11.1 Exposure Routes, Scenarios & Pathways (Occupational, Dietary, Environmental, Consumer)
Key Takeaways
- A conceptual site model traces source to transport medium to exposure point to route to receptor; an incomplete pathway contributes no dose no matter how high the source concentration.
- Occupational, residential/general-public, consumer-product, and dietary scenarios use different calendars and contact rates; swapping a worker 8-hour shift for a child soil-ingestion rate is an exposure-assessment error.
- Incidental soil and dust ingestion, dermal loading, vapor inhalation, and particle inhalation are different routes; coarse particles that deposit in the throat are largely swallowed and become a secondary oral dose.
- Aggregate exposure is one chemical across all operating routes and pathways; cumulative risk is several chemicals that share a common mechanism of toxicity.
- Handbook III.B.2 C requires the measured species to match the toxicity value: parent versus metabolite, and unbound versus total circulating concentration.
Exposure is the missing half of risk
Handbook III.B is exposure assessment: who contacts the agent, by which route, over what duration and frequency, and at what concentration at the point of contact. Hazard identification asked whether the agent can cause a defined adverse effect. Dose–response will ask how much is too much. Neither question yields a risk number until exposure is quantified. Exposure assessment is 8% of the DABT examination inside Domain III (38% overall). Independent OpenExamPrep teaching in this section covers conceptual site models (CSMs), the four common exposure scenarios (occupational, dietary, environmental/general public, and consumer product), the physical difference between vapor and particle inhalation, a preview of aggregate versus cumulative combination, and handbook III.B.2 C distinctions between parent and metabolite and between unbound and total internal measures. It is not an ABT product and does not claim official approval, review, or partnership with ABT or EPA.
Conceptual site model: source, pathway, receptor
A CSM is a narrative plus a diagram of how a chemical moves from where it is released to the people or ecological receptors who can take it up. The classic triplet is source → pathway → receptor. Superfund-style practice expands that triplet:
- Source — origin of the mass (leaking underground storage tank, stack, pesticide application, consumer spray can, tailings pile).
- Release mechanism — leak, volatilization, runoff, overspray, abrasion of a painted surface, tracking of soil on shoes.
- Transport medium — soil, groundwater, surface water, outdoor air, indoor air, dust, the food web.
- Exposure point — where contact occurs (kitchen tap, bedroom air, play soil, cafeteria fish, warehouse breathing zone).
- Exposure route — ingestion, dermal contact, or inhalation (rarely injection or ocular).
- Receptor — resident child, indoor worker, subsistence fisher, aquatic invertebrate, nesting bird.
A completed CSM is a hypothesis about complete pathways. An incomplete pathway—contaminated groundwater with no well and no vapor intrusion into occupied indoor air—does not contribute human dose, however ugly the source looks. Drawing the CSM before running a fate model is how you avoid quantifying a path that does not exist, and how you catch a path a laboratory package never measured (indoor air from a shallow plume under a slab).
Route versus pathway. A route is the portal into the body (oral, dermal, inhalation). A pathway is the environmental story that delivers chemical to that portal (vapor intrusion → indoor inhalation; garden soil → incidental ingestion). One route can be fed by several pathways. Listing “inhalation” without naming outdoor vapor, indoor vapor intrusion, and fugitive dust is incomplete.
The diagram in this section is a teaching CSM for gasoline that has leaked from a tank into soil and a shallow aquifer. Soil supports incidental ingestion and dermal contact, fugitive-dust inhalation, and soil-vapor inhalation outdoors. Groundwater supports a potable-well ingestion path and vapor intrusion into indoor air. Receptors sit at the end of every complete arrow. An arrow that stops in the aquifer with no well and no occupied slab is not a completed human path.
Four scenario classes
Occupational (workplace). The receptor is an adult worker. Time-use is not 24 hours a day, 365 days a year. Teaching reasonable-maximum workplace calendars in EPA RAGS-style Superfund work are often 250 days/year and 25 years at a single facility, with an 8-hour shift. Industrial-hygiene practice may instead use a working lifetime (for example 40–45 years) and a measured 8-hour time-weighted average. The chemical may be a vapor, a mist, a dust, or a dermal film under gloves. Clothing and occlusion change dermal dose. Children, the elderly, and 24-hour residential occupancy are out of scope unless the facility actually houses people.
Environmental / general public. Residential, recreational, and bystander scenarios. RAGS-style residential teaching values were 350 days/year (two weeks away from home) and 30 years at one residence, with 24-hour occupancy for air. Children have different soil-ingestion and body-weight defaults than adults. Recreational swimmers and anglers are separate receptors with their own contact rates. A trespasser on a fenced site is not a resident.
Consumer product. Intermittent, product-specific use: paints, cleaners, cosmetics, treated textiles, do-it-yourself pesticides. Dose is set by use pattern (mass per event, events per year, room volume, minutes in the room), not by a Superfund well concentration. Screening tools in the spirit of EPA SHEDS and European ConsExpo combine those use patterns with emission rates. A consumer scenario is not an occupational 8-hour TWA and is not a lifetime tap-water concentration unless the product actually contaminates water.
