If I remember correctly oil in oxygen lines should be avoided due to the possibility of hazardous combustion. However I've seen recently that engineers use oil to lubricate pneumatic air tools. Can someone explain why this is allowed??

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    $\begingroup$ The oxygen lines are in the airplane. The air tools use shop air. From the shop compressor. They are not related. And nobody uses air tools around open oxygen lines anyway. $\endgroup$
    – John K
    Sep 8, 2019 at 15:50
  • $\begingroup$ I edited the title to transform it into a question (this is a Q&A website) and to make it more specific. I had to assume your question was about airplanes. feel free to edit it again if it is not the case. $\endgroup$
    – Manu H
    Sep 8, 2019 at 17:56
  • $\begingroup$ Perhaps better suited to the chemistry site, but air is only ~20% oxygen. Diluting it with nitrogen makes combustion much more difficult. $\endgroup$
    – jamesqf
    Sep 8, 2019 at 18:02

2 Answers 2


This demonstrates the difference between oxygen and air.

Shop air is compressed air. It's about 20% oxygen. Compressed oxygen used in aviation is pure oxygen supplement, not compressed air, to save weight and volume of the gas you have to carry.

For breathing, the important thing is the partial pressure of oxygen, not the absolute oxygen content or the proportion of other gases.


shop air lines do not carry pure oxygen, they carry compressed air. any oil mist mixed into the compressed air will be far less flammable than oil mist mixed with oxygen.


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