A colourless, odourless gas that makes up 21% of the atmosphere, which is half-way between 17% (below which breathing for unacclimatised people becomes difficult) and 25% (above which many organic compounds are highly flammable). The element is essential for all forms of life since it is a constituent of DNA, and it comprises over half the mass of the average person. Indeed it is the most common element in the Earth’s crust, in the form of silicon-oxygen minerals, and in the sea since it accounts for 89% of the mass of water. Oxygen gas is fairly soluble in water, which makes life in rivers, lakes and oceans possible. We breathe in oxygen so that it can react with energy stores in our bodies to provide the heat and motion that keeps us alive, and we return it to the atmosphere as carbon dioxide. Industrially, oxygen is produced on a large scale from liquid air, and used for steel making, metal cutting (oxy-acetylene torches) and by the chemicals industry.