Whether oxygen levels at 19.5% are safe for work in confined space?

Many people consider that the oxygen levels at or above 19.5% are safe to work in a confined space. However, I read somewhere that before assuming oxygen level of 19.5% is safe, one should find out why the levels are below that in atmosphere. If the balance 1.5% is of some toxic chemical, it might be fatal also. CO is fatal at 5000 ppm (0.5%). Similarly ammonia. However, if the chemical present has some odour, we can immediately detect and come out.

Otherwise, measuring oxygen levels alone is not sufficient and we have to analyse for presence of other chemicals and purge the confined space with fresh air till oxygen levels are near 21%, say 20.8-20.9%. And fresh air supply should not be stopped during the work. Each and every time when work is stopped for rest or tea break or lunch or end of the day, before resuming the work the levels are to be checked at the place of the work.

I came to know of an accident in which three workers were involved in cleaning the oily floor at a basement were overcome by the toxic vapours present when they resumed work after a tea break. First two persons one after another entered inside the basement and became unconscious. The third one too was overcome but before that he shouted for help and other coworkers present at 0.0 m level immediately rescued them and were alright after a day's treatment.

The unfortunate observation during the rescue opertions was workers lack of knowledge about the switch of fresh air supply blower for the basement. Even the shopfloor supervisor does not know about its location. Awareness about location of emergency systems is the most important aspect and a sketch showing the same should be displayed at important locations in the shopfloor and mock drills should be conducted before start of work in a confined space, every time.