The Meaning and Major Types of Air Pollution
Air pollution is the release into the atmosphere of various gases, finely divided solids, or finely dispersed liquid aerosols at rates that exceed the natural capacity of the environment to dissipate and dilute or absorb them.
These substances may reach concentrations in the air that cause undesirable health, economic, or aesthetic effects. Air pollution is divided into two namely; outdoor and indoor air pollution
1. Outdoor Air Pollution
Outdoor air pollution is a growing problem. Its estimate indicates that urban outdoor air pollution has risen by eight percent globally between 2008 and 2013. Urbanization, which is often associated with rising air pollution, is increasing too, by 2050, up to two thirds of the global population is expected to live in urban areas (WHO, 2016).
This estimation is already a reality in 2021 especially in the war prone countries, Nigeria for instance is becoming urbanized as result of unabated insurgency in the rural areas. Consequently, there are high influx of population from countryside to urban areas for safety.
Unless action is taken to control outdoor air pollution, studies show that outdoor air pollution will become the leading cause of environment-related child death by 2050.
Air pollution can be high in parts of North America and Europe, but it has improved slightly over the past decade with new environmental regulations and progress in technology. Meanwhile, China and India have been frequently cited as areas where air pollution is at its worst.
In some cities in Asia, it has exceeded 20 times the World Health Organization (WHO) guidelines. Nevertheless, there is considerable variation within those countries both regionally and locally, especially in recent years.
In Beijing, for example, latest estimates show an improvement compared to previous years. In fact, data from WHO (2016) unveil a striking new analysis: the most polluted city in the world (by PM10 measurements) is Onitsha, Nigeria.
Two other Nigerian cities, Kaduna and Aba, are also among the top 10 most polluted cities. Zabol, Iran, is the most polluted city by PM2.5 measurements. Asian cities continue to occupy the majority of the top of the list for PM2.5 measurements.
2. Indoor Air Pollution
Burning of solid fuels for household cooking, heating and lighting is a major cause of household, or indoor, air pollution. Indoor air pollution puts nearly 3 billion people worldwide at risk of ill health and early death.
Indoor pollutants include particulate matter (PM10 and PM2.5), mould, dust mites and bacteria, as well as chemicals and Volatile Organic Compounds (VOCs) [such as formaldehyde and benzene] from paints, personal care products and building materials.
The effects of indoor air pollution kill more children globally than outdoor air pollution, especially in Africa and Asia. Eighteen of the nineteen countries where 95 per cent or more of the population use solid fuels for cooking are in sub-Saharan Africa.
Clean, dry air consists primarily of nitrogen and oxygen, 78 percent and 21 percent respectively, by volume. The remaining 1 percent is a mixture of other gases, mostly argon (0.9 percent), along with trace (very small) amounts of carbon dioxide, methane, hydrogen, helium, and more.
Water vapour is also a normal, though quite variable, component of the atmosphere, normally ranging from 0.01 to 4 percent by volume; under very humid conditions the moisture content of air may be as high as 5 percent.
The gaseous air pollutants of primary concern in urban settings include sulfur dioxide, nitrogen dioxide, and carbon monoxide; these are emitted directly into the air from fossil fuels such as fuel oil, gasoline, and natural gas that are burned in power plants, automobiles, and other combustion sources.
Ozone (a key component of smog) is also a gaseous pollutant; it forms in the atmosphere via complex chemical reactions occurring between nitrogen dioxide and various volatile organic compounds (e.g., gasoline vapours).
Airborne suspensions of extremely small solid or liquid particles called ―particulates‖ (e.g., soot, dust, smokes, fumes, mists), especially those less than 10 micrometres (μm; millionths of a metre) in size, are significant air pollutants because of their very harmful effects on human health.
They are emitted by various industrial processes, coal- or oil-burning power plants, residential heating systems, and automobiles. Lead fumes (airborne particulates less than 0.5 μm in size) are particularly toxic and are an important pollutant of many diesel fuels.
