The Toxicity (TWA and STEL) data quoted in the pure gas PDF files are taken from the National Occupational Health and Safety Council (NOHSC) of Australia document :
Exposure Standards for Atmospheric Contaminants in the Occupational Environment.
National Exposure Standards
This document may be obtained from here
The determination of toxicity as expressed in terms of a compound’s LC50 (Lethal concentration for 50%). The method and data are provided in:
International Standard ISO 10298 1995(E) “Determination of toxicity of a gas or gas mixture.