23-CS-3 Sustainability, Engineering and the Environment · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 11-CS-3 Sustainability, Engineering and the Environment. Open book; non-communicating 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. A coal plant emits continuously, so the likelihood of exposure is high and the consequence per exposure low–medium (chronic health effects)—a high-likelihood, low-consequence chronic risk. A nuclear plant releases little normally, so the likelihood of a harmful release is low but the consequence of a major accident would be high—a low-likelihood, high-consequence catastrophic risk. Coal causes more routine harm; nuclear's risk is dominated by rare, severe events.
Reduce the hazard (act on the source, per the hierarchy of controls): (1) substitute or reformulate to reduce the most volatile/carcinogenic components (e.g. lower-benzene fuel); (2) install vapour-recovery systems on pumps and nozzles so vapour is captured rather than released; and (3) reduce vapour generation through sealed systems and lower-vapour-pressure handling. Reduce the exposure (protect the worker): (1) engineering controls—canopy/local exhaust ventilation and vapour-recovery nozzles that keep vapour out of the breathing zone; (2) administrative controls—rotate staff and limit time at the pump to cut exposure duration, and enforce no-smoking and safe procedures; and (3) PPE—gloves and, where warranted, respiratory protection. Hazard reduction is preferred because it lowers the risk at source rather than relying on the worker; since gasoline contains non-threshold carcinogens (benzene), exposure should be minimized as far as reasonably achievable (ALARA), not merely kept under a limit.
Convert 170 ppb to µg/m³ (µg/m³ = ppb × MW ÷ 24.45 at 25 °C):
A risk of about 4 × 10⁻³ (nearly 4,000 in a million) vastly exceeds the 10⁻⁶ acceptable level, so this is not a safe exposure. (The inhalation unit risk is defined for a 70 kg adult breathing 20 m³/day, so it is applied directly to the air concentration. If we scale for this woman's lower body weight, her dose per kilogram is 70/50 = 1.4 times higher, giving a risk of about 5.2×10⁻³. The conclusion is the same either way: the exposure is not safe.) In the Canadian frame, Health Canada regards an incremental lifetime cancer risk of 10⁻⁵ to 10⁻⁶ as essentially negligible; 3.7×10⁻³ exceeds even the 10⁻⁴ upper bound sometimes tolerated by a factor of about 37, so exposure must be reduced.
9 ppb = 9×10⁻³ mg/L; dose = (9×10⁻³ × 2)/70 = 2.57×10⁻⁴ mg/(kg·day):
Since $HQ < 1$ (though close to it), the exposure is below the reference dose and is considered safe for non-carcinogenic effects—but with little margin, so it warrants monitoring. In the Canadian frame, 9 µg/L sits just below Health Canada's maximum acceptable concentration of 10 µg/L for arsenic in drinking water. Arsenic is also a known human carcinogen, and the HQ addresses only non-cancer effects, so the water should still be kept as low in arsenic as reasonably achievable.
