Oxygen is a highly soluble element that is critical to biological functions, notably respiration. It comprises about 16 to 21 percent of Earth's atmosphere, allowing it to be inhaled and converted to carbon dioxide to power the cells of aerobic organisms. Most animals and plants on Earth require oxygen to survive. Generally, plants convert carbon dioxide to oxygen, and animals convert oxygen to carbon dioxide, during respiration.
As an element, oxygen builds the critical components of the biosphere: water, soil and air. According to the Royal Society of Chemistry, oxygen accounts for "two-thirds of the human body and nine-tenths of water." Almost half of Earth's crust is made of oxygen compounds such as silicon. It is critical for the process of combustion and is extremely flammable, making oxygen as potentially destructive as it is necessary for life.
The way that oxygen reacts with almost all elements has inspired a variety of medical and industrial applications. Oxygen is often given to medical patients to ease distressed respiration processes. The steel industry utilizes oxygen to transform iron into steel, as well as to improve the welding capability of materials. In recent years, oxygen has gained application in the sewage treatment and industrial waste disposal.