23-CS-3 Sustainability, Engineering and the Environment · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 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 = likelihood × consequence. A coal plant emits pollutants continuously, so the likelihood of exposure is high while the consequence per exposure is low–medium (chronic, population-wide health effects). A nuclear plant releases little in normal operation, so the likelihood of a harmful release is low, but the consequence of a major accident would be high. Coal thus presents a high-likelihood, lower-consequence chronic risk that causes more routine harm, while nuclear presents a low-likelihood, high-consequence catastrophic risk.
Limit/eliminate the hazard (act on the source): (1) replace the oil-lubricated compressor with an oil-free (or properly maintained) breathing-air compressor so CO is not generated by oil breakdown at high temperature; and (2) relocate the air intake to clean air well away from the diesel engine exhaust and any other combustion source, preventing intake of contaminated air. Reduce the exposure (protect the worker): (1) install a CO monitor/alarm and a high-efficiency CO-removal filter on the breathing-air line, with regular air-quality testing against the CSA 5 ppm limit; and (2) never work alone—provide a trained attendant/buddy and maintain the equipment to standard so a failure is detected and the worker rescued. Source control (oil-free compressor, clean intake) is decisive, since PPE fed with contaminated air offers no protection—indeed it delivered the lethal dose here.
This statement expresses the non-threshold assumption used for carcinogens: because cancer can in principle be initiated by a single molecular event that damages DNA, it is assumed there is no safe threshold—any exposure, however small, carries some finite (if tiny) probability of causing cancer. Risk is therefore modelled as linear with dose down to zero, which is why carcinogen risk is expressed as a probability (via the slope factor) and judged against a very small acceptable level rather than a "safe dose."
Concentration 0.5 µg/L = 5×10⁻⁴ mg/L. Intake dose when exposed:
The incremental cancer risk of about 9 × 10⁻⁶ exceeds the commonly accepted 10⁻⁶ (one-in-a-million) threshold by nearly an order of magnitude, so this is not a safe exposure on the cancer criterion; treatment to lower the arsenic would be warranted. For Canadian context: 0.5 µg/L is one-twentieth of Health Canada's 10 µg/L maximum acceptable concentration for arsenic, and Health Canada generally treats lifetime risks in the 10⁻⁵ to 10⁻⁶ range as “essentially negligible,” so on that looser benchmark this water would sit just inside the acceptable band. The verdict above follows the 10⁻⁶ criterion the question intends.
The paper says “the woman exposed as described in part d.”; part (d) describes a 70 kg man, so the same 70 kg body mass and 2.0 L/day intake are used here.
Here the dose on an exposure day is compared with the RfD, which is slightly conservative; averaging it over the exposure period (non-cancer doses are averaged over the exposure duration, not a lifetime) scales it by 350/365, giving $HQ = (1.43\times10^{-5})(350/365)/(3.0\times10^{-4}) \approx 0.046$—the same conclusion. Since $HQ \ll 1$, the exposure is well below the reference dose and is safe for non-carcinogenic (nerve-damage) effects. As is characteristic of arsenic, the carcinogenic risk is the governing concern here even though the non-carcinogenic hazard is acceptable.
