TWA

8-hour time weighted average (TWA) exposure limits: For practical reasons, OELs are generally established in relation to a reference period of a typical 8-hour working day, and they are generally set on the basis of a nominal 40-hour working week and for a working lifetime of 40 years (48 weeks/year; 5 days/week i.e. 9600 days or 76,800 hours). 

SCOEL
Methodology for derivation of occupational exposure limits of chemical agents  The General Decision-Making Framework of the Scientific Committee on Occupational Exposure Limits (SCOEL) 2017