Air pollution can threaten the health of human beings, trees, lakes, crops, and animals. Abundant amounts of air pollution changes natural atmospheric processes, causing acid rains, ozone hole, and enhancing greenhouse effect. Additionally, it causes economic losses.

Pollutants are classified either as primary pollutants, or those that enter the atmosphere directly from various sources, or as secondary pollutants, or those that are formed when primary pollutants react with each other or with other compounds present in the atmosphere.

Industrial or gray smog is considered the most serious type of air pollution. Smoke and oxides of that are released by burning coal and oil containing minor amounts of sulfur is the cause. The smoke gives the air a gray color. Industrial smog has been known to cause air pollution disasters. One of the worst occurred in London in December of 1952. Five days of stagnant air brought about high-pressure systems caused between 3,500 and 4,000 deaths.

Another main cause of air pollution is exhaust fumes from cars and other vehicles that run on gasoline or diesel. Until a few years ago, lead was added to most gasolines to make car engines run better. The lead passes through the engine and out of the exhaust system into the air. Lead is a highly toxic metal and can cause nervous system damage and digestive problems.

Road traffic emissions, particularly from diesel vehicles are a major source of particulate matters and nitrogen oxides. PM10 particles (the fraction of particulate in air of very small size (<10 µm)) are of major current concern, as they are small enough to penetrate deep into the lungs and so potentially pose significant health risks. Particles are often classed as either primary or secondary pollutants.

Sulfur and nitrogen oxides (primary pollutants) from power plants, industry, cars and other sources cause rain, snow and fog to become acidic. The most serious damage caused by acid rain today is acidification of water lakes and rivers. In some cases they become so acidic that they can no longer support fish and their food chains. Forest health can also be affected from this in areas where the soils are being acidified.

Ozone depletion is another result of pollution. The ozone layer in the stratosphere protects the earth from harmful ultraviolet radiation from the sun. Release of chlorofluorocarbons (CFC's) from aerosol sprays, polystyrene containers, refrigerator coolant and air conditioning units removes some of the ozone, causing "holes"; to open up in this layer and allowing the radiation to reach the earth.

Ozone in the in the upper level occurs naturally and protects life on earth but ozone at ground level is a noxious pollutant. It is the major component of photochemical smog (also know as brown smog) and presents the most intractable urban air quality problem. Ozone is not emitted directly. It is secondary pollutant formed in the atmosphere through a complex set of chemical reactions involving hydrocarbons, oxides of nitrogen, and sunlight. The rate at which the reactions proceed is related to both temperature and intensity of the sunlight.

The greenhouse effect, also referred to as global warming, is generally believed to come from the build up of carbon dioxide (CO₂) gas in the atmosphere. Carbon dioxide is produced when fuels are burned. Plants convert carbon dioxide back to oxygen, but the release of carbon dioxide from human activities is higher than the world's plants can process. The situation is made worse since many of the earth's forests are being removed, and plant life is being damaged by acid rain.

Finally, air pollution can also occur indoors. Common indoor air pollutants include carbon monoxide and nitrogen dioxide from faulty gas heaters and cookers, carbon monoxide and benzene from cigarette smoke, and volatile organic compounds (VOCs) from synthetic furnishings, vinyl flooring and paints. In addition, there are biological pollutants such as dust mites and mould.

Adequate ventilation is a key to controlling exposure to indoor air pollution. Home and work environments should be monitored for adequate airflow and proper exhaust systems installed.

Air pollution generates various economic losses but their detailed estimation is extremely difficult. They may be divided into four groups: expenses for air quality protection; expenses generated by the worse health condition of the society; losses of raw materials which become air pollution; losses caused by the increased corrosion of machines, buildings, and damage of historical buildings and monuments.

The problem of air pollution is worldwide and transcends national boundaries. Though air pollution is still a serious problem, in many countries in the world, steps are being taken to stop the damage to our environment from air pollution. Many electric power plants, factories, and facilities that burn wastes are equipped with devices called scrubbers. Scrubbers remove sulfur dioxide and other pollutants before the wastes are released into the air. Also, today's cars are designed to create less pollution. Many are equipped with anti-pollution devices called catalytic converters, which reduce the amount of pollution from automobile engines.

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