Jun 01 2012

What is the Hierarchy of Controls?

Workplace safety, like many other specialty area (even yours) develops its own set of jargon and the hierarchy of controls is one of the more useful bits of safety jargon particularly for safety management systems structured around hazard management. So what is it?

A hierarchy is …?

A hierarchy is just a fancy name for a list. Usually a list of things that have something in common and usually in some sort of order – big to small, good to bad, useful to useless – you get the idea, right? So a hierarchy of controls is just a list of types of controls from really good to less useful. Oh, did I hear you ask what the blazes is a control? Even if I didn’t I’ll explain it anyway.

And Controls, what are they?

A control is some type of action that reduces the chance that a risk will happen. So, using a seat-belt when you’re driving a car reduces the chance that your head will smack into the windscreen if you’re unfortunate enough to have a ding. So a seatbelt’s a type of control but it’s not as effective as an air bag in protecting your head because you don’t have to do anything for the airbag to do its job but you actually have to remember to put your seat belt on and if you forget then it doesn’t work.  So some types of controls are better than others and the hierarchy of controls just lists the better types of risk controls at the top and the not so good ones at the bottom.

Types of Controls in the Hierarchy


If you stop and think about it you can probably figure out yourself what the best type of control is – avoiding the risk by eliminating it. If there’s no risk there’s no damage and that’s why it’s at the top of the hierarchy of controls.

So the best way of avoiding smacking your head against your windscreen is not to have an accident while driving your car and the best way of avoiding an accident is to not drive the car. Problem solved, right? Wrong.

It’s not really practical in our society to not drive a car or at least be a passenger in one. So elimination, while the best type of risk control, is not always the most practical. So what other options are there?


If you can’t stop getting around in a car what other options do you have if you don’t want your head getting up close and personal with your windscreen? You could substitute riding in the front seat to riding in the rear seat but then you couldn’t drive. That’s the balancing act involved when we’re talking about controlling risk … sometimes the alternative, while safer, is not always something we want. The other thing to think about is that substitution doesn’t get rid of the risk; it just reduces it and it sometimes introduces different risks. So rather than smacking against the windscreen you now smack into the back of the front seats. Is that better or worse? Probably a bit better but still not really something you want to go through.

Sometimes, though, substitution is a great idea. In the construction industry, for example, they substituted dry cement deliveries to either real small (5 kg) packages or really big ones (250 kg) to reduce manual handling injuries (the small ones are less likely to cause injury while people won’t try to lift the big ones without help). Often really dangerous chemicals can be replaced with safer ones that do the same job just as effectively.

So substitution reduces the risk to people without them having to do anything which it’s the second choice in the hierarchy of controls.


OK, back to the car. Elimination isn’t a real option and being a back seat driver doesn’t sound too appealing either. What if we put a barrier between your head and the windscreen? That would be great if you could see through it. But what if the barrier was only there if you were in an accident – I hope this ringing some bells because it sounds a lot like an air bag.

If you’re in an accident while driving your car the air bag pops out and cushions your noggin preventing it from smacking into the steering wheel or the windscreen. Sounds great doesn’t it? But for every silver lining there’s gonna be a cloud and in this case it’s the airbag. These things fully inflate in less than 60 milliseconds (there’s a thousand milliseconds in a second) at a speed of about 250 kph so any part of your body that’s in its way is going to get hurt. And the powder, have you ever seen the inside of a car after the airbags have gone off … it’s like a talcum powder factory.

So again, isolation isn’t a perfect solution and you have to consider if new and different risks are introduced into the process due to the method of isolation.

In the real world of work, such things as guards around rotating shafts, splash screens where liquids are churned up, cages around auto-operating equipment such as robotic assemblers and acoustic enclosures around noisy plant and equipment are examples of isolation.

When in place and working properly, isolation works great. But if the machine guard isn’t put on properly, if the door on the acoustic enclosure is closed properly they don’t work as intended. So some human intervention is necessary which is why it comes behind substitution in the hierarchy of controls.


