Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide and small amounts of other gases. Air also contains a variable amount of water vapour on average around 1%. In fact, air contains slightly less than 1% argon making it the third most abundant gas in air after nitrogen and oxygen. Argon is the most abundant and most used of the noble gases.

Photosynthesis is a process used by plants in which energy from sunlight is used to convert carbon dioxide and water into molecules needed for growth. These molecules include sugars, enzymes and chlorophyll. In this process, green plants and some bacteria change light energy and carbon dioxide gas into usable chemical energy in the form of a sugar called glucose. Carbon dioxide plus water yields glucose and oxygen gas.

For combustion to occur (that is a fire to start), oxygen must be present. Fire is a chemical reaction that creates light and heat from oxygen and fuel. A lit candle needs to draw oxygen from the air in order to continue burning. If you limit the amount of air available, the candle's flame eventually goes out once it uses up all the oxygen. When a candle burns, the wax is reacting with oxygen in the air to give out heat and light. If you give the candle more oxygen, then it will burn brighter but it will burn out more quickly. It all depends whether the candle has a high or low concentration of oxygen.

When you breathe in, nitrogen gas will enter the blood and circulate around the body without being used up or added to. When it reaches the lungs, it will pass out again. The percentage of nitrogen inhaled is roughly 79% but it is only 74% when exhaled. Nitrogen is a rather inert gas that does not change density depending on temperature as readily as oxygen does. Nitrogen itself is not actually used up by the body. It is simply a byproduct of the nitrates and various emissions involved in photosynthesis from plants and various algae that get pumped with whimsical abandon into the atmosphere.

Tape grass, pond weed and hydrilla are underwater plants which can take carbon dioxide from the water around them and give out oxygen. Their leaves have no pores or stomata. So, the oxygen dissolved in the air is taken in through their body surface.

Nitrogen is an essential macronutrient needed by all plants to thrive. In plants much of the nitrogen is used in chlorophyll molecules which are essential for photosynthesis and further growth. It is needed for the production of amino acids which make up proteins. Nitrogen often comes from fertilizer application and from the air. Plants cannot use nitrogen in the gaseous form so it therefore needs to be turned into either nitrate or ammonium before it can be utilised by the plant. This can be done through nitrogen fixation.

The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere it results in an elevation of the average surface temperature above what it would be in the absence of the gases. Plants reduce the greenhouse effect by absorbing carbon dioxide rather than excess heat radiation from the air while human activities like manufacturing increase the greenhouse effect by adding various greenhouse gases.

Dolphins breathe through a hole at their top which is called blowhole. The blowhole is the external opening (like a nasal passage) to the lungs. Through its blowhole the dolphin inhales air containing vital oxygen and exhales respiratory wastes including carbon dioxide.

When inhaled air pollutants affect the lungs and respiratory tract first but can also be taken up and transported by the bloodstream throughout the body. As respiratory tract is the major entry point of air pollutants, many respiratory diseases including lung cancer are often linked to air pollution. The respiratory system is particularly sensitive to air pollutants because it is made up of a mucous membrane covering its internal surface. The lungs are designed to absorb large amounts of air in close contact with the bloodstream and facilitate the transport of oxygen. The cells of the lung tissue can be damaged by air pollutants such as ozone, metals and free radicals.

The thermal power plants generate significantly large quantities of solid byproduct namely fly ash. Fly ash also known as flue-ash is one of the residues generated in combustion and comprises the fine particles that rise with the flue gases. The fine particles of fly ash reach the pulmonary region of the lungs and remain there for long periods of time where they behave like cumulative poisons.

Global warming is the rise in the average temperature of earth's atmosphere and oceans. Warming of the climate system is primarily caused by increasing concentrations of greenhouse gases produced by human activities such as the burning of fossil fuels and deforestation. The heat caused by infrared radiation is absorbed by greenhouse gases such as water vapour, carbon dioxide, ozone and methane which slow its escape from the atmosphere. Although greenhouse gases make up only about 1 percent of the earth's atmosphere, they regulate our climate by trapping heat and holding it in a kind of warm air blanket that surrounds the planet. This phenomenon is what scientists call the greenhouse effect. Without it, scientists estimate that the average temperature on earth would be colder by approximately 30 degrees, far too cold to sustain our current ecosystem.

Starfish breathe by absorbing oxygen from special finger-like structures that are found on their body surface. They contain fuzzy patches which perform the same function of gills. Each one of those patches can contain dozens of papulae which are extended when the animal is comfortable and feels secure. Each of these permits the animal to collect oxygen for the starfish to breathe. They are generally found only on the top and side surfaces, generally not on the underside.

A frog's webbed feet help it move through the water. The frog's webbed feet are an adaptation that helps a frog swim in the water by pushing the water away and helping the frog propel itself faster through the water. People sometimes wear flippers in the water when swimming for the same reason.

Compressed natural gas (CNG) is a fossil fuel substitute for gasoline (petrol), diesel fuel and propane/LPG. Although CNG's combustion does produce greenhouse gases, it is widely considered a more environmentally clean alternative to conventional fuels. The pollution loads have been calculated and compared by considering usage of the compressed natural gas (CNG) in place of conventional diesel buses. One study reveals that the use of CNG shows tremendous reduction in various pollutants in gm/km. By the use of CNG there is 84% reduction in CO, 58% reduction in oxides of nitrogen and 97% reduction in particulate matter.

Carbon and oxygen can combine to form two gases. When combustion of carbon is complete in the presence of plenty of air the product is mainly carbon dioxide (CO₂). When combustion of carbon is incomplete in the presence of limited supply of air, only half as much oxygen is added to the carbon and the product is carbon monoxide (CO). Carbon monoxide is also formed as a pollutant when hydrocarbon fuels (natural gas, petrol and diesel) are burned. So, it has no relation with the presence of carbon dioxide which is found to be very essential for the existence of life on earth.