicle emmissions). Emissions from vehicular traffic includes nitrous oxides and hydrocarbons which forms ozone, and is an indicator of  photochemical smog. The excessive emission of diesel fumes from heavy-duty vehicles is particularly problematic due to a lack of regular vehicle maintenance and law enforcement.

The Greater Johannesburg area contains over 6 700 km of tarred roads and 650 km of untarred roads. The public transport system as outlined in table 2 consist of rail (45%), bus (26%) and minibus taxi (29%).

The transport mode for home-to-work trips includes taxi (55%), bus (12%), train (10%) and car (3%).

DOMESTIC SOURCES OF AIR POLLUTION

Economic progress normally involves a progression through what is called the "fuel ladder" where communities start off by using coal, paraffin, liquid petroleum gas and eventually electricity. In the South African context all of the above mentioned fuels, either individually or a combination of these, have been used for cooking and heating. A British delegate at a Soweto workshop on air pollution in June 1990, Bill Muirhead of Falkirk, said: "Stoves don't smoke. It is the coal that smokes. So, clean the coal! Whatever you do, don't oblige people to chuck out perfectly good coal-burning stoves to replace them with expensive electric stoves." (Clarke, 1991). The fuel being used is influenced by the economic status of a community or individual. This then constitutes a range of environmental impacts in terms of environmental quality, environmental health and the sustainable use of non-renewable energy resources. A study conducted by Graham et.al. (1999), highlighted the emissions and cost effectiveness of traditional and transitional household fuel burning appliances in South Africa. This study is of particular relevance in relation to the number of informal settlements within the Greater Johannesburg metropolitan area.

MINING

Mining processes are mainly responsible for air pollution in the form of dust, especially where these mine dumps are not covered with vegetation. The prevailing wind direction on the reef (north-northwest) also plays a major role.

INDUSTRIES

These contribute significantly to air pollution as industries give off various types of pollutants, for example plants that produce aluminium discharge ammonia, hydrocarbons, organic acids and sulphur oxides. The level of air pollution by industrial sources depends on the distance from the industry, the properties of the chemical, the local topography and the atmospheric conditions. The direction, distance and speed of air, and in turn the dispersion of air pollutants are influenced by the direction, distance and speed that the air mass travel (Gauteng SoER,1995).

POWER STATIONS

Power stations are mostly related to the emission of fossil fuel air pollution. Electricity itself is very clean but the production of electricity can have highly adverse impacts on the environment. The most highly utilised substance in the generation of electricity is coal. Power stations are responsible for sulphur oxides and particulate matter in the atmosphere, which in turn impacts negatively on our health and the environment.

INDOOR AIR POLLUTION

Indoor air pollution includes chemical sprays, household products such as paint strippers, cigarette smoke and various building materials. Infectious agents such as viruses and bacteria may also be more concentrated indoors especially where overcrowding exists. Sick Building Syndrome often results from air pollution especially in buildings with a lack of proper ventilation and fresh air.

POPULATION GROWTH

Greater Johannesburg currently has a population of 2.5 million (Census 96, Stats SA), which could double in the next 25 years. An increase in the affluence of the middle to higher income sector is associated with higher use of natural resources particularly energy. In contrast to this the poor population suffer ineffective urban management and inadequate provision of municipal services, which results in significant local degradation of the environment (SMLC, 1997)