In this module
Objectives covered: LO 2.5.1, LO 2.5.2. By the end you will be able to:
- Rate likelihood and severity using a 5 × 5 risk matrix.
- Decide whether a risk is acceptable, tolerable or unacceptable.
- Explain ALARP and residual risk.
5.1 Two questions
For each hazard, ask two questions:
- Likelihood: how likely is it that harm will happen?
- Severity: if it does happen, how bad is the most likely serious outcome?
Risk = Likelihood × Severity. Many companies use a 5 × 5 matrix, giving a score from 1 to 25.
| Score | Likelihood | Severity |
|---|---|---|
| 1 | Rare: almost never | Minor: first aid only |
| 2 | Unlikely: could happen sometime | Medical treatment |
| 3 | Possible: has happened in the industry | Lost-time injury |
| 4 | Likely: has happened here | Permanent disability |
| 5 | Almost certain: happens often | Fatality or multiple fatalities |
| Risk score | Level | Action |
|---|---|---|
| 1 to 4 | Low | Acceptable. Manage with normal procedures. |
| 5 to 9 | Medium | Reduce where reasonably practicable. Supervisor approves. |
| 10 to 16 | High | Add controls before work. Manager approves. |
| 20 to 25 | Extreme | Do not start or continue the work until the risk is reduced. |
5.2 Worked example
Workers carry 50 kg cement bags up a ladder to a first-floor slab. Falls with a load have happened on this site before (likelihood 4) and could cause permanent injury (severity 4). Risk = 4 × 4 = 16, High.
Controls: use a hoist or material lift (engineering), use 25 kg bags (substitution), and keep the ladder for people only. New likelihood 2, severity 4: residual risk = 8, Medium, and acceptable with supervision.
5.3 ALARP and residual risk
Residual risk is the risk that remains after controls are in place. It is never zero. ALARP means "As Low As Reasonably Practicable": keep reducing the risk until the cost, time and effort of further reduction is grossly out of proportion to the benefit. A cheap control that prevents a fatality is always reasonably practicable.
Rate the risk twice: before controls (initial risk) and after controls (residual risk). If the residual risk is still high, the controls are not enough.
- ISO 31000:2018 and IEC 31010:2019: risk management and risk assessment techniques, including the consequence and likelihood matrix.
- ISO 45001 clause 6.1.2.2: assessment of OH&S risks.
- The ALARP principle comes from UK law and is widely used in oil and gas worldwide.
In June 2022 a fire and explosion at the BM Container Depot in Sitakunda killed around 50 people, including firefighters. Containers of hydrogen peroxide were stored without proper hazard classification or separation. A simple risk assessment would have rated the likelihood of fire as possible and the severity as multiple fatalities, an extreme risk that demanded segregation, labelling and an emergency plan before storage.
Learning Check
Answer these two questions to check your understanding. You can try again as many times as you like.