Risk Assessment and Regulatory Toxicology
This domain tests whether you can derive a health-based guidance value and then use it correctly. Expect questions that require you to recognise the four-step paradigm, apply uncertainty factors, compute a margin of exposure or hazard quotient, and state what a value does and does not mean. Regulatory framing - enforceable versus advisory limits - is also fair game.
The four-step paradigm
Hazard identification, dose–response assessment, exposure assessment, and risk characterization. Hazard identification asks whether an agent can cause harm; risk characterization asks how likely harm is at the actual exposure.
Noncancer values
- Reference dose: RfD = NOAEL (or BMDL) ÷ composite uncertainty factors. Defaults are 10 for interspecies and 10 for intraspecies variability, with additional factors of up to 10 for subchronic-to-chronic extrapolation, database gaps, and a modifying factor.
- Benchmark dose modelling (BMDL) is preferred over NOAEL because it uses the whole curve and is not limited to the tested doses.
- Margin of exposure = point of departure ÷ exposure; an MOE of 100 or more is conventionally acceptable for noncancer endpoints when the default factors are used.
- Hazard quotient = exposure ÷ RfD; a quotient below 1 is not proof of safety, only that exposure is below the derived value.
Cancer values
Cancer slope factors come from low-dose linear extrapolation by default and yield an excess-risk estimate = slope factor × dose. The linear default reflects a conservative policy for genotoxic carcinogens, not a demonstrated mechanism at low dose.
Worked example. A NOAEL of 5 mg/kg/day with factors of 10 (interspecies), 10 (intraspecies) and 3 (database) gives RfD = 5 ÷ 300 = 0.0167 mg/kg/day. A 70 kg adult drinking 2 L/day of water containing 0.05 mg/L receives 0.1 mg/day = 0.00143 mg/kg/day. Hazard quotient = 0.00143 ÷ 0.0167 ≈ 0.086, comfortably below 1; MOE ≈ 3,500. The correct statement is that exposure is well below the guidance value and the database factor already covers some uncertainty - not that the water is proven safe for every individual.
Traps that cost marks
- Mixing units: RfD is mg/kg/day, not mg/L or mg/m3.
- Applying uncertainty factors to a cancer slope factor, or treating an excess-risk estimate as a threshold.
- Treating a hazard quotient below 1 as evidence of no risk.
- Confusing OSHA permissible exposure limits (enforceable) with ACGIH threshold limit values (advisory).