Step 1 — the inventory
The calculation starts by counting every dangerous substance present or anticipated at the establishment. The inventory is not only raw materials:
- intermediates and by-products,
- product and finished stock,
- wastes — frequently forgotten,
- substances that could be generated during an accident,
- maintenance and laboratory stocks.
The quantity taken is the maximum that may be present at one time — not the average.
Step 2 — placing the substance
There are two routes, and they work together:
- Named substance: if it appears in the list by name, its own threshold is used.
- Hazard category: if it does not, it is placed in a category by its classification and that category's threshold applies.
A named substance's threshold replaces its category threshold; the two are not counted twice.
Step 3 — comparing with thresholds
Every threshold has two values: lower tier and upper tier. Even where no single substance exceeds a threshold, substances under the same hazard heading are assessed together by the summation rule — a calculation without it is incomplete.
Three common errors
- Leaving out waste.
- Recording today's stock instead of the maximum capacity.
- Skipping the summation rule and concluding “no substance exceeds a threshold”.
All three place the establishment outside scope on paper; at an inspection the missing notification is then added to the finding.