# Cryogenic air separation

WHO’s Foundations of medical oxygen systems (17 February 2023) describes cryogenic fractional distillation in air separation units (ASU), which separate nitrogen and oxygen, and sometimes argon and other rare inert gases, from atmospheric air and generate liquid oxygen; Ph. Eur. monograph 0417 covers oxygen produced this way, at a minimum of 99.5%.

The same WHO document lists cryogenic fractional distillation, which generates liquid oxygen (LOX), alongside pressure swing adsorption, which generates a gas oxygen product. ASU plants for production and bulk LOX storage are situated off site and typically managed by private companies, and they produce LOX in large quantities, 300 to 5,500 tonnes a day. Liquid oxygen has a boiling point of −183 °C, and its production, handling and storage need special technologies to keep it insulated from environmental heat. ASU production serves both industry and medicine, the medical sector having more stringent GMP and good distribution practice requirements.

EDQM records that when Ph. Eur. 0417 was revised in 1997, cryogenic distillation of ambient air was the most common method for this grade, and the minimum nominal content was raised to 99.5% V/V; since issue 12.3 the monograph’s definition states that it covers oxygen produced via cryogenic distillation. The USP Oxygen monograph sets not less than 99.0%. The ULTRACON concentrator in ULTRAOX plants is, in its datasheet’s words, a low pressure non-cryogenic solution.

## Sources

- [WHO, Foundations of medical oxygen systems (17 February 2023), sections 1.6 and 4.1](https://iris.who.int/server/api/core/bitstreams/4ad6f5fd-089e-4142-8439-b64e98d816d8/content)
- [EDQM, Ph. Eur. monograph 3098 Oxygen (98 per cent): history (background on monograph 0417)](https://extranet.edqm.eu/4DLink1/Web_mono_hist?3098)
- [EDQM, Ph. Eur. monograph 0417 Oxygen: history](https://extranet.edqm.eu/4DLink1/Web_mono_hist?417)
- [USP monograph Oxygen](https://doi.usp.org/USPNF/USPNF_M59550_02_01.html)
- [ULTRACON datasheet, p. 2](https://www.ultracontrolo.com/products/ultracon)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/glossary/cryogenic-air-separation/