11-CS-3 Engineering Management · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2018 — 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 depends on both hazard (the intrinsic harmfulness of the contaminant) and exposure (how much is contacted, and for how long). Indoor air often carries higher risk than outdoor air because the exposure is greater: pollutants from indoor sources accumulate in an enclosed, poorly ventilated space (so concentrations can exceed outdoor levels), and people spend most of their time indoors—raising both the concentration contacted and the duration. Even where the hazard is similar, the elevated exposure indoors increases the risk, which is why ventilation and indoor source control are so important.
A risk of about 5 × 10⁻³ vastly exceeds the 10⁻⁶ acceptable level, so this is not safe. (The inhalation unit risk assumes standard breathing, so the 20 m³/day and 70 kg are not needed.)
6 ppb = 6×10⁻³ mg/L; intake dose when exposed = (6×10⁻³ × 2)/50 = 2.4×10⁻⁴ mg/(kg·day). Cancer risk (lifetime-averaged):
Hazard quotient (exposure-averaged dose ≈ 2.4×10⁻⁴ × 350/365 = 2.30×10⁻⁴):
The cancer risk (~1.5×10⁻⁴) is unsafe (over a hundredfold above 10⁻⁶), while the hazard quotient (~0.77) is below 1, so the non-carcinogenic exposure is acceptable. The carcinogenic endpoint governs, so the exposure is unsafe overall.
LC₅₀: the concentration of a substance (in air or water) that is lethal to 50% of a test population—a measure of acute inhalation/aquatic toxicity (analogous to LD₅₀ for dose). Environmental justice: the fair treatment and meaningful involvement of all people in environmental decisions, so no group bears a disproportionate share of environmental burdens. Dose-response curve: the graph relating the dose of a substance to the magnitude or frequency of the biological effect. NOAEL: the No-Observed-Adverse-Effect Level—the highest dose at which no adverse effect is observed. Mutagen: an agent that causes mutations (heritable changes) in DNA, potentially leading to cancer or hereditary effects.
At the source: de-energize and isolate (lock out) the overhead line, or have the utility install temporary insulating covers, so the lethal energy is simply not present while work proceeds nearby—the only certain protection. Along the path: maintain the mandated limit of approach (3 m for this voltage) by physical means—position equipment so the boom cannot reach the lines, install boom-height limiters/insulated barriers and proximity alarms, and mark exclusion zones; ground/bond the truck and keep workers out of contact with it (and its metal-mesh hose, the conductor that killed the passenger-side worker) whenever the boom is moving near the lines. At the worker: assign a dedicated signaller/spotter to watch the boom and warn the operator (absent here), train the crew on the overhead-line hazard and the limit of approach, and control the remote so its toggles cannot be triggered accidentally. Best action: source control—de-energizing the line—is best, because it removes the hazard entirely and does not depend on human vigilance; the fatality occurred precisely because a live 27,600 V line was left energized and no one was monitoring the boom's movement. Where de-energizing is impossible, a dedicated spotter and boom limit devices enforcing the 3 m limit-of-approach are the essential engineering/administrative backups.
