Risk Assessment and Toxicology (Dichloromethane): 23-CS-3 Sustainability, Engineering and the Environment, December 2013, Q5

Question 5: Risk Assessment and Toxicology (Dichloromethane) (25 marks)

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.

(a) The Four Steps in Risk Assessment

1. Hazard identification—determining whether a substance can cause harm and what effects it produces. 2. Dose–response assessment—quantifying how the probability or severity of effect varies with dose (yielding slope factors or reference doses). 3. Exposure assessment—estimating the magnitude, frequency, and duration of exposure and thus the dose received. 4. Risk characterization—combining dose–response and exposure to estimate the overall risk, with its uncertainties.

(b) Reducing Hazard and Exposure to Gasoline Vapour

Reduce the hazard (act on the source, per the hierarchy of controls): (1) substitute with a lower-volatility or less-toxic product where possible; (2) use vapour-recovery systems on pumps and storage tanks so vapours are captured rather than released; and (3) reduce volatility (e.g. fuel formulation, cooler storage) to lessen vapour generation. Reduce the exposure (act on the worker): (1) engineering controls—local exhaust ventilation and enclosed dispensing to remove vapour from the breathing zone; (2) administrative controls—job rotation and limiting time at the pump to cut exposure duration; and (3) personal protective equipment—an organic-vapour respirator and gloves. Hazard reduction is preferred because it lowers the risk at source rather than relying on the worker.

(c) Carcinogens versus Non-Carcinogens

Carcinogens are assessed as non-threshold—any dose is assumed to carry some finite cancer probability—so risk is a probability computed from a slope factor, judged against a very small acceptable level (e.g. 10⁻⁶). Non-carcinogens are assessed as having a threshold below which no adverse effect occurs, so a reference dose is set and safety is judged by whether exposure stays below it (the hazard quotient).

(d) Daily Dose of Dichloromethane

Concentration 40 µg/m³ = 0.040 mg/m³; inhalation rate 20 m³/day; body weight 70 kg:

\text{Dose} = \frac{C \times IR}{BW} = \frac{(0.040\ \text{mg/m}^3)(20\ \text{m}^3/\text{day})}{70\ \text{kg}} = \frac{0.80\ \text{mg/day}}{70\ \text{kg}} \approx \boxed{0.0114\ \text{mg/(kg}\cdot\text{day)}}

(e) Cancer Risk (Lifetime-Averaged)

For a carcinogen the dose is averaged over a 70-year lifetime, prorated by the exposure frequency and duration:

\text{LADD} = 0.0114 \times \frac{350\ \text{d/yr} \times 30\ \text{yr}}{365\ \text{d/yr} \times 70\ \text{yr}} = 0.0114 \times \frac{10{,}500}{25{,}550} \approx 4.70\times10^{-3}\ \text{mg/(kg}\cdot\text{day)}

\text{Risk} = \text{LADD} \times SF = (4.70\times10^{-3})(8.5\times10^{-5}) \approx \boxed{4.0\times10^{-7}}

The incremental lifetime cancer risk is about 4 × 10⁻⁷, which is below the commonly accepted threshold of 10⁻⁶ (one in a million), so on the carcinogenic criterion this exposure is acceptable.

(f) Hazard Quotient (Non-Carcinogenic)

For the threshold (liver) effect, the hazard quotient uses the chronic daily dose during exposure:

HQ = \frac{\text{Dose}}{RfD} = \frac{0.0114}{6.0\times10^{-3}} \approx \boxed{1.9}

Because $HQ > 1$, the exposure exceeds the reference dose and is not safe for non-carcinogenic (liver-damage) effects. (Strictly, a RAGS-style chronic dose that also prorates the 350 d/yr exposure frequency gives $HQ = 1.9 \times 350/365 \approx 1.83$—still above 1, so the verdict is unchanged.) Note the important contrast: the same exposure is acceptable on cancer risk (4 × 10⁻⁷) yet unacceptable on non-cancer effects (HQ ≈ 1.9)—so the non-carcinogenic hazard governs the overall judgment here, and controls would be required.

Question 5 of 5: Risk Assessment and Toxicology (Dichloromethane)