AS published by the Safe Work Australia.
TWA – The time-weighted average airborne concentration over an eight-hour working day, for a five-day working week over an entire working life.
STEL (Short Term Exposure Limit) – the average airborne concentration over a 15-minute period, which should not be exceeded at any time during a normal eight-hour workday.
These Exposure Standards are guides to be used in the control of occupational health hazards. All atmospheric contamination should be kept too as low a level as is workable. These exposure standards should not be used as fine dividing lines between safe and dangerous concentrations of chemical. They are not a measure of relative toxicity.
If the directions for use on the product label are followed, exposure of individuals using the product should not exceed the above standard. The standard was created for workers who are routinely, potentially exposed during product manufacture.