23-CS-3 Sustainability, Engineering and the Environment · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2015 — 11-CS-3 Sustainability, Engineering and the Environment. Closed book; approved calculator permitted. Any four questions constitute a complete paper; all questions are of equal value (25 marks each).
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.
Risk = likelihood × consequence. Living downwind of a coal plant: routine emissions make the likelihood of exposure/harm high while the consequence per exposure is low–medium (chronic health effects)—a continuous, diffuse risk. Living just downstream of a large hydro dam: the likelihood of a dam failure is low, but the consequence of a failure (catastrophic flooding) would be high—a rare, catastrophic risk. Thus coal is a high-likelihood/low-consequence chronic risk that causes more routine harm, while hydro is a low-likelihood/high-consequence acute risk.
At the source: substitute a less-hazardous, low-fume cleaner and/or eliminate the need for confined-space entry (e.g. clean the tank with an external automated spray system), removing the toxic-atmosphere hazard itself. Along the path: force-ventilate the tank and continuously monitor the atmosphere—purge and supply fresh air and test for toxic vapour/oxygen before and during entry so the hazardous atmosphere never reaches the worker. At the worker: require proper PPE and a confined-space entry procedure—supplied-air respirator, gloves, coveralls, and a harness/lifeline with a trained attendant stationed outside and a rescue plan. Best action: the source control—eliminating or substituting the hazard (and avoiding entry)—is best, because it removes the danger for everyone regardless of behaviour, whereas the tragedy here was compounded precisely because control relied on the workers, who entered without PPE and without a rescue system. (A rigorous confined-space program is the essential backstop, since a would-be rescuer becoming the second victim is a classic confined-space failure.) In Ontario these controls are codified in the Occupational Health and Safety Act and the Confined Spaces regulation (O. Reg. 632/05), which requires a written program, atmospheric testing, ventilation or purging, an entry permit, an attendant, and a rescue plan with rescuers who are trained and equipped — every one of which was missing here.
The statement expresses the non-threshold assumption for carcinogens: because a single molecular event could in principle initiate a cancer-causing mutation, it is assumed there is no completely safe dose—any exposure carries some finite probability of cancer, so risk is modelled as linear down to zero. A threshold dose for non-carcinogens is the dose below which no adverse effect occurs, because the body can tolerate, detoxify, or repair damage up to that point; only above the threshold do effects appear. This is why non-carcinogens are managed by keeping exposure below a reference (threshold) dose, while carcinogens are managed by minimizing exposure to keep risk acceptably small.
Concentration 0.5 µg/L = 5×10⁻⁴ mg/L; body weight 50 kg. Intake dose when exposed:
The cancer risk of about 6 × 10⁻⁶ exceeds the 10⁻⁶ acceptable threshold, so this is not a safe exposure on the cancer criterion (though it is only a few times above the benchmark). In a Canadian frame the verdict is borderline rather than alarming: Health Canada treats lifetime cancer risks in the 10⁻⁵ to 10⁻⁶ range as “essentially negligible,” and 0.5 µg/L is one-twentieth of the Canadian maximum acceptable concentration for arsenic (10 µg/L). On the strict 10⁻⁶ benchmark the question implies, however, the exposure does not qualify as safe.
Since $HQ \ll 1$, the exposure is well below the reference dose and is safe for non-carcinogenic (nerve-damage) effects. Strictly, the non-cancer average daily dose averaged over the exposure period carries EF/365 = 350/365, giving HQ = 0.064 instead of 0.067; either way HQ ≪ 1 and the verdict is unchanged. As usual for arsenic, the carcinogenic risk governs the overall judgment.
