Institute of Work Health and Safety / Risk

Hierarchy of control, and why substitution is skipped

Everybody can recite it. Most implementations start two thirds down.

The ordering is well known: eliminate the hazard, substitute something less hazardous, engineer it out, separate people from it administratively, and only then rely on what a person wears. It is taught everywhere and it is sound. What is less discussed is that in practice a great many controls land in the bottom two categories, and the reason is not ignorance.

The upper levels cost money and require decisions that belong to somebody other than the safety function. Eliminating a hazard usually means changing the process or the design, which is a capital decision. Substitution means changing a material or a supplier. Engineering means buying something. Administrative controls and protective equipment can be implemented by the person who identified the risk, this week, within their own authority.

So the hierarchy is often applied not as a ranking of effectiveness but as a ranking of what can be done without asking. That is an organisational problem rather than a technical one, and it is not solved by more training on the hierarchy. It is solved by the risk conversation having access to the people who can authorise the upper levels, early enough that the answer is not already fixed.

The tell is a control register in which almost everything is a procedure or an item of equipment. That pattern does not mean the assessments were done badly. It usually means they were done by people without a budget.

The pattern also has a diagnostic use for anybody reviewing another organisation's arrangements. A control register weighted toward the lower levels tells you something specific about how decisions are made there: that the risk conversation happens without access to capital, that it happens after design decisions are fixed, or that it happens at a level too junior to commit anything. Those are three different problems with three different remedies, and none of them is addressed by asking for better assessments.