9.3 Environmental Toxicology & Occupational Health

Environmental Toxicology & Occupational Health Engineering

Environmental toxicology and occupational safety form the regulatory and physiological basis of environmental protection. Engineers must quantify exposure risks, interpret toxicological dose-response studies, design engineering hazard controls (local exhaust ventilation, acoustic abatement), and enforce OSHA workplace safety standards.

Pharmacokinetics & Toxicokinetics (ADME)

Toxicokinetics describes the quantitative movement of xenobiotics through a biological organism, divided into four phases (ADME):

1. Absorption: Chemical uptake across biological membranes (lungs via inhalation, gastrointestinal tract via ingestion, skin via dermal absorption). Governed by Fick's Law of diffusion and octanol-water partitioning ($K_{ow}$).
2. Distribution: Transport of absorbed chemicals via blood plasma to tissue storage sites. Modeled by the Volume of Distribution ($V_d$) ($L$ or $L/kg$): $V_d = \frac{\text{Total Mass of Chemical in Body } (mg)}{\text{Plasma Concentration } C_p (mg/L)}$ High $V_d$ ($> 1 \ L/kg$) indicates extensive tissue binding and accumulation in adipose tissue (e.g., PCBs, organochlorine pesticides).
3. Metabolism (Biotransformation): Enzymatic alteration primarily in the liver. Phase I reactions (Cytochrome P450 oxidation, reduction, hydrolysis) introduce polar functional groups. Phase II reactions (glucuronidation, sulfation) conjugate polar molecules for excretion. Biotransformation can result in bioactivation—generating toxic intermediates (e.g., metabolism of methanol to formaldehyde/formic acid, or benzo[a]pyrene to reactive epoxides).
4. Excretion & Elimination Kinetics: Removal of chemicals via urine, feces, or exhaled air. Elimination is typically first-order: $C_p(t) = C_0 e^{-k_e t} \quad \implies \quad t_{1/2} = \frac{\ln 2}{k_e} = \frac{0.693}{k_e}$ Where $k_e$ is the elimination rate constant ($h^{-1}$) and $t_{1/2}$ is biological half-life.

Quantitative Dose-Response Metrics & EPA Toxicity Assessment

Dose-response curves establish quantitative safe exposure limits for human health risk assessment.

Non-Carcinogenic Metrics & Uncertainty Factors

Non-carcinogenic toxicity operates under a threshold hypothesis—a dose below which biological repair mechanisms prevent adverse effects.

• NOAEL (No-Observed-Adverse-Effect Level): The highest tested dose ($mg/kg\cdot day$) that produces no statistically significant adverse effect compared to controls.
• LOAEL (Lowest-Observed-Adverse-Effect Level): The lowest tested dose producing a statistically significant adverse effect.
• Reference Dose ($RfD$): EPA's daily oral exposure limit deemed safe over a human lifetime: $RfD = \frac{NOAEL \text{ or } LOAEL}{UF \times MF}$ Where Uncertainty Factors ($UF$) account for interspecies extrapolation ($UF_{interspecies} = 10$), human population variability ($UF_{intraspecies} = 10$), subchronic to chronic duration ($UF_{subchronic} = 10$), and LOAEL-to-NOAEL conversion ($UF_{LOAEL} = 10$). Modifying Factor ($MF$) ranges from $1 - 10$ based on professional judgment.

Carcinogenic Slope Factors

Carcinogens follow a non-threshold model assuming linear multi-stage low-dose response. The Cancer Slope Factor ($SF$) ($[mg/kg\cdot day]^{-1}$) represents the upper 95% confidence bound of cancer risk per unit intake.

OSHA HAZWOPER & Site Control Engineering (29 CFR 1910.120)

The OSHA Hazardous Waste Operations and Emergency Response (HAZWOPER) standard governs health and safety at Superfund sites, RCRA corrective action sites, and chemical spill emergencies.

