ISO/TS 16976-4:2019

This document describes how to calculate the work performed by a person's respiratory muscles with and without the external respiratory impediments that are imposed by RPD of all kinds, except diving equipment. This Document describes how much additional impediment people can tolerate and contains values that can be used to judge the acceptability of an RPD.
NOTE       These calculations are explained in some textbooks on respiratory physiology (in the absence of an RPD), but most omit them or are incomplete in their explanations.