When every substance passes and the mixture does not WEL

Every reading under its own limit, and the mixture over

Exposure limits are published one substance at a time, and people check them one substance at a time. For almost every real job that is the wrong arithmetic.

Three solvents each at forty per cent of their own limit are not three passes. Where substances act on the same organ or by the same mechanism the effect is additive, and the test is the sum of the fractions:

C₁/L₁ + C₂/L₂ + C₃/L₃ + …  must not exceed 1

Forty per cent three times over is 1.2. The mixture is over the limit while every single reading on the sheet is comfortably under, and nobody working substance by substance will ever see it. That is the whole reason this page exists: not to do a division anybody could do, but to do the addition nobody does.

Deciding what goes in which group

The additive rule applies to substances acting on the same target, so the grouping is a judgement and it is yours. Solvents that are all central nervous system depressants go together. Substances that irritate the same part of the respiratory tract go together. A respiratory irritant and a liver toxin do not, and adding them would be wrong in the other direction.

Get the grouping from the safety data sheets and from the toxicology rather than from the fact that the substances happen to be in the same room. Where you are unsure, grouping them is the cautious error and it is the one to make.

Keep the groups, the readings and the reasoning together. The question asked later is not what the sum was, it is why those substances were in the same group, and that is not reconstructable from a number.

The limits themselves

Use the current list for your jurisdiction, and use it in the same averaging period as your reading: an eight-hour time-weighted average against the eight-hour limit, a short-term reading against the fifteen-minute limit. Comparing a fifteen-minute peak against an eight-hour limit is the other common error, and it goes the reassuring way.

Limits change. A substance reclassified downwards takes every historical assessment of it with it, so the date of the list you used is part of the record.

And remember what a limit is. It is not a line with safety on one side: it is a level below which most people are not expected to be harmed, with known exceptions for sensitised individuals, and the duty is to reduce exposure as far as reasonably practicable rather than to reach the limit. A sum of 0.9 is not a pass, it is a number that should prompt the question of what would bring it down.

Everything stays in this browser. Nearby: NOISE and VIBRATION handle the physical exposures the same way, HEAT covers thermal strain, and RISK records the assessment and what was known at the time.