Air pollution is a type of environmental pollution that affects the air and is usually caused by smoke or other harmful gases, mainly oxides of carbon, sulphur and nitrogen.

In other words, air pollution is the contamination of air due to the presence or introduction of a substance which has a poisonous effect.

Many of the world's large cities today have polluted air or low air and complain about it’s daily 
quality. 

Even 2,000 years ago, the Romans were complaining about the polluted air in their cities. At that time, the air was thick with smoke from fires and the smell of sewers. Air pollution has been classified as a danger to human health and Earth's many ecosystems for a long time.

Air quality 
Pollution can be gas , liquid or solid. It can also be classified chemically, such as:- oxide, hydrocarbon, acid or other kinds. Pollution can also be classified by what makes it.

Many pollutants go into the air from natural sources. These pollutants include dust, sea salt, volcanic ashes and gases, smoke from forest fires, pollen, and many other materials. In fact, there are many more natural pollutants than pollutants that humans make. However, humans and other living things have adapted to most of these natural pollutants.

Air pollution is usually described as either primary pollutants or secondary pollutants.Primary pollutants are pollutants that are put directly into the air by humans or natural sources. Examples of primary pollutants are exhaust fumes (gas) from cars, soot from smoke, dust storms and ash from volcanic eruptions (as seen in the picture on the left).

Secondary pollutants are pollutants that are made from chemical reactions when pollutants mix with other primary pollutants or natural substances like water vapor. Many secondary pollutants are made when a primary pollutant reacts with sunlight. Ozone and smog are secondary pollutants. Ozone is a gas that stops harmful ultraviolet rays from the sun.  When it is near the ground, though, it can poison people and other organisms.
    

Human-made air pollution comes from many things. Most air pollution made by humans today is because of transportation. Automobiles , for instance, make about 60% of the human-made air pollution. The gases inside car exhaust, like nitrogen oxide, make smog and acid rain.

Farmlands and forests sometimes burn in wildfires producing soot (a black powder composed mainly of carbon, produced when coal, grasses, wood etc. is burned) from smoke. Soot can affect people and other life. People start many of those fires.

Industrial air pollution 
Many industrial power plants burn fossil fuels to get their energy. However, burning fossil fuels can make a lot of oxides (chemical compounds that have oxygen and other elements inside). In fact, the burning of fossil fuels makes 96% of the sulphur oxides in the atmosphere. Some industries also make chemicals that make poisonous fumes (smoke).

Indoor air pollution 
Air pollution is not only on the outside. Homes, schools, and buildings can also have air pollution. Sometimes the air inside a building is even worse than the air outside. Many things which humans use every day can pollute the air. Compounds inside carpets, paints, building materials and furniture also pollute the air, especially when they are new.

In buildings where the windows are tightly shut to stop air leaks, the air inside can be polluted more than the air outside.

Acid precipitation 
Acid precipitation is precipitation, like rain, sleet, or snow, that contains acids from air pollution. When fossil fuels are burned, they let out oxides into the air. When these oxides mix with water in the atmosphere, they make acid, which fall as precipitation. Acid precipitation can kill living things like fish and trees, by making the place where they live too acidic. Acid rain can also damage buildings made of limestone and concrete.

Ozone hole 
A global concern is the hole in the ozone layer in the stratosphere.  The Earth's ozone layer protects life from the Sun's harmful ultraviolet rays, but in the 1970s, scientists found out that some chemicals let out into the atmosphere makes the ozone turn into oxygen.  This lets more ultraviolet rays reach the Earth.  During the 1980s, scientists found that the ozone layer above the South Pole had thinned by 50 to 98%.

Human health 
On March 17, 1992, in Mexico City, all children under the age of 14 could not go to school because of air pollution. This does not often happen, but being exposed to air pollution every day can make people have many health problems. Children, elderly (old) people, and people with allergies especially, can have a lot of problems because of air pollution. Studies from the University of Birmingham showed that deaths because of pneumonia and air pollution from motor vehicles like cars are related.

The World Health Organization said that 2.4 million people died because of the direct problems of air pollution. Some of the problems include:

Head
Dizziness
Headaches

Face
Burning, scratchy eyes
Runny nose
Coughing and hard breathing

Other
Lung cancer and lung diseases
Sore throat
Chest pains, colds, and allergies

Agricultural effects 
In India in 2014, it was reported that air pollution had cut crop yields in the most affected areas by almost half in 2010 when compared to 1980 levels. There can also be increased yield of crops due to some air quality conditions.

References

Other websites 

 Air quality science and general information
 UNEP Urban Issues 
 European Commission > Environment > Policies > Air >Air Quality.
 UNEP Partnership for Clean Fuels and Vehicles 

 Air quality modelling
 Stuff in the air Standard air quality modelling procedure for industrial sources.
 Wiki on Atmospheric Dispersion Modelling. Addresses the international community of atmospheric dispersion modellers — primarily researchers, but also users of models. Its purpose is to pool experiences gained by dispersion modellers during their work.
 Air Dispersion Modeling Conversions and Formulas One of six technical articles devoted to air quality and air pollution dispersion modeling.

 Effects on human health