11.4 Human Health & Ecological Risk Assessment (HQ, Hazard Index, Cancer Risk)

Human Health & Ecological Risk Assessment (HQ, Hazard Index, Cancer Risk)

Risk assessment is the scientific process of evaluating the likelihood that adverse ecological or human health effects may occur as a result of exposure to one or more hazardous chemical substances. Under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and the Resource Conservation and Recovery Act (RCRA), risk assessment provides the quantitative basis for establishing site cleanup levels, selecting engineering remedies, and justifying "No Further Action" determinations.

The United States Environmental Protection Agency (EPA) formalizes risk assessment through two landmark guidance frameworks:

1. EPA Risk Assessment Guidance for Superfund (RAGS), Volume I: Human Health Evaluation Manual (Part A) (EPA/540/1-89/002).
2. Ecological Risk Assessment Guidance for Superfund (ERAGS): Process for Designing and Conducting Ecological Risk Assessments (EPA 540-R-97-006).

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1. The 4-Step Human Health Risk Assessment Paradigm

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|                        THE EPA 4-STEP RISK ASSESSMENT PARADIGM                          |
|                                                                                         |
|   [STEP 1] HAZARD IDENTIFICATION                                                        |
|   - Screen Chemicals of Potential Concern (COPCs), identify target organs & endpoints. |
|                                     |                                                   |
|                                     v                                                   |
|   [STEP 2] DOSE-RESPONSE ASSESSMENT                                                     |
|   - Non-Carcinogens: Reference Dose (RfD, mg/kg-day) & Reference Conc (RfC, mg/m^3)     |
|   - Carcinogens: Cancer Slope Factor [CSF, (mg/kg-day)^-1] & Inhalation Unit Risk (IUR) |
|                                     |                                                   |
|                                     v                                                   |
|   [STEP 3] EXPOSURE ASSESSMENT                                                          |
|   - Delineate Conceptual Site Model (CSM) pathways, receptors, and intake rates.        |
|   - Calculate Chronic Daily Intake: CDI = (C * IR * EF * ED) / (BW * AT)                |
|                                     |                                                   |
|                                     v                                                   |
|   [STEP 4] RISK CHARACTERIZATION                                                        |
|   - Non-Cancer Risk: Hazard Quotient (HQ = CDI / RfD) & Hazard Index (HI = SUM HQi)     |
|   - Cancer Risk: Excess Lifetime Cancer Risk (ELCR = CDI * CSF)                         |
|   - Compare to Superfund benchmarks: HI <= 1.0 and ELCR within 10^-6 to 10^-4           |
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Step 1: Hazard Identification

Identifies the Chemicals of Potential Concern (COPCs) present in environmental media (soil, groundwater, surface water, sediment, air) by comparing site maximum concentrations against background levels and EPA Regional Screening Levels (RSLs). Toxicological profiles are reviewed to identify adverse toxic endpoints (e.g., hepatotoxicity, nephrotoxicity, neurotoxicity, developmental toxicity, carcinogenicity).

Step 2: Dose-Response Assessment

Establishes the quantitative mathematical relationship between the administered dose of a chemical and the incidence or severity of adverse biological effects in exposed populations:

1. Non-Carcinogenic Toxicity (Threshold Model)

Non-carcinogens operate under a biological threshold: homeostatic repair mechanisms protect the body until a threshold dose is exceeded.

• NOAEL (No Observed Adverse Effect Level): The highest experimental exposure level at which there is no statistically or biologically significant increase in frequency or severity of adverse effects.
• LOAEL (Lowest Observed Adverse Effect Level): The lowest exposure level at which adverse effects are observed.
• Reference Dose ($RfD$, $\text{mg/kg-day}$): An estimate of daily oral exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime: $RfD = \frac{\text{NOAEL (or LOAEL)}}{\prod \text{UF} \times \text{MF}}$ Where Uncertainty Factors (UFs) account for scientific uncertainties (each typically $10\times$ or $3\times$):
  • $UF_A$: Animal-to-human interspecies extrapolation ($10\times$)
  • $UF_H$: Intraspecies human sensitivity variation ($10\times$)
  • $UF_S$: Subchronic-to-chronic exposure duration extrapolation ($10\times$)
  • $UF_L$: LOAEL-to-NOAEL extrapolation if NOAEL is unavailable ($10\times$)
  • $UF_D$: Incomplete toxicological database deficiency ($10\times$)
  • $MF$: Modifying Factor based on professional judgment ($0.1\text{ to }10$, default $= 1.0$)
• Reference Concentration ($RfC$, $\text{mg/m}^3$): Equivalent benchmark for continuous inhalation exposure.

