[paul cotter (OP)]

This question involves both chemistry and physiology-I am not quite sure where to place it. Nitric oxide is a common signalling molecule in the body, produced enzymatically from arginine, one of it's functions being vasodilation. Quite some time ago I heard of a treatment for sick children that  involved inhaled nitric oxide: this puzzled me as nitric oxide is converted to the noxious nitrogen dioxide on contact with the oxygen in air. About ten years ago I was working in a children's hospital and I noticed gas cylinders containing nitric oxide, •1% in nitrogen: obviously this mixture would rapidly lead to hypoxia if used as the sole inhalational gas and before administering such a gas there would be residual oxygen in the lungs. PS when I was young and foolish( possibly still so ) I was often exposed to clouds of nitrogen dioxide from dissolving various metals in nitric acid and the occasional runaway of a nitration reaction-I guess my lungs never heard of aseptic pneumonia.

[vhfpmr]

That reminds me of the day when someone dropped a penny into a dish of nitric acid, and the teacher whisked it away to the fume cupboard saying it causes 'septic pneumonia'. We thought he was making it up to frighten us. 

[Bored chemist]

The reaction of nitric oxide an oxygen is just about slow enough for most of what you inhale to have not reacted yet.
I'm also fairly sure that the rate of reaction depends on the square of the NO concentration, so, as you dilute it, the reaction becomes disproportionately slower.
It's complicated.
https://www.researchgate.net/publication/351126888_Mechanism_of_the_oxidation_of_nitric_oxide_with_oxygen

[paul cotter (OP)]

BC, as I have said before I am not a chemist, so ill equipped to argue the kinetics of the reaction. I was going on my rather old copy of "Cotton&Wilkinson" where it is stated the reaction is instantaneous. I did come across a reference somewhere( I can't remember where ) to rates of conversion at different concentrations but it was a long time ago and I had not heard of nitric oxide therapy at the time.

[Bored chemist]

I was going on my rather old copy of "Cotton&Wilkinson"

I'm willing to bet that my 1st  edition is not later than yours. :-)
Science moves on.
To be fair, if you do the reaction on a macroscopic scale with "visible" amounts of gas, it looks instant.

[paul cotter (OP)]

BC, it's the third edition. I must see what Mellor has to say, though I am missing some volumes and from memory( very degraded ) I suspect the missing ones are the halogens and the pnictogens.

[paul cotter (OP)]

Much as feared I am missing Mellor #2 and #8.

[ezrahidaya]

I would like to know that too