Having had a play with one of my black masks today, I thought of the various masks seen at dental surgeries over the years.
The first one was the standard black mask used on me in the 1950's followed by a black nasal inhaler with its whistling exit valve and twin feeder pipes wrapped around the head in the early 1960's.
I went to this practice for several years and noticed on one visit that their old McKesson machine had been replaced by a new modern machine which had both a green mask & nasal inhaler attached to it but never had the need to 'sample' it. It was usually covered up but one day it was standing near the door in full uncovered view! Had it just been used? Who had been gassed? I shall never know.
There was a second surgery downstairs in the basement used by a part-time lady dentist and one day when the door was open I noticed an old BOC Walton machine with a full black mask standing behind the chair.
My current dentist does not have a gas machine of course so nothing to look out for now.