Question 1 of 6: Accident Causation and Exposure-Limit Compliance

Question 1: Accident Causation and Exposure-Limit Compliance (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) The accident process and its three main steps

Part (a) — An accident process is the causal chain of events that begins with a root cause embedded in the organisation or the individual and ends in an unwanted loss (injury, damage, or a near-miss). Heinrich's domino model (still the clearest teaching device for this) resolves the chain into a short sequence of dominoes; for exam purposes this collapses to three main steps:

1. Step 1 — Root cause / unsafe condition or act. A pre-existing deficiency (poor training, a missing guard, time pressure, complacency) produces an unsafe act by a person or an unsafe physical condition in the workplace. This step creates the potential for loss but has not yet released any energy or hazardous material.
2. Step 2 — Triggering event (the accident itself). The unsafe act/condition combines with an immediate trigger and an uncontrolled release of energy, material, or motion occurs — the instant of loss of control.
3. Step 3 — Consequence / loss. The released energy contacts a person or asset and produces the injury, illness, or property damage (or, if no contact occurs, a recorded near-miss).

Example. A forklift operator is under schedule pressure and has not been re-certified (Step 1, root cause: inadequate training/supervision, an unsafe condition). The operator takes a blind corner too fast in a congested aisle (Step 2, triggering event: loss of control of the vehicle). The forklift strikes a pedestrian stepping out from a cross-aisle (Step 3, consequence: injury). Removing any single domino — recertifying the operator, installing a corner mirror/pedestrian barrier, or simply routing pedestrian traffic away from the aisle — would have interrupted the sequence before the loss occurred.

(b) TLV-TWA / STEL compliance check

Part (b) —

Given. Ten spot concentration readings across the 8:10 AM–5:00 PM monitoring window (table above); TLV-TWA = 100 ppm; STEL = 200 ppm.

Find. Whether the worker's exposure exceeded the TLV-TWA and/or the STEL.

Approach. Treat each reading as representative of the interval since the previous sample (a standard industrial-hygiene time-weighting convention for periodic spot samples), compute the exposure-weighted average over the monitored period, and separately compare the single highest reading against the STEL (an independent 15-minute-scale ceiling, not captured by the TWA).

1. Weight each reading by its time interval. For reading $i$ taken $\Delta t_i$ minutes after the previous one: $$C_i\,\Delta t_i$$ e.g. the 9:05 AM reading (150 ppm) is held for the 55 minutes since 8:10 AM, contributing $150\times55=8250$ ppm·min; the 2:03 PM reading (250 ppm) is held for the 43 minutes since 1:20 PM, contributing $250\times43=10750$ ppm·min. Summing all nine intervals: $$\sum C_i\,\Delta t_i = 92885\ \text{ppm}\cdot\text{min}$$
2. Divide by the total monitored time. The window spans 8:10 AM to 5:00 PM $=530$ minutes ($\approx8.83$ h): $$TWA=\frac{\sum C_i\,\Delta t_i}{\sum \Delta t_i}=\frac{92885}{530}=\boxed{175.3\ \text{ppm}}$$
3. Compare to the TLV-TWA. $175.3\ \text{ppm} > 100\ \text{ppm}$ — the time-weighted average exceeds the TLV-TWA by roughly 75%.
4. Check the STEL independently. The STEL is a 15-minute ceiling, not a period average, so it is checked against the single worst reading rather than the TWA: the 2:03 PM reading of $\boxed{250\ \text{ppm}}$ exceeds the 200 ppm STEL on its own.

Both checks fail independently, so the worker's exposure is non-compliant on two separate grounds: the shift-average dose is too high, and there was at least one short-term excursion well above the ceiling. Either finding alone would already require corrective action (engineering controls or respiratory protection) before the area is re-entered.