23-CS-3 Sustainability, Engineering and the Environment · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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.
1. Hazard identification—determining whether a substance is capable of causing harm and what health effects it produces. 2. Dose–response assessment—quantifying the relationship between the dose received and the probability or severity of the effect (yielding slope factors or reference doses). 3. Exposure assessment—estimating the magnitude, frequency, and duration of human exposure and thus the dose actually received. 4. Risk characterization—combining dose–response and exposure to estimate the overall risk to the exposed population, with its uncertainties.
Reading of the printed question (assumption declared under NOTE 1). The printed part (b) asks for “two ways to reduce the hazard of toluene vapour, and two ways to reduce his exposure to gasoline vapour.” No gasoline appears anywhere else in the scenario, which describes only spray-painting with a toluene-based paint, so “gasoline” is taken to be a slip carried over from an earlier version of the question. The answer below treats the second half as the painter’s exposure to the toluene (solvent) vapour. The distinction the examiner is testing — reducing the hazard (the intrinsic harmfulness of the agent) versus reducing the exposure (the dose the worker receives) — is unaffected by which solvent is named, and the same four controls apply to a petroleum-solvent vapour.
Reduce the hazard (following the hierarchy of controls): (1) substitute the toluene-based paint with a lower-toxicity water-based coating; and (2) reformulate or reduce the solvent content, using a product with lower vapour pressure so less toluene volatilizes. Reduce the exposure: (1) apply engineering controls—spray in a ventilated booth with local exhaust that captures the vapour at source; and (2) use personal protective equipment—a properly fitted organic-vapour respirator—together with administrative controls such as limiting spraying time. Substitution and engineering controls are preferred because they reduce the hazard itself rather than relying on the worker.
The two are assessed on fundamentally different assumptions. Carcinogens are treated as having no threshold—any dose is assumed to carry some finite probability of cancer—so risk is expressed as an incremental probability using a slope factor, and the acceptable level is a very small risk (e.g. 10⁻⁶). Non-carcinogens are assumed to have a threshold below which no adverse effect occurs, so a reference dose (safe dose) is defined, and safety is judged by whether exposure stays below it (the hazard quotient).
Units in the printed question (assumption declared under NOTE 1). The paper prints the arsenic concentration as “0.005 mg/m³”, but the exposure route is drinking water and the intake is given in litres per day, so the concentration must be a liquid-phase concentration: mg/m³ is an air-concentration unit and is a slip for mg/L. Two checks confirm this. First, 0.005 mg/L (= 5 µg/L) is half the Canadian maximum acceptable concentration for arsenic in drinking water (0.010 mg/L), i.e. exactly the kind of realistic, just-compliant value an examiner would choose; read literally, 0.005 mg/m³ would be 5 × 10−6 mg/L (5 ng/L), a two-thousandth of the Canadian limit and below the detection limit of routine drinking-water arsenic analysis — not a concentration an examiner would set for a risk calculation. Second, the literal reading makes both parts (e) and (f) return “safe” by many orders of magnitude, which destroys the contrast between threshold and non-threshold effects that parts (e) and (f) are plainly written to elicit. The solution therefore adopts C = 0.005 mg/L, and states the assumption as NOTE 1 of the paper directs. (For completeness: taken literally, 2 L/day = 0.002 m³/day would give a dose of 1.43 × 10−7 mg/(kg·day), a cancer risk of 2.1 × 10−7 and a hazard quotient of 4.8 × 10−4 — both “safe”.)
Daily intake dose (normalized to body weight):
For a carcinogen, incremental lifetime cancer risk = dose × slope factor:
This risk of about 2 in 10,000 exceeds the commonly accepted threshold of 10⁻⁶ (one in a million) by more than two orders of magnitude, so this is not a safe exposure and mitigation would be required.
For non-carcinogenic effects, the hazard quotient is the dose divided by the reference dose:
Because $HQ < 1$, the exposure is below the threshold for non-carcinogenic effects and is considered safe in that respect. Note the contrast: the same arsenic exposure is acceptable for its non-cancer (threshold) effects yet unacceptable for its cancer (non-threshold) risk—so the carcinogenic risk governs the overall judgment.