2. Carcinogenic Toxicity (Non-Threshold Linear Multistage Model)

EPA assumes that for genotoxic carcinogens, there is no absolute threshold dose; any non-zero exposure confers some theoretical probability of initiating DNA damage leading to carcinogenesis.

• Cancer Slope Factor ($CSF$ or $q_1^*$, expressed in $(\mathbf{\text{mg/kg-day})^{-1}}$): The upper-bound 95% confidence limit of the probability of a carcinogenic response per unit daily intake of a chemical over a lifetime.
• Inhalation Unit Risk ($IUR$, expressed in $(\mathbf{\mu\text{g/m}^3)^{-1}}$): The upper-bound excess lifetime cancer risk resulting from continuous lifetime exposure to an agent at a concentration of $1,\mu\text{g/m}^3$ in air.

[!NOTE] Authoritative toxicity values are published in the EPA Integrated Risk Information System (IRIS), Provisional Peer-Reviewed Toxicity Values (PPRTVs), and Agency for Toxic Substances and Disease Registry (ATSDR) Toxicological Profiles.

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2. Step 3: Exposure Assessment & Chronic Daily Intake ($CDI$)

Susceptible Receptors and Route Logic

Identify receptors before selecting exposure factors. Infants and children can receive higher soil/dust dose per kilogram and have developmental vulnerability; pregnant workers or residents may present fetal-development concerns; older adults or people with cardiopulmonary disease can be more vulnerable to irritants and asphyxiants; immunocompromised people may be more susceptible to biological hazards; and workers can have high task-specific inhalation or dermal contact. Susceptibility is chemical- and endpoint-specific—connect the toxic endpoint, route, duration, and life stage. Screen ingestion, inhalation, dermal absorption, injection, and transplacental or lactational pathways only where a source and transport mechanism make them plausible.

Exposure Assessment quantifies the magnitude, frequency, duration, and pathways of human contact with COPCs across current and reasonably foreseeable future land uses (Residential, Commercial/Industrial, Construction Worker, Recreational).

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|                   COMPLETE EXPOSURE PATHWAY (5 CONCEPTUAL ELEMENTS)          |
|                                                                             |
|   [1. Contaminant Source]      (e.g., Leaking Underground Solvent Tank)     |
|              |                                                              |
|              v                                                              |
|   [2. Release Mechanism]       (e.g., Leaching to Groundwater)              |
|              |                                                              |
|              v                                                              |
|   [3. Environmental Medium]    (e.g., Saturated Sand Aquifer)               |
|              |                                                              |
|              v                                                              |
|   [4. Exposure Point & Route]  (e.g., Potable Tap Water / Ingestion & Inhal)|
|              |                                                              |
|              v                                                              |
|   [5. Receptor Population]     (e.g., On-Site Residential Occupants)        |
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The Chronic Daily Intake ($CDI$) Equation

Chemical intake received via ingestion or dermal contact is calculated as the Chronic Daily Intake ($CDI$) (also termed Average Daily Dose, $ADD$), expressed in $\mathbf{\text{mg/kg-day}}$:

$CDI = \frac{C \cdot IR \cdot EF \cdot ED}{BW \cdot AT}$

Where:

• $C$ = Chemical concentration in exposure medium (e.g., $\text{mg/L}$ for water, $\text{mg/kg}$ for soil; typically evaluated as the 95% Upper Confidence Limit of the arithmetic mean, 95% UCL)
• $IR$ = Intake / Ingestion Rate ($2.0,\text{L/day}$ drinking water for adults; $100,\text{mg/day}$ soil for adults, $200,\text{mg/day}$ soil for young children; $20,\text{m}^3/\text{day}$ inhalation)
• $EF$ = Exposure Frequency ($\text{days/year}$; e.g., $350,\text{days/yr}$ for residential, $250,\text{days/yr}$ for commercial/industrial worker)
• $ED$ = Exposure Duration ($\text{years}$; e.g., $26,\text{years}$ residential standard [$6,\text{yr child} + 20,\text{yr adult}$], $25,\text{years}$ industrial worker)
• $BW$ = Body Weight ($\text{kg}$; standard EPA values: $80,\text{kg}$ adult, $15,\text{kg}$ young child)
• $AT$ = Averaging Time (days):
  • For Non-Carcinogenic Effects: Averaged over the exact period of exposure: $AT_{\text{non-cancer}} = ED \times 365\,\text{days/year}$
  • For Carcinogenic Effects (Lifetime Average Daily Dose - LADD): Averaged over a theoretical human lifespan of 70 years: $AT_{\text{cancer}} = 70\,\text{years} \times 365\,\text{days/year} = \mathbf{25,550\,\text{days}}$

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3. Step 4: Quantitative Risk Characterization Calculations

Risk Characterization integrates dose-response toxicity values with exposure intake estimates to generate quantitative numerical risk metrics.

