When Every Substance Passes and the Mixture Does Not WEL
Three solvents each at forty per cent of their own limit are not three passes. The sum of the fractions is 1.2, and nothing checked one at a time will ever show it.
The shift
SubstanceMeasuredUnitHoursMinutesLimit
The limit is the long-term one from the current list for your jurisdiction, in the same unit as the reading. “Acts with” is a label of your choosing: put substances that act on the same organ in the same group and they are added together.
What the shift comes to
The additive test
The working
Questions people ask about WEL