Isolation used to be thought of as a type of engineering control but times change and so do the definitions. If we return to the car, the engineering controls would be things like the airbag not going off unless the impact speed is more than a pre-set limit. The change from windscreen’s being made of safety glass to laminated glass prevents you getting sprayed with high speed glass particles in a collision or when a stone hits your windscreen. The head restraint on the back of the seat reduces the chance of whiplash injury by restricting head’s movement. Crush zones built into the car absorb much of the impact which would otherwise be transferred directly into the bodies of people in the car.

Engineering controls are those that you can’t necessarily see but which work to reduce the chance of an injury happening when an unwanted event happens. Limit switches that prevent mechanical plant from moving beyond pre-set limits or key capture systems that switch off power to equipment when a key is removed from a lock are examples of engineering controls that are found in workplaces.

Engineering controls are subject to failure if not looked after properly. The limit switch might stick or corrode, the laminated windscreen may prevent bits of glass flying around at high-speed but if you hit one with your head you’ll bounce rather than go through it. Don’t know what’s worse really. So these types of controls aren’t as good as those that preceded them in the hierarchy of controls but they’re better than those that follow.


Now we’re getting into the really dodgy options in the hierarchy of controls. These are things like defensive driver training (or any form of training) or telling people to drive more carefully. Those ads you see on the TV telling you not to speed or not to drive when drunk or high on drugs or tired. That annoying beeper that goes off when you go a little too fast or signs on the side of the road telling you how fast you’re supposed to drive or even those cops hiding behind billboards with their speed cameras. These are all attempts to persuade you to behave differently and so avoid an accident and by avoiding an accident you prevent your head coming into contact with the windscreen.

But how often do hear or see these things and ignore them? How often do you think, “I’m not that tired” or “I’m not that drunk … I’m OK to drive”? How often do people curse the cops when they’re caught speeding? Exactly! While these things have a benefit, would you rather rely on these to protect your head or on the air bag? Me, I’ll go with the airbag because even if I’m doing everything right there’s no guarantee that some other fool (and there’s always another person to blame and they’re always a fool) is doing the same.

If you’ve been at work for any length of time you’ll have seen examples of these types of controls. Warning signs telling you the surface is hot or cold or that the tank has dangerous chemicals. Safety posters on the walls, safety training courses, policies and procedures and so the lists go on. Sure they’re useful but certainly nowhere near as good as the other options from the hierarchy of controls that we’ve talked about.

Personal Protective Equipment

Now for the last and probably the most unreliable option in the hierarchy of controls – personal protective equipment (PPE ).

Continuing with our car, PPE is the seat belt. You have to consciously use it, wrap it around you, click the end into the buckle, make sure it’s located properly and re-tension the belt. Then it’ll work and, if you do have an accident, at least your head and the windscreen aren’t going to get acquainted. But, if you forget or just decide not to use it then it does nothing and you are going to get an up close view of that gunk you’ve been meaning to clean off your windscreen – just before you bounce off it. If you have an airbag – as that’s inflating in one direction you’ll be flying into it from the other at a great rate of knots. Probably not as bad as hitting the windscreen but certainly not something you’d want to repeat too often.

At work, you’ve probably got safety glasses, safety helmet, safety boots, gloves, ear muffs or plugs, overalls and so on. All are PPE and all are only effective if used and used properly as well as being maintained properly. Safety glasses pushed to the top of your head or ear muffs strung around your neck aren’t going to do the job they’re supposed to.

That’s The Hierarchy of Controls

So that’s the Hierarchy of Controls, sorry it took so long.

Starting at the top is Elimination followed by Substitution, Isolation, Engineering, Administrative and lastly PPE. The ones at the top are there because they don’t need you to do anything for them to work. As you move down the hierarchy of controls there is a greater reliance on people doing the right thing. So only select lower order controls (those further down the list) when higher order controls (those further up the list) can’t be implemented.

As with the example of the car, often the best solution is to combine a several types of risk control from throughout the hierarchy of controls.

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