Environmental Health Sciences and Exposure Assessment
Environmental Health Sciences and Exposure Assessment
This domain tests the scientific engine behind every environmental health decision: how a contaminant leaves a source, moves through air, water, soil or food, reaches a receptor, and what dose the receptor actually receives. Questions usually pair a calculation with a judgement about which pathway dominates or which default assumption is defensible.
Source-to-receptor logic
- Chain: source → release → transport and fate → exposure point → route → dose → effect.
- Fate and transport are governed by advection, dispersion, partitioning (K_ow for lipids and biota, K_oc for soil and sediment), volatilisation (Henry's law constant), degradation half-life, and accumulation (BCF/BAF, biomagnification).
- Routes are inhalation, ingestion (drinking water, soil, food, dust) and dermal. Aggregate exposure sums across media; cumulative risk sums across chemicals sharing a mechanism.
- The pathway contributing most in a model is not automatically the true driver - confirm with measurements or biomonitoring.
Dose metrics
Potential (applied) dose → internal dose → target-tissue dose → biologically effective dose. The chronic intake equation is:
ADD (mg/kg-day) = (C × IR × EF × ED) / (BW × AT)
C is concentration in the medium, IR intake rate, EF exposure frequency (days/year), ED exposure duration (years), BW body weight (kg), AT averaging time (days). For carcinogens AT = 70 years × 365 days; for non-carcinogens AT = ED × 365.
Characterising exposure
Point estimates (deterministic) versus probabilistic (Monte Carlo) analysis. Reasonable maximum exposure pairs upper-percentile intake rates with a central or upper-bound concentration and must be stated explicitly. Variability is real heterogeneity in a population; uncertainty is ignorance about a value - regulators require them reported separately.
Worked example
A 6-year-old (15 kg) ingests 200 mg soil/day, 350 days/year, in soil containing 500 mg/kg lead. Convert: 200 mg/day = 0.0002 kg/day. ADD = (500 × 0.0002 × 350 × 6) / (15 × 2190) = 210 / 32,850 = 0.0064 mg/kg-day. Reasoning: the numerator totals mass ingested over the exposure period; the denominator spreads it over body weight and averaging time. For lead this value is screening only - EPA uses the IEUBK model or the Adult Lead Methodology because lead has no threshold and the timing of exposure matters.
Traps that cost marks
- Using ED × 365 as the averaging time for a carcinogen.
- Unit slips: ppm to mg/m³ uses mg/m³ = ppm × MW / 24.45 at 25 °C, 1 atm.
- Labelling variability as uncertainty, or vice versa.
- Quoting a single number as "the" exposure without a percentile or scenario.
- Forgetting dermal and non-dietary ingestion when they are plausible.