1. Non-Carcinogenic Risk: Hazard Quotient ($HQ$) and Hazard Index ($HI$)

Non-carcinogenic risk is quantified by dividing the calculated chemical intake by the compound's Reference Dose:

$\mathbf{HQ = \frac{CDI}{RfD}}$

For inhalation exposures: $HQ = \frac{C_{\text{air}}}{RfC}$

To evaluate multiple chemicals across an exposure pathway, individual Hazard Quotients are summed to calculate the Hazard Index ($HI$):

$\mathbf{HI = \sum_{i=1}^{n} HQ_i = HQ_1 + HQ_2 + \dots + HQ_n}$

[!IMPORTANT] Non-Cancer Regulatory Benchmarks:

• If $\mathbf{HQ \le 1.0}$ and $\mathbf{HI \le 1.0}$: the screening calculation does not indicate that the reference level is exceeded; uncertainty and exposure-pathway completeness still matter.
• If $\mathbf{HQ > 1.0}$ or $\mathbf{HI > 1.0}$: the reference level is exceeded and adverse effects cannot be ruled out. The assessor refines exposure assumptions and segregates HQs by target organ / toxicological mechanism where appropriate. An HQ or HI above 1 is a risk-management concern, not proof that effects will occur or an automatic remediation mandate.

2. Carcinogenic Risk: Excess Lifetime Cancer Risk ($ELCR$)

Carcinogenic risk represents the incremental probability that an individual will develop cancer over a lifetime as a direct result of chemical exposure:

$\mathbf{\text{Risk (ELCR)} = CDI \times CSF}$

For multiple carcinogens across multiple exposure routes, risks are summed: $\text{Total Cancer Risk} = \sum \text{Risk}_i = \sum (CDI_i \times CSF_i)$

[!IMPORTANT] EPA Superfund Target Cancer Risk Range (40 CFR § 300.430):

• Target Risk Range: Under the NCP, EPA generally uses $\mathbf{10^{-6}\text{ to }10^{-4}}$ cumulative excess lifetime cancer risk as the risk-management range for Superfund remedies. A value inside the range does not by itself mean no action is needed; site context, uncertainty, exposure, ARARs, and remedy-selection factors still apply.
• Point of Departure: $\mathbf{10^{-6}}$ (1 in 1,000,000) is EPA's point of departure for determining remediation goals for carcinogens under 40 CFR § 300.430(e)(2)(i)(A)(2); it is not a separate statutory bright-line standard.
• Above $10^{-4}$: Risk above the upper end generally warrants further evaluation and risk management, but the NCP remedy decision is not reduced to an automatic engineering-control mandate.

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|              WORKED EXAMPLE: NON-CANCER (HQ) & CANCER RISK (ELCR)           |
|                                                                             |
|   Scenario: Residential adult drinking groundwater contaminated with:       |
|   - Arsenic: C = 0.020 mg/L | RfD = 0.0003 mg/kg-day | CSF = 1.5 (mg/kg-d)^-1|
|   - Toluene: C = 1.200 mg/L | RfD = 0.0800 mg/kg-day | Non-Carcinogen       |
|   - Adult Intake: IR = 2.0 L/day, EF = 350 days/yr, ED = 26 yr, BW = 80 kg  |
|                                                                             |
|   Step 1: Calculate Non-Carcinogenic CDI (AT = 26 yr * 365 = 9,490 days)     |
|     CDI_factor = (2.0 L/d * 350 d/yr * 26 yr) / (80 kg * 9,490 d)           |
|     CDI_factor = 18,200 / 759,200 = 0.02397 L/kg-day                        |
|                                                                             |
|     CDI_arsenic = 0.020 mg/L * 0.02397 L/kg-day = 0.000479 mg/kg-day        |
|     CDI_toluene = 1.200 mg/L * 0.02397 L/kg-day = 0.02877 mg/kg-day         |
|                                                                             |
|   Step 2: Calculate Hazard Quotients & Hazard Index                         |
|     HQ_arsenic = 0.000479 / 0.0003 = 1.60                                   |
|     HQ_toluene = 0.02877 / 0.0800 = 0.36                                    |
|     HI_total = 1.60 + 0.36 = 1.96                                           |
|     Finding: HI > 1.0 (driven by arsenic HQ = 1.60) -> refine/evaluate risk   |
|                                                                             |
|   Step 3: Calculate Carcinogenic Intake for Arsenic (AT = 25,550 days)      |
|     LADD_arsenic = (0.020 mg/L * 2.0 L/d * 350 d/yr * 26 yr) /              |
|                    (80 kg * 25,550 days)                                    |
|     LADD_arsenic = 364 / 2,044,000 = 1.781 * 10^-4 mg/kg-day                |
|                                                                             |
|   Step 4: Calculate Excess Lifetime Cancer Risk (ELCR)                      |
|     ELCR = LADD * CSF = (1.781 * 10^-4 mg/kg-day) * 1.5 (mg/kg-day)^-1      |
|     ELCR = 2.67 * 10^-4 (approx 2.7 in 10,000)                              |
|     Finding: ELCR exceeds the 10^-4 upper threshold -> Action Required!     |
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4. Ecological Risk Assessment (ERA) Framework (EPA ERAGS)

