23-CS-3 Sustainability, Engineering and the Environment · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2014 — 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 is evaluated as the product of consequence and likelihood (risk = probability of an event × its severity/consequence). For a coal-fired plant, the routine emissions (particulates, SOₓ, NOₓ, mercury) cause chronic health harm continuously, so the likelihood of exposure/harm is high while the consequence of any single exposure is low–medium (incremental respiratory/cardiovascular effects across the population). For a nuclear plant, normal operation releases very little, so the likelihood of a harmful release is low, but the consequence of a major accident (radiation release) would be high. Thus the two present opposite risk profiles: coal is a high-likelihood, lower-consequence chronic risk that harms health routinely, while nuclear is a low-likelihood, high-consequence catastrophic risk—coal actually causing more routine public-health harm, while nuclear's risk is dominated by the small chance of a severe event.
Limit/eliminate the hazard (act on the source): (1) relocate or re-mount the motor away from the operator's body—move it to the side or rear of the machine rather than directly beneath the worktable; and (2) shield the motor with appropriate electromagnetic shielding, or substitute a lower-emission motor design. Reduce the exposure (act on the worker): (1) increase the distance between the worker and the motor (field strength falls steeply with distance)—for example by extending the machine frame or adding a barrier that keeps the abdomen farther from the motor; and (2) administrative controls—reassign pregnant workers to machines or duties away from the motor, or limit time at that station during pregnancy. Source control (relocation/shielding) is preferred because it protects all workers regardless of behaviour.
For carcinogens the dose–response is assumed non-threshold and linear—risk is proportional to dose all the way down to zero, so any dose carries some probability of cancer (quantified by a slope factor). For non-carcinogens the dose–response has a threshold—no adverse effect occurs below a certain dose (the reference dose), and effects appear only above it.
Assume a 70 kg adult. Concentration 0.005 µg/L = 5×10⁻⁶ mg/L. Intake dose when exposed:
Lifetime-averaged dose (350 d/yr × 30 yr, averaged over 70 yr):
The incremental cancer risk of about 9 × 10⁻⁸ is well below the 10⁻⁶ acceptable threshold, so this exposure is safe on the carcinogenic criterion (the arsenic concentration of 0.005 µg/L is extremely low—far below the 10 µg/L drinking-water standard).
Since $HQ \ll 1$, the exposure is far below the reference dose and is safe for non-carcinogenic (nerve-damage) effects as well. (Strictly, the non-cancer average daily dose also carries the exposure frequency averaged over the exposure period, 350/365, giving $HQ \approx 4.6\times10^{-4}$; the verdict is unchanged.) Both endpoints are acceptable here because the arsenic concentration is so low.
