Air Pollution

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Air pollution began in prehistoric times when humans first used fire. Discoveries within prehistoric caves show evidence of soot on the ceilings caused by inadequate ventilation from open fires. In addition, outdoor air pollution was present when early humans began to forge metals. In fact, core samples taken by scientists from glaciers in Greenland confirm pollution from metal production by the ancient Chinese, Greeks, and Romans.

Since historic times, air pollution has been a matter of concern in large urban areas. Indeed, there were complaints about smoke in ancient Rome. The use of coal throughout the centuries caused cities to produce dense smoke. Along with smoke, large concentrations of sulfur dioxide (SΟ2) were produced. The mixture of smoke and sulfur dioxide typified the foggy streets of nineteenth-century, industrialized London, England. Such situations are far less common in the cities of North America and Europe today. However, until recently, they were evident in other cities, such as Ankara, Turkey, and Shanghai, China, that rely heavily on the use of coal.

The most common sources of air pollution are the combustion processes. The most obvious pollutant is smoke. However, the widespread use of fossil fuels have made sulfur (S) and nitrogen oxides (NΟ_x) pollutants of great concern. With increasing use of petroleum-based fuels, a range of organic compounds have become widespread in the atmosphere.

Primary pollutants such as sulfur dioxide or smoke are the direct emission products of the combustion process. Many key pollutants in the urban atmospheres are secondary pollutants, produced by processes initiated through photochemical reactions. The photochemical smog in Los Angeles, California, is now characteristic of urban atmospheres dominated by secondary pollutants.

Although motorized vehicles, such as automobiles, are the main source of air pollution in contemporary cities, there are other equally significant sources. Stationary sources are still important and the oil-burning furnaces that have replaced the older coal-burning ones are responsible for a range of gaseous emissions and fly ash. Incineration is also an important source of complex combustion products, especially where incineration burns a wide range of refuse. These emissions can include chlorinated hydrocarbons such as dioxin. When plastics, which often contain chlorine (Cl), are incinerated, the result is hydrochloric acid (HCl) in the waste gas stream. Metals, especially where they are volatile at high temperatures, can migrate to smaller, respirable particles. The accumulation of toxic metals, such as cadmium (Cd), on fly ash gives rise to concern about harmful effects from incinerator emissions. Many questions have been raised about the completeness of the destruction process in specialized incinerators designed to destroy toxic compounds such as polychlorinated biphenyls (PCBs). Even under optimum conditions where the furnace operation has been properly maintained, great care needs to be taken to control leaks and losses during transfer operations (fugitive emissions).

The enormous range of compounds used in modern manufacturing processes have also meant that there has been an ever-widening range of emissions from both the industrial processes and the combustion of their wastes. Although the amounts of these exotic compounds are often rather small, they add to the complex range of compounds found in the urban atmosphere. The deliberate loss of effluents through discharge from pipes and chimneys is not the only source that needs attention. Fugitive emissions of volatile substances that leak from valves and seals require careful control.

Air pollution control procedures are increasingly an important part of civic administration, although their goals are far from easy to achieve. It is also troubling that, although many urban concentrations of primary pollutants (for example, smoke and sulfur dioxide) are on the decline in developed countries, such is not the case in developing countries. Here the desire for rapid industrial growth has often lowered urban air quality. Secondary air pollutants are generally proving more difficult to eliminate than primary pollutants such as smoke.

Air pollution was the cause of a major disease outbreak in London, England, in 1952. According to the U.K. Ministry of Health, early in December 1952, severe air pollution caused by airborne pollutants from the use of coal was made worse by exceptionally cold temperatures and windless conditions. The air pollution event caused about 4,000 people to die, primarily from the airborne particles being inhaled. The air pollution quickly disappeared when the weather changed. However, over the following few months, another estimated 8,000 people may have died as the result of the pollution event. In all, about 100,000 people were sickened by the air pollution. The historic incident is often referred to as the Great Smog of '52.

According to the World Health Organization (WHO), the cities with the world's worst air pollution in 2016 were: (1) Zabol, Iran; (2) Gwalior, India; (3) Allahabad, India; (4) Riyadh, Saudi Arabia; (5) Al Jubail, Saudi Arabia; (6) Patna, India; (7) Raipur, India; (8) Bamenda, Cameroon; (9) Xingtai, China, and (10) Baoding, China.