Dietary. Intake equals food concentration times consumption rate, summed across commodities. Residue files come from monitoring programs or field-trial tolerances. Subsistence fish or wild-game diets use much higher fish-ingestion rates than the general-population mean. Breast milk is a dietary path for infants. Dietary assessment is still an ingestion route; it is a different scenario from incidental soil ingestion at a waste site.
| Scenario | Typical receptor | RAGS-style teaching calendar (not a live default) | Dominant routes |
|---|---|---|---|
| Occupational | Adult worker | Often 8 h/day, 250 d/year, 25 years at one site | Workplace air; dermal film; incidental dust ingestion |
| Residential / general public | Adult and child residents | Often 24 h, 350 d/year, 30 years at one residence | Tap water, soil/dust, indoor and outdoor air, homegrown produce |
| Consumer product | User of a specific article | Events per year × minutes per event | Indoor air during use; dermal film; hand-to-mouth |
| Dietary | General population or subsistence subgroup | Daily consumption, often averaged over a year | Residues in food and drinking water |
Those calendars are RAGS Part A teaching values. Live assessments should pull current Exposure Factors Handbook age bins rather than treating 250/25/350/30 as physics.
Routes the examination likes to mix
Incidental ingestion of soil and dust. Children mouth hands and objects; adults ingest far less. The soil pathway is often the risk driver at metal-contaminated yards even when outdoor air looks clean. Indoor dust can be outdoor soil tracked in, or a consumer-product residue. Unit trap: soil ingestion is usually tabulated in mg/day and must be converted to kg/day (divide by 1,000,000) before it is multiplied by a soil concentration in mg/kg.
Dermal. Contact with soil, water, or a product film. Absorption depends on loaded area, adherence (soil loading in mg/cm²), contact time, occlusion, and an absorption fraction. Intact stratum corneum limits uptake of ions and large polar molecules; solvents and damaged skin raise flux. A dermal dose is not “the soil concentration” without area and adherence.
Inhalation of vapors versus particles. Vapors and gases are molecularly dispersed. Highly water-soluble or reactive gases deposit in the extra-thoracic airways (the Category 1 portal-of-entry story in section 11.3). Poorly soluble gases reach alveoli and blood. Particles deposit by impaction, sedimentation, and diffusion according to mass median aerodynamic diameter (MMAD). Coarse particles (greater than about 10 μm) stay in the nose and throat and are largely swallowed, converting an “inhalation” sample into a secondary oral dose. Respirable particles (about 1–5 μm) reach the pulmonary region. The same mg/m³ of dust is not the same internal dose as the same mg/m³ of vapor.
Aggregate versus cumulative (preview)
Aggregate exposure combines one chemical across all routes and pathways that actually operate for that receptor: diet plus water plus indoor air plus soil, for example. A pesticide risk cup that adds food, drinking water, and residential use is an aggregate problem.
Cumulative risk combines multiple chemicals that share a common mechanism of toxicity. Organophosphate insecticides and acetylcholinesterase inhibition is the teaching class. Cumulative is not a synonym for aggregate. Mixing the words is a Domain III error. Chapter 14 returns to hazard index and toxic equivalency factors; here you only need to name which combination problem you are in before you add doses.
Parent versus metabolite, unbound versus total (III.B.2 C)
Exposure assessment must state which chemical species is being quantified and which measurement will be compared with the toxicity value.
Parent versus metabolite. An air concentration of parent solvent is not the same quantity as urinary metabolite mass. If the reference dose was derived from parent administered dose, reconstructing intake from a metabolite requires a molar yield and a collection interval. If toxicity is driven by a reactive metabolite, the relevant internal dose may be that metabolite in the target tissue, not parent in air. Measuring the wrong species is an exposure-assessment error, not a laboratory inconvenience.
Unbound versus total. Many plasma assays report total concentration (bound plus free). For receptor-mediated effects and for clearance, the unbound (free) concentration is usually the driving force. A highly albumin-bound chemical can show a large total blood number and a small free number. Comparing a free tissue model to a total biomonitoring number, or the reverse, silently mis-scales dose. Section 11.4 picks this up when urine and blood become reconstructed intakes.
Scenario
A former smelter yard has lead in soil, a groundwater plume of a chlorinated solvent, and a warehouse still used by workers. The CSM must separate receptors. Children in adjacent houses have incidental soil ingestion and indoor dust; they do not work 8-hour shifts. Warehouse workers have occupational particle inhalation and dermal loading; they do not eat 200 mg of yard soil per day. Residents with a private well have a water-ingestion pathway the workers on city water do not. Indoor air in the houses may complete a vapor-intrusion pathway from the plume; the warehouse slab may or may not. Adding every concentration in the remedial-investigation report into one “site dose” without those receptor-specific complete pathways is not an exposure assessment.
A second file is a pyrethroid used in agriculture and in household sprays. Food residues plus drinking-water traces plus indoor-air after a home treatment are aggregate exposure to one active ingredient. Adding a second pyrethroid that shares the same neuronal mechanism is a cumulative question. Calling the second chemical “just another route of the first” collapses two different combination rules.
Traps
- Treating a source as completed exposure because the soil number is high.
- Using a child soil-ingestion rate for an 8-hour industrial hygienist, or a worker 250-day calendar for a resident child.
- Equating mg/m³ of coarse dust with mg/m³ of vapor.
- Calling several organophosphates in one diet aggregate when the problem is cumulative, or calling one pesticide in food plus water cumulative.
- Matching a parent-based RfD to a metabolite biomonitoring number without a yield, or a free-concentration model to a total-plasma measurement.
A groundwater plume sits under vacant land with no wells and no occupied buildings. Soil at an adjacent playground is contaminated, and children play there daily. Which conceptual-site-model statement is correct?
Which pairing correctly matches an exposure scenario with a RAGS-style teaching calendar rather than mixing receptors?
An assessor adds food residues, tap-water traces, and indoor-air after a home treatment of the same insecticide, then adds a second insecticide that inhibits acetylcholinesterase by the same mechanism. Which labeling is accurate, and which internal-dose pairing matches handbook III.B.2 C?