23-CS-3 Sustainability, Engineering and the Environment · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2019 — 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.
This expresses the non-threshold assumption for carcinogens: because a single molecular event could in principle damage DNA and initiate a cancer, it is assumed there is no completely safe threshold—any exposure, however small, carries some finite (if tiny) probability of causing cancer. Risk is therefore modelled as increasing linearly with dose down to zero, which is why carcinogen risk is expressed as a probability (via a slope factor or unit risk) rather than judged against a "safe dose."
Rearranging the given ppb = C·R·T/(MW·P) at 25°C and 1 atm (R·T/P = 24.45 L/mol) gives C = ppb × MW ÷ 24.45: $C = 70 \times 131.4/24.45 \approx 376\ \text{µg/m}^3$. Cancer risk = concentration × inhalation unit risk:
A risk of about 1.5 × 10⁻³ greatly exceeds the acceptable level (10⁻⁴ per this paper; Health Canada treats 10⁻⁵ to 10⁻⁶ as “essentially negligible”), so this is not a safe exposure by any criterion. (The inhalation unit risk already assumes standard breathing, so the 20 m³/day and body weight are not needed.)
Average man: BW = 70 kg; residential exposure IR = 2 L/day, EF = 350 d/yr, ED = 30 yr; C = 7 µg/L = 7×10⁻³ mg/L. Cancer risk (AT = 70-yr lifetime = 25,550 d):
This slightly exceeds the acceptable 10⁻⁴, so the cancer risk is unsafe (marginally). Hazard quotient (AT = exposure period = 30 yr = 10,950 d):
Since $HQ < 1$, the non-carcinogenic exposure is acceptable. So the cancer risk is (marginally) unsafe while the non-cancer hazard is safe—the carcinogenic endpoint governs, so the exposure is unsafe overall. Against the Canadian frame the verdict is firmer still: 1.2 × 10⁻⁴ is about 12 times the upper end of Health Canada’s 10⁻⁵ to 10⁻⁶ “essentially negligible” range. Note that 7 µg/L is below Canada’s arsenic maximum acceptable concentration (MAC) of 10 µg/L; that MAC is set at what treatment can reliably achieve, not at a negligible-risk level, so meeting it does not make the cancer risk negligible, and Health Canada asks that arsenic be kept as low as reasonably achievable.
At the source: fit vapour-recovery nozzles and systems on the pumps (and lower-benzene fuel) so the carcinogenic vapour is captured rather than released—removing the hazard at its origin. Along the path: provide ventilation and canopy/open-air dispersion and keep the attendant's breathing zone away from the fill point, so any vapour is diluted before it reaches him. At the worker: limit exposure time (rotate duties), train on the benzene hazard, and provide respiratory protection where warranted. Best action: the source control—vapour recovery (and reduced-benzene fuel)—is best, because it removes the carcinogen at its origin and protects everyone without depending on worker behaviour. Because benzene is a non-threshold carcinogen, exposure should be minimized as far as reasonably achievable (ALARA), not merely kept under a limit, which makes eliminating the vapour at source especially important.
