Which noun is used to describe the study of the efficiency of people in their working environment?

Ergonomics, possibly? Oops - just seen I should have posted a reference. Well, the free online dictionary (among others) seem to agree with me if you Google 'definition ergonomics'. HTH

Ergonomics is more to do with the equipment people use in a working enviromment (or even in the car). (Times English Dictionary) To me the question calls more for a name for the work done by those people who follow you around with a clipboard making arcane notes. Or perhaps Taylorism https://en.wikipedia.org/wiki/Scientific_management which may need to be cut-and-pasted if that _ causes problems.
For ergonomics, see https://en.wikipedia.org/wiki/Ergonomics which quotes a definition tallying with my reference above to the TED.
There's probably another fancy name for Time and Motion Study which Monkey or ZB will find.

Ergonomics is the study of the efficiency of people in their working environment.

https://ios-inc.com/ergonomics-designing-healthy-work-environments-for-your-employees/

http://www.championseating.com/cgi-bin/champion.cgi/ergonomics.html

It is both people and environment.

http://dictionary.reference.com/browse/ergonomics

According to the Thesaurus there are no synonyms for this word.

"Human Factors Engineering" "Anthropometrics"