Health Risk Assessment and Management
Core concepts
The classic four-step risk assessment is hazard identification, dose–response assessment, exposure assessment, and risk characterization. Noncancer effects use a reference dose/concentration (RfD/RfC) and a hazard quotient: HQ = exposure ÷ RfD. HQs above 1 are a screening flag, not proof of harm. Cancer effects use a slope factor or unit risk: risk = exposure × unit risk. Margin of exposure (MOE) = point of departure ÷ exposure; larger is better, and there is no universal "safe" MOE.
Risk characterization should separate variability (real differences among people) from uncertainty (lack of knowledge). Reasonable maximum exposure (RME) typically pairs a high-end exposure estimate with central-tendency toxicity values, not worst case on both.
Traps
- Using an RfD to estimate cancer risk, or a slope factor for noncancer effects.
- Summing HQs across agents that affect different target organs (sum only same-endpoint HQs; cancer risks may be summed).
- Treating an RfC as a bright-line threshold rather than a benchmark with uncertainty factors built in.
- Calling an MOE "acceptable" by rule of thumb without context.
- Selecting the average exposure when an RME was specified.
Worked example
Air concentration is 0.02 mg/m³, unit risk = 1×10⁻⁶ per µg/m³. Convert: 0.02 mg/m³ = 20 µg/m³. Excess lifetime cancer risk = 20 × 1×10⁻⁶ = 2×10⁻⁵, i.e., about 2 in 100,000.
Rule of thumb
Typical acceptable-risk ranges cited in management decisions run from about 1×10⁻⁶ to 1×10⁻⁴; use ALARP (as low as reasonably practicable) when risk falls in the gray zone, and document the trade-offs.