Pollution is the introduction of contaminants into an environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms.  Pollution can take the form of chemical substances or energy , such as noise, heat, or light. Pollutants, the elements of pollution, can be foreign substances or energies, or naturally occurring; when naturally occurring, they are considered contaminants when they exceed natural levels. Pollution is often classed as point source or nonpoint source pollution . The Blacksmith Institute issues annually a list of the world's worst polluted places. In the 2007 issues the ten top nominees are located in Azerbaijan , China , India , Peru , Russia , Ukraine and Zambia . [ citation needed ]
The greenhouse effect is a process by which radiative energy leaving a planetary surface is absorbed by some atmospheric gases, called greenhouse gases . They transfer this energy to other components of the atmosphere, and it is re-radiated in all directions, including back down towards the surface. This transfers energy to the surface and lower atmosphere, so the temperature there is higher than it would be if direct heating by solar radiation were the only warming mechanism  .
This mechanism is fundamentally different from that of an actual greenhouse , which works by isolating warm air inside the structure so that heat is not lost by convection .
The greenhouse effect was discovered by Joseph Fourier in 1824, first reliably experimented on by John Tyndall in 1858, and first reported quantitatively by Svante Arrhenius in 1896. 
If an ideal thermally conductive blackbody was the same distance from the Sun as the Earth, it would have an expected blackbody temperature of 5.3 °C. However, since the Earth reflects about 30%  (or 28%  ) of the incoming sunlight, the planet's actual blackbody temperature is about -18 or -19 °C  , about 33°C below the actual surface temperature of about 14 °C or 15 °C.  The mechanism that produces this difference between the actual temperature and the blackbody temperature is due to the atmosphere and is known as the greenhouse effect.
Global warming , a recent warming of the Earth's surface and lower atmosphere,  is believed to be the result of a strengthening of the greenhouse effect mostly due to human-produced increases in atmospheric greenhouse gases
Ozone depletion describes two distinct, but related observations: a slow, steady decline of about 4 percent per decade in the total volume of ozone in Earth 's stratosphere (the ozone layer ) since the late 1970s , and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same period. The latter phenomenon is commonly referred to as the ozone hole . In addition to this well-known stratospheric ozone depletion, there are also tropospheric ozone depletion events , which occur near the surface in polar regions during spring.
The detailed mechanism by which the polar ozone holes form is different from that for the mid-latitude thinning, but the most important process in both trends is catalytic destruction of ozone by atomic chlorine and bromine.  The main source of these halogen atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds, commonly called freons , and of bromofluorocarbon compounds known as halons . These compounds are transported into the stratosphere after being emitted at the surface.  Both ozone depletion mechanisms strengthened as emissions of CFCs and halons increased.
CFCs and other contributory substances are commonly referred to as ozone-depleting substances ( ODS ). Since the ozone layer prevents most harmful UVB wavelengths (270–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere , observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol that bans the production of CFCs and halons as well as related ozone depleting chemicals such as carbon tetrachloride and trichloroethane . It is suspected that a variety of biological consequences such as increases in skin cancer , cataracts ,  damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion
Acid rain is a rain or any other form of precipitation that is unusually acidic , i.e. elevated levels of hydrogen ions (low pH ). It can have harmful effects on plants, aquatic animals, and infrastructure through the process of wet deposition. Acid rain is caused by emissions of compounds of ammonium , carbon , nitrogen , and sulfur which react with the water molecules in the atmosphere to produce acids. Governments have made efforts since the 1970s to reduce the production of sulfur dioxide into the atmosphere with positive results. However, it can also be caused naturally by the splitting of nitrogen compounds by the energy produced by lightning strikes, or the release of sulfur dioxide into the atmosphere by volcano eru ption Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog, cloudwater, and dew) and dry (acidifying particles and gases) acidic components. A more accurate term is “acid deposition”. Distilled water , once carbon dioxide is removed, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are Alkaline. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid , but unpolluted rain also contains other chemicals. 
H 2 O (l) + C O 2 (g) -> H 2 C O 3 (aq) Carbonic acid then can ionize in water forming low concentrations of hydronium and carbonate ions:
2 H 2 O (l) + H 2 C O 3 (aq) C O 3 2− (aq) + 2 H 3 O + (aq) Acid deposition as an environmental issue would include additional acids to H 2 C O
Air is very important to us because it is the most important material to keep us alive.Without clean air,we will have health problems like lung disease and asma.We will need to spend more on medicine and installation of air-filtering system.