23-CS-3 Sustainability, Engineering and the Environment · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2017 — 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 the product of two factors: hazard (the intrinsic toxicity of the agent, i.e. how much harm a given dose can do) and exposure (how much of the agent actually reaches a person: concentration × contact time).
Why both are higher indoors. Hazard: indoor air contains a wider range of more toxic pollutants than outdoor air. These include combustion products (CO, NO₂ and particulates from gas stoves, furnaces and attached garages), VOCs and formaldehyde released from furnishings, paints and cleaning products, environmental tobacco smoke, mould and allergens, and radon seeping from soil into basements, which is a significant issue in many parts of Canada. Exposure: these pollutants are released into a small, enclosed volume with limited air exchange, especially in tightly sealed, energy-efficient buildings, so indoor concentrations are often several times outdoor levels. Canadians also spend about 90% of their time indoors, so the contact time is much longer.
Effect on risk. Because risk is the product of the two, raising both hazard and exposure multiplies the risk. For many people the dominant inhaled risk therefore comes from indoor rather than outdoor air, even though outdoor pollution gets more public attention. Risk is reduced by attacking each factor: source control (substituting low-emitting products, venting combustion appliances, radon mitigation) lowers the hazard present, and ventilation (heat-recovery ventilators, range-hood exhaust) lowers exposure.
At the source: eliminate the CO source—use electric power-washers instead of gasoline-powered ones, so no carbon monoxide is produced in the enclosed garage. Along the path: ensure adequate ventilation and monitoring—do not work when the exhaust fans have failed; provide forced mechanical ventilation and continuous CO monitoring/alarms so rising CO is detected and work stops before it becomes dangerous. At the worker: train the crew on the CO hazard and safe procedures (a key failure here—the workers had no CO training), and provide CO personal alarms and, if needed, supplied-air respiratory protection. Best action: the source control—switching to electric equipment—is best, because it removes the hazard entirely and does not depend on ventilation working or workers reacting; the tragedy occurred precisely because a CO-emitting engine was run in an enclosed space and work continued after the ventilation failed. (Critically, the crew should never have kept working once the exhaust fans stopped.)
Carcinogens are assessed as non-threshold (any dose carries some cancer probability, quantified by a slope factor or unit risk), judged against a very small acceptable lifetime risk (e.g. 10⁻⁶; Health Canada treats 10⁻⁵ to 10⁻⁶ as essentially negligible). Non-carcinogens are assessed as having a threshold (a reference dose below which no effect occurs), judged by the hazard quotient (dose ÷ RfD).
A lower LD₅₀ means a smaller dose is lethal, i.e. greater toxicity. Table salt (LD₅₀ = 4,000 mg/kg) has a lower LD₅₀ than ethanol (10,000 mg/kg), so table salt is more toxic than ethanol on this measure (it takes less to kill per kg of body weight). A lethal dose of salt is 2.5 times smaller than one of ethanol (10,000 ÷ 4,000). Toxicity describes the dose needed for harm, not how often people are harmed: we are rarely poisoned by salt only because we never ingest anything near 4 g/kg.
Convert 370 ppb to a mass concentration (at 25 °C, µg/m³ = ppb × MW ÷ 24.45):
Cancer risk = concentration × inhalation unit risk:
A risk of about 8 × 10⁻³ (roughly 8,000 in a million, or about 8,000 times the 10⁻⁶ criterion) is enormously higher than the acceptable level. It is also far above the 10⁻⁴ upper bound that regulators sometimes tolerate, and far above the 10⁻⁵ to 10⁻⁶ range that Health Canada regards as essentially negligible. This is therefore emphatically not a safe exposure: the trichloroethylene concentration is far too high. The inhalation unit risk already assumes a standard adult (70 kg, 20 m³/day). The breathing rate given here matches that assumption, but the lower body weight gives a larger dose per kilogram. Scaling the risk by 70/50 gives about 1.14 × 10⁻², which only reinforces the conclusion.
7 ppb = 7×10⁻³ mg/L; dose = (7×10⁻³ × 2)/70 = 2.0×10⁻⁴ mg/(kg·day):
Since $HQ < 1$, the exposure is below the reference dose and is safe for non-carcinogenic effects. The 7 µg/L concentration is also below Health Canada's maximum acceptable concentration for arsenic in drinking water (10 µg/L). Note, however, that arsenic is also a human carcinogen, so an HQ below 1 settles only the non-cancer question; a complete assessment would add a cancer-risk estimate (dose × oral slope factor), which at this dose would typically exceed 10⁻⁶.
