Question 1 of 6: Acceptable Risk/ALARP and Benzene Exposure Compliance

Question 1: Acceptable Risk/ALARP and Benzene Exposure 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) Acceptable risk and ALARP

Part (a) — Acceptable risk is the level of residual risk that a society, regulator, or organisation is willing to tolerate in exchange for the benefits an activity provides, given that reducing risk to absolute zero is neither technically achievable nor economically rational. It is not a single universal number: acceptable risk is set by regulation, industry norm, or informed stakeholder judgement, and it is always weighed against the cost, difficulty, and benefit of further risk reduction.

ALARP ("As Low As Reasonably Practicable") is the risk-acceptance criterion most commonly used to operationalise "acceptable risk" in practice. It divides the risk spectrum into three bands: an unacceptable region at the top (risk is intolerable regardless of cost — the activity must be modified or stopped); a broadly acceptable region at the bottom (risk is already so low that further reduction is not warranted); and, in between, a tolerable-if-ALARP band, where risk must be reduced further unless the person responsible can demonstrate that the cost, time, or effort of any additional reduction is grossly disproportionate to the safety benefit gained. ALARP therefore requires an explicit, documented cost–benefit judgement for every risk that falls in the middle band, not a one-time pass/fail check.

(b) Benzene exposure — TWA, STEL, and Ceiling compliance

Part (b) —

Given. A six-interval exposure schedule totalling 9 hours (table above); TLV-TWA = 225 ppm; TLV-STEL = 350 ppm; TLV-C (ceiling) = 400 ppm.

Find. Whether the workplace is safe, checking all three ACGIH exposure-limit criteria independently.

Approach. Compute the 8-hour time-weighted average per OSHA's standard formula (which normalises to a nominal 8-hour reference shift regardless of the actual monitored duration), then separately check the worst 30-minute readings against the STEL (a 15-minute-scale limit that must never be exceeded, applied here to the two half-hour intervals since each spans at least one 15-minute window) and against the instantaneous ceiling TLV-C.

1. Time-weighted dose. Sum concentration × duration over all six intervals: $$\sum C_i T_i = (300)(1)+(200)(2)+(100)(4)+(300)(1)+(450)(0.5)+(400)(0.5)=1825\ \text{ppm}\cdot\text{h}$$
2. 8-hour TWA. Per OSHA 29 CFR 1910.1000's TWA formula, the dose is normalised to a nominal 8-hour reference shift regardless of the actual total monitored time (9 h here): $$TWA=\frac{\sum C_iT_i}{8}=\frac{1825}{8}=\boxed{228.1\ \text{ppm}}$$ Since $228.1>225$, the TWA already exceeds the TLV-TWA.
3. STEL check. Both half-hour intervals (450 ppm and 400 ppm) are held long enough to contain a full 15-minute STEL-averaging window at that concentration, and both exceed the 350 ppm STEL: $$450\ \text{ppm} > 350\ \text{ppm},\qquad 400\ \text{ppm} > 350\ \text{ppm}$$
4. Ceiling check. The single highest reading, $\boxed{450\ \text{ppm}}$, also exceeds the instantaneous ceiling TLV-C of 400 ppm — a value the worker should never be exposed to at all, even momentarily.

The workplace is not safe — and fails on all three independent criteria, not just one. Even the TWA alone (228.1 vs. 225 ppm) is a narrow but real exceedance; the STEL and ceiling violations are the more serious findings, since they flag two half-hour periods with acute exposure well above levels the worker should never experience, benzene being a known human carcinogen where the ceiling/STEL protections exist specifically to prevent short, high-concentration excursions that a shift-average alone would mask.