Mandatory Worker Training Hours

• General Site Worker (40-Hour): Requires 40 hours of off-site classroom instruction and 3 days of supervised field experience.
• Occasional / Restricted Worker (24-Hour): Requires 24 hours of classroom instruction and 1 day of field experience for workers entering un-excavated areas for limited tasks.
• Annual Refresher: All HAZWOPER-certified personnel must complete 8 hours of annual refresher training.

Site Work Zones

Sites are organized into three concentric safety zones to control contaminant migration:

1. Exclusion Zone (Hot Zone): Area of active contamination. Full PPE required; strict entry log.
2. Contamination Reduction Zone (CRZ / Warm Zone): Buffer zone containing the Decontamination Corridor (Decon Line) for personnel and equipment.
3. Support Zone (Cold Zone): Uncontaminated administrative area housing command post and medical support.

EPA Personal Protective Equipment (PPE) Ensembles

• Level A: Maximum respiratory, skin, and eye protection. Fully encapsulating vapor-tight chemical suit + Self-Contained Breathing Apparatus (SCBA) or supplied-air respirator.
• Level B: Maximum respiratory protection + high skin splash protection. Chemical-resistant splash suit + SCBA. (Used when atmosphere is IDLH or oxygen-deficient $< 19.5%$).
• Level C: Air-purifying protection. Chemical splash suit + Air-Purifying Respirator (APR / PAPR) with combination organic vapor/HEPA cartridges. (Used when oxygen $> 19.5%$ and contaminants are known and $< IDLH$).
• Level D: Basic work uniform (hard hat, safety glasses, steel-toe boots, coveralls). No respiratory protection.

Occupational Noise Exposure & 5 dBA Exchange Rate

OSHA (29 CFR 1910.95) sets the Permissible Exposure Limit (PEL) for noise at $90 \ dBA$ for an 8-hour Time-Weighted Average (TWA) and utilizes a $5 \ dBA$ exchange rate (doubling/halving allowed exposure time for every $5 \ dBA$ change).

Allowable Exposure Time ($T_i$)

$T_i = \frac{8}{2^{(L_i - 90)/5}}$

• $90 \ dBA \implies T = 8 \text{ hours}$
• $95 \ dBA \implies T = 4 \text{ hours}$
• $100 \ dBA \implies T = 2 \text{ hours}$

Cumulative Noise Dose ($D$) & Equivalent TWA ($L_{TWA}$)

$D = 100 \times \sum_{i=1}^{n} \frac{C_i}{T_i}$ $L_{TWA} = 16.61 \log_{10}\left(\frac{D}{100}\right) + 90$

Where $C_i$ is actual exposure duration at sound level $L_i$. An Action Level of $85 \ dBA$ (Noise Dose $D = 50%$) triggers mandatory inclusion in a Hearing Conservation Program.

Industrial Hygiene & Local Exhaust Ventilation (LEV)

Engineering controls are preferred over PPE under OSHA's Hierarchy of Controls. LEV captures airborne contaminants at the generation source.

DallaValle Capture Velocity Equations

To capture contaminants at distance $x$ from a hood face of area $A$:

• Plain Opening Unflanged Hood: $v_x = \frac{Q}{10 x^2 + A}$
• Flanged Hood (baffle blocks rear air draw, saving $25%$ airflow): $v_x = \frac{Q}{0.75 (10 x^2 + A)}$

Where $v_x$ is required capture velocity ($ft/min$ or $m/s$), $Q$ is exhaust flow rate ($cfm$), and $x$ is centerline distance to contaminant source ($ft$).

Minimum Duct Transport Velocity

Exhaust duct velocities must be maintained high enough to prevent particulate settlement:

• Gases / Vapors: $1000 - 2000 \ fpm$
• Fine Dusts (wood flour, cement): $3000 - 3500 \ fpm$
• Heavy Metal Dusts (lead, grinding dust): $4000 - 4500 \ fpm$