Air composition

Atmospheric air is formed by various gases, water vapor, microorganisms and impurities (dust and soot).

The gases in the atmosphere include oxygen, noble gases (helium, neon, argon, krypton, radon, xenon), nitrogen, and carbon dioxide. The following table shows the amount (percentage) of each gas in the atmosphere, nitrogen being the most abundant.





Noble Gases






The amount of water vapor, microorganisms and impurities depends on some factors such as weather, pollution and winds. So these are variable components of atmospheric air.

Constant Air Components


It is the most abundant gas in the air (78%). Its chemical formula is N2, that is, a nitrogen molecule is formed by two nitrogen atoms.

Animals and plants do not take nitrogen from the air, but there are some living things that can make use of it and turn it into nitrogenous salts such as nitrates. These living things are the bacteria that live in leguminous plant roots (beans, soybeans and peas).

The cycle begins with nitrogen gas entering the soil. Bacteria absorb it into nitrates that are given away in parts to plants. These plants use nitrates to produce proteins, which are part of the plant body.

Herbivorous animals eat these plants by acquiring proteins for themselves. Carnivorous animals eat herbivores by transferring their proteins to them. When an animal dies it is decomposed by bacteria and fungi, which return to the soil and later absorbed by another plant. And so, starting the nitrogen cycle again.


About 21% of the air in the atmosphere is oxygen gas. Our body cannot stay long without breathing. We need atmospheric air because it contains oxygen, which is responsible for breathing. Oxygen acts in the "burning" of food, producing energy necessary for the functioning of our organs, so they can stay active. It also serves as oxidising gas, which fuels combustion (burning).

When a living being uses oxygen gas for breathing, we call it aerobic beings (plants and animals). When they don't use oxygen gas to breathe or burn their food, we call them anaerobic beings (some bacteria).

O O2 It can, however, cause harm to man. When it comes in contact with iron (Fe) it causes the so-called rust, which destroys cars, gate machines, ships and so on.

4Fe +3 O2 → 2 Fe2O3

Carbon dioxide

This gas, with chemical formula CO2, is essential for plant life in photosynthesis, which produces glucose and energy. Glucose is stored in starch form and can be used in cellulose production. To perform photosynthesis is necessary:

- chlorophyll (green light-absorbing substance);
- Sun light;
- Water;
- carbon dioxide

When sunlight is present (absorbed by chlorophyll), the carbon dioxide from the air and soil water taken from the roots is carried to the leaves and becomes glucose and oxygen gas. Glucose is used as a source of energy or to make another substance and oxygen is released into the environment.

Noble gases

They are hardly combined with other substances, so they are noble. They are helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn). They are isolated and used by man:

- in flashes, cameras (Xe);
- on illuminated signs (Ne, Kr);
- to fill balloons (He);
- in apparatus used for cancer treatment (Rn);
- inside light bulbs (Air).

Helium gas is very light. Neon gas is also called neon gas. It produces red and orange light.

The krypton produces a bluish-green light.

Variable Air Components

Water vapor

Atmospheric water vapor comes from the evaporation of water from seas, rivers and lakes; breathing of living beings; plant perspiration; soil water evaporation and manure water evaporation (animal feces and urine).

This moisture (water vapor) is important for living things because it helps in cloud formation. In some places where there is low humidity, many people have difficulty breathing. This is the case of the Midwest region of Brazil. In such cases it is recommended to place water containers near the bed. This is so that water vapor moistens the airway mucosa (nose, pharynx).


It is formed by various solid particles that deposit on furniture, household items, roads, roofs, etc. In the atmosphere, it is possible to see the dust.


The biggest smokers with soot are the factories that have no filters in their chimneys. Soot, which is dark in color, is formed by substances such as lead (Pb). Causes serious damage to respiratory system.

Smoke from cars, buses and trucks contains sulfur dioxide (SO2), carbon monoxide (CO), nitrogen dioxide (NO2) and hydrocarbons.


They are in large quantities in the atmosphere. Many are responsible for diseases such as tetanus, tuberculosis and flu.

Some do not cause disease and aid in the decomposition of dead organisms and the manufacture of antibiotics. Others, such as the lactic bacillus, develop in milk, producing the curd.