The six major air pollutants aforementioned have been designated by the U.S. Environmental Protection Agency (EPA) as ―criteria‖ pollutants—criteria meaning that the concentrations of these pollutants in the atmosphere are useful as indicators of overall air quality. The sources, acceptable concentrations, and effects of the criteria pollutants are summarized in the table.
Except for lead, criteria pollutants are emitted in industrialized countries at very high rates, typically measured in millions of tons per year. All except ozone are discharged directly into the atmosphere from a wide variety of sources.
They are regulated primarily by establishing ambient air quality standards, which are maximum acceptable concentrations of each criteria pollutant in the atmosphere, regardless of its origin. The six criteria pollutants are described in subsequent sections.
The very small fragments of solid materials or liquid droplets suspended in air are called particulates. Except for airborne lead, which is treated as a separate category, they are characterized on the basis of size and phase (i.e., solid or liquid) rather than by chemical composition.
For example, solid particulates between roughly 1 and 100 μm in diameter are called dust particles, whereas airborne solids less than 1 μm in diameter are called fumes.
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The particulates of most concern with regard to their effects on human health are solids less than 10 μm in diameter, because they can be inhaled deep into the lungs and become trapped in the lower respiratory system.
Certain particulates, such as asbestos fibres, are known carcinogens (cancer- causing agents), and many carbonaceous particulates e.g., soot are suspected of being carcinogenic.
Major sources of particulate emissions include fossil-fuel power plants, manufacturing processes, fossil-fuel residential heating systems, and gasoline-powered vehicles.
Carbon monoxide is an odourless, invisible gas formed as a result of incomplete combustion. It is the most abundant of the criteria pollutants.
Gasoline-powered highway vehicles are the primary source, although residential heating systems and certain industrial processes also emit significant amounts of this gas.
Power plants emit relatively little carbon monoxide because they are carefully designed and operated to maximize combustion efficiency.
Exposure to carbon monoxide can be acutely harmful since it readily displaces oxygen in the bloodstream, leading to asphyxiation (oxygen deprivation) at high enough concentrations and exposure times.
A colourless gas with a sharp, choking odour, sulfur dioxide is formed during the combustion of coal or oil that contains sulfur as an impurity.
Most sulfur dioxide emissions come from power-generating plants; very little comes from mobile sources. This pungent gas can cause eye and throat irritation and harm lung tissue when inhaled.
Sulfur dioxide also reacts with oxygen and water vapour in the air, forming a mist of sulfuric acid that reaches the ground as a component of acid rain.
Acid rain is believed to have harmed or destroyed fish and plant life in many thousands of lakes and streams in parts of Europe, the northeastern United States, southeastern Canada, and parts of China.
It also causes corrosion of metals and deterioration of the exposed surfaces of buildings and public monuments.
Of the several forms of nitrogen oxides, nitrogen dioxide, a pungent, irritating gas is of most concern. It is known to cause pulmonary edema, an accumulation of excessive fluid in the lungs.
Nitrogen dioxide also reacts in the atmosphere to form nitric acid, contributing to the problem of acid rain. In addition, nitrogen dioxide plays a role in the formation of photochemical smog, a reddish brown haze that often is seen in many urban areas and that is created by sunlight- promoted reactions in the lower atmosphere.
Nitrogen oxides are formed when combustion temperatures are high enough to cause molecular nitrogen in the air to react with oxygen. Stationary sources such as coal-burning power plants are major contributors of this pollutant, although gasoline engines and other mobile sources are also significant.
Inhaled lead particulates in the form of fumes and dusts are particularly harmful to children, in whom even slightly elevated levels of lead in the blood can cause learning disabilities, seizures, or even death.
Sources of airborne lead particulates include oil refining, smelting, and other industrial activities. In the past, combustion of gasoline containing a lead-based antiknock additive called tetraethyl lead was a major source of lead particulates.