While human health risk assessment focuses on protecting individual humans, Ecological Risk Assessment (ERA) evaluates adverse chemical impacts on populations, ecological communities, and ecosystem structure/function.

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|                 EPA 8-STEP ECOLOGICAL RISK ASSESSMENT (ERAGS)               |
|                                                                             |
|   [TIER 1: SCREENING-LEVEL ECOLOGICAL RISK ASSESSMENT (SLERA)]              |
|   - Step 1: Screening-Level Problem Formulation & Environmental Setting     |
|   - Step 2: Screening-Level Exposure Estimate & Risk Calculation (EHQ)      |
|             * EHQ = EEC / TRV (If EHQ <= 1.0, Stop; If EHQ > 1.0, Proceed)  |
|                                     |                                       |
|                                     v                                       |
|   [TIER 2: BASELINE ECOLOGICAL RISK ASSESSMENT (BERA)]                      |
|   - Step 3: BERA Problem Formulation & Conceptual Model                     |
|             * Define Assessment Endpoints vs. Measurement Endpoints         |
|   - Step 4: Study Design and Data Quality Objectives                        |
|   - Step 5: Field Verification of Sampling Design                           |
|   - Step 6: Site Investigation & Laboratory Ecotoxicity Bioassays           |
|   - Step 7: Risk Characterization (Food-Web Bioaccumulation Modeling)       |
|   - Step 8: Risk Management & Remedial Target Selection                     |
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Screening-Level ERA (SLERA - Steps 1 & 2)

• Calculates conservative Ecological Hazard Quotients ($EHQ$ or $HQ_{\text{eco}}$): $EHQ = \frac{EEC}{TRV}$ where $EEC$ is the Expected Environmental Concentration (maximum detected environmental concentration) and $TRV$ is the Toxicity Reference Value (conservative ecotoxicity benchmark, e.g., EPA Region 4 ecological screening values, NOAA Screening Quick Reference Tables / SQiRTs, EPA Ambient Water Quality Criteria / AWQC).
• If $EHQ \le 1.0$, the screening comparison generally does not identify a benchmark exceedance; confirm pathway completeness and uncertainty before concluding the screen.
• If $EHQ > 1.0$, ecological risk cannot be ruled out; project transitions into a full Baseline Ecological Risk Assessment (BERA).

Baseline ERA (BERA) Core Concepts:

1. Assessment Endpoints: Explicit operational expressions of the actual environmental value to be protected (e.g., "Maintenance of self-sustaining, reproducing populations of native salmonid fish in the river" or "Protection of soil invertebrate communities from toxic mortality").
2. Measurement Endpoints: Measurable ecological characteristics, field surveys, or laboratory test responses used to infer changes in the assessment endpoint (e.g., "Laboratory 10-day sediment toxicity bioassay survival rate of Hyalella azteca" or "Fish tissue PCB concentration measurements").
3. Food-Web Bioaccumulation & Biomagnification Modeling: Quantifies the uptake and trophic transfer of hydrophobic contaminants ($\log K_{ow} > 3.0$) through ecological food chains using:
   • Bioconcentration Factor ($BCF$): Direct chemical uptake from water to organism: $BCF = \frac{C_{\text{tissue}}}{C_{\text{water}}}$.
   • Bioaccumulation Factor ($BAF$): Chemical uptake from all environmental sources (water, sediment, and dietary food ingestion).
   • Biota-Sediment Accumulation Factor ($BSAF$): Normalizes tissue lipid content to sediment organic carbon: $BSAF = \frac{C_{\text{tissue}} / f_{\text{lipid}}}{C_{\text{sediment}} / f_{oc}}$.