Urban air pollutants have a wide range of effects, with health problems being the most enduring concern. In atmospheres filled with smoke and sulfur dioxide, various bronchial diseases were intensified. In atmospheres where the air pollutants are not so obvious, the effects may be more difficult to identify and address. For example, in photochemical smog, eye irritation from the secondary pollutant peroxyacetyl nitrate (PAN) is one on the most characteristic direct effects of the smog. High concentrations of carbon monoxide in cities where automobiles operate at high density force the human heart to work harder to make up for the oxygen displaced from the blood's hemoglobin by carbon monoxide. This extra stress appears to reveal itself by increased incidence of complaints among people with heart problems. Many suspect that contemporary air pollutants are involved in the increases in asthma, but the links between asthma and air pollution are complex and related to a whole range of factors. Lead (Pb), from automotive exhausts, is thought by many to be a factor in lowering the intelligence quotients (IQs) of urban children.

Air pollution also affects materials in the urban environment. Soiling has long been regarded as a problem, originally the result of the smoke from wood or coal fires, but in modern times increasingly the result of fine black soot from diesel exhausts. The acid gases, particularly sulfur dioxide, erode building materials. This effect is most noticeable with calcareous stones, which are the predominant building material of many important historic structures. Metals also suffer from atmospheric acidity. In modern photochemical smog, natural rubbers crack and deteriorate rapidly.

Health problems relating to indoor air pollution have existed since ancient times. Anthracosis, or black lung disease, has been found in mummified lung tissue. The late 1990s and early 2000s witnessed a shift from the concern about outdoor air pollution to worry about indoor air quality.

The production of energy from combustion and the release of solvents is so pervasive in the contemporary world that it causes air pollution problems on a regional and global nature. Acid rain, which is widely observed throughout the world, is produced when emissions of sulfur oxides (SO_x) and nitrogen oxides react with water and oxygen in the air to form acids. Precipitation and fog deposit these acids in water bodies, on the ground, on structures, and on plants, which can all be damaged by these acidic compounds. Carbon dioxide (CO2) emitted in the combustion process is increasing the concentration of carbon dioxide in the atmosphere and enhancing the greenhouse effect, which is the increased temperature of Earth as a result of greenhouse gases, absorbing and trapping heat in the lower atmosphere (troposphere). The warming of Earth is referred to as climate change. Power plants are responsible for about one-half of the energy-producing fossil fuel emissions, whereas vehicles contribute approximately one-third of these emissions.

There are also important issues surrounding the effects of indoor air pollutants on materials. Many industries, especially the electronics industry, must take great care over the purity of indoor air where a speck of dust can destroy a microchip or low concentrations of air pollutants change the composition of surface films in component design. Museums must care for objects over long periods of time, so precautions must be taken to protect delicate dyes from the effects of photochemical smog; and paper and books from sulfur dioxide; and metals from sulfide gases.

In many countries, including the United States, steps are being taken to minimize and even prevent air pollution from damaging the environment. Scientists are studying the damaging effects on animals and plants, while politicians are writing laws to control pollution emissions. At the same time, teachers are teaching their students about the effects of air pollution. In the United States, the Environmental Protection Agency (EPA) is required by the Clean Air Act (CAA) of 1990 to set standards for air quality termed the national ambient air quality standards (NAAQS). The NAAQS are established for pollutants that are deemed harmful to public health and the environment.

Solvents such as carbon tetrachloride (CCl4) and aerosol propellants such as chlorofluorocarbons (CFCs) are detectable all over the globe and are responsible for such problems as ozone layer depletion. Because of their effects, these compounds have been targeted under the CAA for phaseout to prevent further breakdown of the ozone layer. Although air quality in the United States improved between 1980 and 2010, many problems remain to be addressed with regards to the public health and the environment.

The effects on the public health with respect to air pollution are considerable. In fact, as of 2018, the World Health Organization (WHO) estimated that approximately 4.6 million people around the world die each year from the effects of air pollution. Further, respiratory diseases in children are exacerbated due to air pollution.

According to the Scientific American article “5 Steps to Clean Up Air Pollution,” the American Lung Association and many other public health and environmental organization recommend the following five ways to curb air pollution in the United States:

Peter Brimblecombe, PhD
Revised by William A. Atkins, BB, BS, MBA