Discovering the air that feeds fire
18th-century chemists believed burning released a substance called phlogiston. In Sweden, Carl Wilhelm Scheele obtained "fire air" around 1771–72 by heating mercury oxide and other compounds, but his book appeared only in 1777. In England Joseph Priestley independently made the gas in 1774 and reported that a mouse lived longer in it. Antoine Lavoisier showed it is an element that combines with substances in burning and breathing, toppled phlogiston theory and named it oxygène, "acid-maker". Who "discovered" oxygen is still an interesting debate.
Photosynthesis, respiration and oxygen's history
Almost all atmospheric oxygen was made by living things. In 1937 Robert Hill showed isolated chloroplasts release oxygen in light, and in 1941 Samuel Ruben and colleagues used heavy oxygen isotopes to show that oxygen from photosynthesis comes from water, not carbon dioxide. In our bodies haemoglobin in red blood cells carries oxygen to mitochondria, which use it to burn nutrients for energy. The early Earth's air had almost no oxygen, but cyanobacterial photosynthesis led to the Great Oxidation Event about 2.4–2.3 billion years ago, which some suggest was a crisis for life at the time. Changes in oxygen levels are studied for links with the rise of animals and mass extinctions. High in the sky oxygen forms ozone (O₃), which blocks ultraviolet light; after the Antarctic ozone hole was reported in 1985, the Montreal Protocol restricted CFCs.
Oxygen and health
In healthy people blood oxygen saturation is usually 95% or higher and can be checked with a pulse oximeter, though readings may be less accurate with nail polish, tremor or darker skin pigmentation, according to reports. Seek medical help promptly for severe breathlessness, bluish lips or persistently low readings. Medical oxygen saves lives in lung disease, but breathing higher concentrations than needed for long periods can cause oxygen toxicity such as lung injury, and people with conditions like COPD should follow their prescribed flow. Oxygen makes fires burn fiercely, so never smoke or use open flames during oxygen therapy. Health claims for "canned oxygen" or "oxygenated water" lack good evidence. This is general information, not a substitute for medical advice.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- Why do I get breathless on high mountains?
- The air is still about 21% oxygen, but lower pressure means each breath brings in less oxygen. Ascending fast can cause altitude sickness, so acclimatise gradually.
- What colour is oxygen?
- Oxygen gas is colourless and odourless, but liquid oxygen, below −183 °C, is pale blue and is attracted to magnets.
Sources reviewed: 2026-10-26
References
- Scheele, C. W. (1777). Chemische Abhandlung von der Luft und dem Feuer. Uppsala and Leipzig: M. Swederus.
- Priestley, J. (1775). An account of further discoveries in air. Philosophical Transactions of the Royal Society of London, 65, 384–394.
- Lavoisier, A.-L. (1789). Traité élémentaire de chimie. Paris: Cuchet.
- Chapman, S. (1930). A theory of upper-atmospheric ozone. Memoirs of the Royal Meteorological Society, 3(26), 103–125.
- Hill, R. (1937). Oxygen evolved by isolated chloroplasts. Nature, 139, 881–882.
- Ruben, S., Randall, M., Kamen, M., & Hyde, J. L. (1941). Heavy oxygen (O18) as a tracer in the study of photosynthesis. Journal of the American Chemical Society, 63(3), 877–879.
- Farman, J. C., Gardiner, B. G., & Shanklin, J. D. (1985). Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction. Nature, 315, 207–210.
- Holland, H. D. (2006). The oxygenation of the atmosphere and oceans. Philosophical Transactions of the Royal Society B, 361(1470), 903–915.
- Lyons, T. W., Reinhard, C. T., & Planavsky, N. J. (2014). The rise of oxygen in Earth's early ocean and atmosphere. Nature, 506, 307–315.