In many countries there is now a complete ban on the use of lead in gasoline. In the United States, lead concentrations in outdoor air decreased more than 90 percent after the use of leaded gasoline was restricted in the mid-1970s and then completely banned in 1996.
Hundreds of specific substances are considered hazardous when present in trace amounts in the air. These pollutants are called air toxics. Many of them cause genetic mutations or cancer; some cause other types of health problems, such as adverse effects on brain tissue or fetal development.
Although the total emissions and the number of sources of air toxics are small compared with those for criteria pollutants, these pollutants can pose an immediate health risk to exposed individuals and can cause other environmental problems.
Most air toxics are organic chemicals, comprising molecules that contain carbon, hydrogen, and other atoms. Many are volatile organic compounds (VOCs), organic compounds that readily evaporate. VOCs include pure hydrocarbons, partially oxidized hydrocarbons, and organic compounds containing chlorine, sulfur, or nitrogen.
They are widely used as fuels (e.g., propane and gasoline), as paint thinners and solvents, and in the production of plastics.
In addition to contributing to air toxicity and urban smog, some VOC emissions act as greenhouse gases and, in so doing, may be a cause of global warming. Some other air toxics are metals or compounds of metals for example, mercury, arsenic, and cadmium.
In many countries, standards have been set to control industrial emissions of several air toxics. The first hazardous air pollutants regulated in the United States (outside the workplace environment) were arsenic, asbestos, benzene, beryllium, coke oven emissions, mercury, radionuclides (radioactive isotopes), and vinyl chloride.
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In 1990 this short list was expanded to include 189 substances. By the end of the 1990s, specific emission control standards were required in the United States for ―major sources‖—those that release more than 10 tons per year of any of these materials or more than 25 tons per year of any combination of them.
Air toxics may be released in sudden and catastrophic accidents rather than steadily and gradually from many sources.
For example, in the Bhopal disaster of 1984, an accidental release of methyl isocyanate at a pesticide factory in Bhopal, Madhya Pradesh state, India, immediately killed at least 3,000 people, eventually caused the deaths of an estimated 15,000 to 25,000 people over the following quarter-century, and injured hundreds of thousands more.
The risk of accidental release of very hazardous substances into the air is generally higher for people living in industrialized urban areas. Hundreds of such incidents occur each year, though none has been as severe as the Bhopal event.
Other than in cases of occupational exposure or accidental release, health threats from air toxics are greatest for people who live near large industrial facilities or in congested and polluted urban areas. Most major sources of air toxics are so-called point sources that is, they have a specific location.
Point sources include chemical plants, steel mills, oil refineries, and municipal waste incinerators. Hazardous air pollutants may be released when equipment leaks or when material is transferred, or they may be emitted from smokestacks.
Municipal waste incinerators, for example, can emit hazardous levels of dioxins, formaldehyde, and other organic substances, as well as metals such as arsenic, beryllium, lead, and mercury. Nevertheless, proper combustion along with appropriate air pollution control devices can reduce emissions of these substances to acceptable levels.
Hazardous air pollutants also come from ―area sources, which are many smaller sources that release pollutants into the outdoor air in a defined area. Such sources include commercial dry- cleaning facilities, gasoline stations, small metal-plating operations, and woodstoves. Emission of air toxics from area sources are also regulated under some circumstances.
Small area sources account for about 25 percent of all emissions of air toxics. Major point sources account for another 20 percent. The rest more than half of hazardous air-pollutant emissions come from motor vehicles.
For example, benzene, a component of gasoline, is released as unburned fuel or as fuel vapours, and formaldehyde is one of the by-products of incomplete combustion. Newer cars, however, have emission control devices that significantly reduce the release of air toxics.
In summary, outdoor air pollution is a growing problem. Recent estimates indicate that urban outdoor air pollution has risen by 8 per cent globally between 2008 and 2013.
The effects of indoor air pollution kill more children globally than outdoor air pollution, especially in Africa and Asia. Air toxics may be released in sudden and catastrophic accidents rather than steadily and gradually from many sources.
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