Air is the invisible mixture of gases that surrounds Earth. Air contains important substances, such as oxygen and nitrogen, that most species need to survive. Human beings (Homo sapiens), of course, are one of those species. Sometimes, the word "atmosphere" is used instead of the word "air."

Standard Dry Air is the composition of gases that make up air at sea level. It is a standard scientific unit of measurement. Standard Dry Air is made up of nitrogen, oxygen, argon, carbon dioxide, neon, helium, krypton, hydrogen, and xenon. It does not include water vapor because the amount of vapor changes based on humidity and temperature. Because air masses are constantly moving, Standard Dry Air is not accurate everywhere at once.

Nitrogen and oxygen make up about 99 percent of Earth’s air. People and other animals need oxygen to live. Carbon dioxide, a gas that plants depend on, makes up less than 0.04 percent.

Plants and animals each produce the gases that the other needs to live. Plants need carbon dioxide—people and other animals exhale carbon dioxide as a waste product. People and other animals need oxygen—plants produce oxygen during an important process called photosynthesis, which turns the sun’s energy into nutrients.

Water vapor in the air is sometimes visible as clouds. Water enters the atmosphere through the water cycle. The water cycle also brings molecules in the air into oceans, lakes, and rivers.

Some gases in the air come from volcanic eruptions. Volcanic eruptions eject gases from the interior of Earth. The most common gas emitted by volcanoes is water vapor. Other gases, such as carbon monoxide and sulfur dioxide, are toxic to most organisms. A few organisms, however, thrive on these gases. At the bottom of the ocean are bacteria that do not need oxygen or sunlight to survive. In other words, they do not need air. These strange organisms create their own nutrients using hydrogen sulfide, not carbon dioxide. The hydrogen sulfide comes from cracks, or vents, in Earth’s crust.

The air is different as you move higher and higher into the atmosphere. The air gets "thinner" as elevation climbs because there are fewer air molecules up there. Mountain climbers often have to use canisters of oxygen as they climb above 3,800 meters (12,500 feet) because there is not enough oxygen in the atmosphere for most people to breathe. High mountains such as Mount Everest (8,848 meters, or 29,035 feet), in Nepal and China, are littered with empty oxygen canisters that climbers discard when they are used up.

High in the stratosphere, a layer of Earth’s atmosphere, is a special air molecule called ozone. Ozone is made up of three atoms of oxygen. The massive collection of these molecules is called the ozone layer. The ozone layer blocks harmful ultraviolet, or UV, rays so the sun’s powerful radiation does less damage to living things on Earth.

Unfortunately, air pollution has a negative effect on the air we breathe. Air pollution happens when harmful byproducts, like exhaust from cars, enter the air. These pollutants can clog the atmosphere with smog, a combination of smoke and fog. They can also create toxic clouds of dust. Other air pollutants, such as methane and excess amounts of carbon dioxide, can upset the balance of molecules in the air, contributing to global warming.