Question 2 of 6: OSHA Incidence Rates, Inherent Safety, and Risk vs. Hazard

Question 2: OSHA Incidence Rates, Inherent Safety, and Risk vs. Hazard (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) OSHA incidence rates — injuries and lost workdays

Part (a) —

Given. 1,200 full-time employees; 38 lost-time injuries in the year; 274 lost workdays resulting from those injuries.

Find. The OSHA incidence rate based on (i) injuries and (ii) lost workdays.

Approach. OSHA's standard incidence-rate formula normalises the count of a chosen event to a base of 100 full-time-equivalent workers, using the statistical convention that 100 FTE work 200,000 hours per year (100 workers × 2,000 h/yr each):

1. Total hours worked. $$H=1200\times2000=\boxed{2{,}400{,}000\ \text{hours}}$$
2. Injury incidence rate. $$IR_{inj}=\frac{N_{inj}\times200{,}000}{H}=\frac{38\times200{,}000}{2{,}400{,}000}=\boxed{3.17\ \text{injuries per 100 FTE-yr}}$$
3. Lost-workday rate. Applying the same normalisation to the lost-day count (not the injury count) gives the average number of workdays lost per 100 employees per year: $$IR_{LWD}=\frac{N_{LWD}\times200{,}000}{H}=\frac{274\times200{,}000}{2{,}400{,}000}=\boxed{22.83\ \text{lost workdays per 100 FTE-yr}}$$

Both rates are reported per 100 full-time-equivalent workers so that facilities of different sizes can be benchmarked against industry averages on a common basis; the lost-workday rate is the more severity-sensitive of the two because a single serious injury can contribute dozens of lost days while the injury count itself only increments by one.

(b) Inherent safety

Part (b) — The inherent safety concept (Kletz) seeks to eliminate or reduce a hazard by the choice of process, material, or design itself, rather than adding layers of protective equipment and procedures to control a hazard that remains present. It is organised around four strategies: minimise (use less of a hazardous material — smaller inventory), substitute (replace a hazardous material or reaction route with a less hazardous one), moderate (use a hazardous material in a less hazardous form or condition — lower pressure, lower concentration, refrigerated instead of pressurised storage), and simplify (design out unnecessary complexity that creates opportunities for operator error or equipment failure).

Practical application. A mineral-processing facility that stores a large pressurised inventory of a hazardous reagent on site (as in Question 3(b) of this paper) can apply the minimise strategy by moving to smaller, more frequent just-in-time deliveries instead of a single large high-pressure storage vessel — this directly shrinks the worst-case release scenario (a smaller available inventory bounds the maximum credible mass flow and evacuation distance) rather than relying solely on administrative controls (permits, PPE) around the existing large inventory.

(c) Risk vs. Hazard

Part (c) — A hazard is an intrinsic property of a substance, process, or situation that has the potential to cause harm — it exists independently of whether anyone is actually exposed to it. Risk is the combination of the likelihood that a harmful event occurs and the severity of its consequences, i.e. risk is a function of both the hazard and the exposure/probability of contact: $$Risk = f(\text{Hazard},\ \text{Probability of exposure},\ \text{Consequence severity})$$

Example. Concentrated sulphuric acid is a hazard everywhere it exists — its corrosive/toxic potential does not change with location. The risk it poses, however, depends entirely on exposure: sealed in a labelled drum inside a locked, bunded storage room it presents very low risk (near-zero probability of contact), while the same acid decanted into an open beaker on an unattended bench presents high risk (a realistic probability of splash contact with severe consequences), even though the hazard itself is identical in both cases.