8.2 Routes of Entry, Toxicology, Dose, and Exposure Limits
Key Takeaways
- The four routes of entry are inhalation, skin/dermal absorption, ingestion, and injection; inhalation dominates most airborne occupational exposures.
- Dose-response is fundamental: 'the dose makes the poison,' and LD50/LC50 quantify acute lethality.
- OSHA PELs are legally enforceable; ACGIH TLVs and NIOSH RELs are recommendations and are often lower.
- OEL categories include 8-hour TWA, STEL (15-minute), and Ceiling (C, never to be exceeded).
From Exposure to Health Effect
Toxicology studies how chemical and physical agents harm living systems. The guiding principle, attributed to Paracelsus, is that the dose makes the poison — even water is toxic at extreme dose, and a hazardous solvent may be tolerable at trace levels. The ASP tests whether you can connect agent, dose, route, time, and susceptibility to a prevention decision.
The Four Routes of Entry
- Inhalation — gases, vapors, fumes, mists, and respirable dust; the dominant route for airborne contaminants.
- Skin (dermal) absorption — solvents and some pesticides cross intact skin; OELs carry a "skin" notation when this route is significant.
- Ingestion — hand-to-mouth transfer via contaminated food, drink, or cigarettes.
- Injection — sharps, high-pressure fluid injection, animal bites, or puncture wounds.
Dose is not just concentration. It integrates how much agent is present, how long contact lasts, how often it repeats, how efficiently it is absorbed, and how the body metabolizes and excretes it. Two workers in the same room can receive different doses if breathing rate, glove use, or task differ.
Core Toxicology Terms
| Term | Meaning | Example |
|---|---|---|
| Acute exposure | Short, often high exposure with rapid effect | Vapor release causing immediate irritation |
| Chronic exposure | Repeated low-level exposure over months/years | Solvent-induced liver effects |
| Local effect | Effect at the point of contact | Acid burn on skin |
| Systemic effect | Effect after absorption and distribution | Lead affecting the nervous system |
| Dose-response | Higher dose increases likelihood/severity | Steeper response above a threshold |
| LD50 / LC50 | Dose/concentration lethal to 50% of test animals | Lower value = more acutely toxic |
| Sensitization | Immune response on re-exposure | Isocyanate-induced occupational asthma |
Occupational Exposure Limits — Know the Source
The ASP frequently tests the difference among the three OEL families. Confusing enforceability is a classic trap.
| Limit | Issuing body | Legal status |
|---|---|---|
| PEL (Permissible Exposure Limit) | OSHA | Legally enforceable |
| TLV (Threshold Limit Value) | ACGIH | Recommendation, not law |
| REL (Recommended Exposure Limit) | NIOSH | Recommendation, not law |
Many OSHA PELs date to 1968 ACGIH TLVs and are considered outdated; NIOSH RELs and current TLVs are frequently lower because they consider only health, whereas PELs also weigh technical and economic feasibility.
Averaging Periods
- TWA (time-weighted average): the 8-hour, 40-hour-week average exposure.
- STEL (short-term exposure limit): a 15-minute TWA not to be exceeded, with no more than four excursions per day, each at least 60 minutes apart.
- Ceiling (C): a concentration that must never be exceeded at any instant during the workday.
A full-shift TWA below the limit can still hide a brief high peak that violates a STEL or Ceiling — time pattern matters.
Worked Mixture Calculation
Mixtures complicate interpretation: when several agents share a target organ, OSHA and ACGIH treat their effects as additive. Compute the mixture exposure as the sum of each measured concentration divided by its own limit; the result must stay at or below 1.0. Example: a worker is exposed to 100 ppm of solvent A (TLV 200) and 50 ppm of solvent B (TLV 100). The equivalent exposure is (100/200) + (50/100) = 0.5 + 0.5 = 1.0 — exactly at the limit, so the combined exposure is overexposed once any further contribution is added, even though neither chemical alone exceeds its own TLV.
Failing to add additive mixtures is a classic ASP trap.
Time-Weighted Average Arithmetic
A TWA is computed as the sum of (concentration x time) divided by total hours (usually 8). If a worker is exposed to 60 ppm for 2 hours, 20 ppm for 4 hours, and 0 ppm for 2 hours, the 8-hour TWA = [(60x2) + (20x4) + (0x2)] / 8 = (120 + 80 + 0) / 8 = 25 ppm. Compare that figure to the 8-hour PEL or TLV-TWA. The exam expects you to fold idle (zero-exposure) hours into the denominator.
Dose-Response and Thresholds
The dose-response curve plots effect against dose. Most non-carcinogens show a threshold below which no adverse effect appears; OELs are set near or below that threshold with a margin of safety. Carcinogens are often modeled as non-threshold, meaning any dose carries some risk — which is why control aims at ALARA-style minimization rather than a "safe" number. The NOAEL (no observed adverse effect level) and LOAEL (lowest observed adverse effect level) from animal or epidemiological studies anchor where limits are derived.
Susceptibility and the Limits' Fine Print
Limits are not magic borders. A result below the OEL does not protect every worker — sensitizers, skin hazards, mixtures, unusual 10- or 12-hour shifts, and individual susceptibility all break the simple comparison. NIOSH and ACGIH limits assume a standard 8-hour shift; extended shifts may require adjustment models. For ASP scenarios, read the exposure story: identify the route, the timing of effect, whether sampling matches the exposure pattern, and whether the control addresses the route. The best answer links toxicology to prevention, not just to a number on a report.
Which statement about occupational exposure limits is correct?
A 15-minute time-weighted average concentration that must not be exceeded, with no more than four excursions per day, describes which type of limit?
A worker develops irritation immediately after a brief, high vapor release, even though the full-shift TWA is well below the PEL. Which concepts best explain this?