General airborne pollution arises from a variety of causes but can usefully be subdivided into pollution from combustion or from other sources. The image shows the silhouette of a power station - an important source of airborne products of combustion.
Combustion of coal and other solid fuels can produce smoke (containing polycyclic aromatic hydrocarbons - PAH) and sulphur dioxide besides other agents such as those also produced by:
Combustion of liquid petroleum products which can generate carbon monoxide, oxides of nitrogen and other agents. Industry and incineration can generate a wide range of products of combustion such as oxides of sulphur and nitrogen, polycyclic aromatic hydrocarbons, dioxins etc. Combustion of any fossil fuel generates varying amounts of particulate matter. It also adds to the environmental burden of carbon dioxide - an important "green house" gas but in these low concentrations it does not affect human health directly. Combustion of fuel can also generate hazardous substances in other ways, besides by chemical oxidation, such as by liberating benzene (from the "cracking" of petrol) or lead (from leaded petrol). Some of the primary pollutants such as nitrogen dioxide can, under the influence of UV light generate secondary pollutants notably ozone (an allotrope of oxygen).
Undoubtedly tens of thousands of deaths have resulted from acute pollution episodes (e.g. the smogs in large cities in the early 1950s). Nowadays some people e.g. asthmatics can be adversely affected by excursions in levels of urban air pollution (notably ozone) in some major cities. What is still unclear is the extent to which urban airborne pollution in the majority of cities complying with current air quality guidelines, contributes to ill health, i.e. whether the air quality guidelines are stringent enough, to protect all the population.
Health effects of concern are asthma, bronchitis and similar lung diseases, and there is good evidence relating an increased risk of symptoms of these diseases with increasing concentration of sulphur dioxide, ozone and other pollutants.
Moreover, there is increasing evidence to suggest that pollution from particulate matter (PM10 and black smoke) at levels hitherto considered "safe" is associated with an increased risk of morbidity and mortality (disease and death) from heart disease as well as lung disease. This is likely especially in people with other risk factors (such as old age, or pre-existing heart and lung disease). These concerns are the subject of current research throughout the world.
Although high occupational exposures to exhaust especially from diesel, and to benzene does increase the risk of some cancers, reliable direct evidence of an increased to cancer risk to the population at large from the lower levels to which they are exposed is lacking.
Incineration can also generate hazardous substances if substances not best suited for disposal by incineration are "disposed" of in this way or if incineration is carried out at too low a temperature (for example this may generate dioxins).
Products of combustion and other harmful airborne pollutants can also arise within the home. Thus nitrogen dioxide generated by gas fires or gas cookers can contribute to an increased respiratory morbidity of those living in the houses. Certain modern building materials may liberate gases or vapours such as formaldehyde at low concentration but which might provoke mild respiratory and other symptoms in some occupants. Modern building standards for asbestos in buildings are such that the resulting airborne fibre concentrations are so small as not to present any risk at all of asbestosis. However some estimates suggest that perhaps one extra death per year might result in the UK from asbestos related cancer as a result of non-occupational exposure in buildings. The image shows an asbestos body i.e. an asbestos fibre which has been coated by ferruginous protein during its residence within the human lung.
Large scale industrial releases with serious acute effects are fortunately rare but you might recollect some events such as in Bhopal (India). Various smaller scale events occur such as leaks from road tankers, or fires in warehouses and factories.Special local environmental exposures can arise for example in communities exposed to drifting pesticide sprays containing say, organophosphates. Some natural phenomena such as volcanic eruptions can present serious risks to health. Fortunately they are rare but can be catastrophic.
You may wish to consult the following internet resources mostly written
by this author for more detailed information in relation to this
Prescott GJ, Cohen GR, Elton RA, Fowkes FGR, Agius RM*. Urban Air Pollution and Cardiopulmonary Ill-Health - A 14.5 year time series study. Occup Environ Med 1998;55: 697-704.
Prescott GJ, Lee RJ, Cohen GR, Elton RA, Lee AJ, Fowkes FGR, Agius RM*. Investigation of factors which might indicate susceptibility to particulate air pollution. Occup Environ Med, 2000; 57: 53-57.
*Author for correspondence