8.3 Human Health & Ecological Risk Assessment

Risk assessment is the scientific process of evaluating the potential for adverse health or ecological effects resulting from exposure to environmental hazards. The standard methodology used in the United States follows the framework established by the EPA in the Risk Assessment Guidance for Superfund (RAGS).

The 4-Step Risk Assessment Framework

1. Hazard Identification: Determines which chemicals are present at a site, at what concentrations, and evaluates whether they have the potential to cause harm. It involves data collection, validation, and selecting "Constituents of Potential Concern" (COPCs).
2. Exposure Assessment: Estimates the magnitude, frequency, and duration of human or ecological exposure to the COPCs. It requires developing a Conceptual Site Model (CSM) to identify complete exposure pathways and quantifying the dose received by receptors.
3. Toxicity Assessment: Evaluates the relationship between the magnitude of exposure (dose) and the likelihood or severity of adverse effects (response). It distinguishes between carcinogenic and non-carcinogenic effects.
4. Risk Characterization: Integrates the exposure and toxicity assessments to quantify the potential risks and evaluate the associated uncertainties.

Exposure Assessment: Chronic Daily Intake (CDI)

The core of the exposure assessment is calculating the dose received by the receptor. For chronic (long-term) exposure, this is expressed as the Chronic Daily Intake (CDI), typically in units of mg of contaminant per kg of body weight per day (mg/kg-day).

The general equation for CDI is:

$CDI = \frac{C \times CR \times EF \times ED}{BW \times AT}$

Where:

• $C$ (Concentration): The representative concentration of the contaminant in the environmental medium (e.g., mg/L in water, mg/kg in soil). The EPA often uses the 95% Upper Confidence Limit (UCL) of the mean as a conservative estimate.
• $CR$ (Contact Rate): The amount of contaminated medium contacted per unit time (e.g., 2 L/day of drinking water, 100 mg/day of soil ingestion).
• $EF$ (Exposure Frequency): How often the exposure occurs (e.g., 350 days/year for a residential scenario).
• $ED$ (Exposure Duration): The number of years the exposure occurs (e.g., 26 years for an adult resident).
• $BW$ (Body Weight): The average body weight of the receptor (e.g., 80 kg for an adult, 15 kg for a child).
• $AT$ (Averaging Time): The time period over which the exposure is averaged. For non-carcinogens, $AT = ED \times 365 \text{ days/year}$. For carcinogens, the risk is averaged over a lifetime, so $AT = 70 \text{ years} \times 365 \text{ days/year} = 25,550 \text{ days}$.

Toxicity Assessment Parameters

Toxicity values are derived by the EPA and published in databases like the Integrated Risk Information System (IRIS).

• Reference Dose (RfD): Used for non-carcinogenic effects. It represents an estimate of a 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. Units are mg/kg-day.
• Slope Factor (SF): Used for carcinogenic effects. It is an upper-bound estimate of the probability of a response per unit intake of a chemical over a lifetime. Units are (mg/kg-day)⁻¹.

Risk Characterization

Non-Carcinogenic Risk

Non-carcinogenic risk is evaluated using the Hazard Quotient (HQ), which compares the estimated dose to the safe threshold (RfD).

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

An $HQ \le 1.0$ indicates that adverse non-carcinogenic health effects are unlikely to occur. To account for exposure to multiple chemicals, HQs for individual chemicals are summed to calculate the Hazard Index (HI).

$HI = \sum HQ_i = HQ_1 + HQ_2 + \dots + HQ_n$

The target safety threshold remains $HI \le 1.0$. If the HI exceeds 1.0, a more detailed analysis is performed to segregate the HQs by the specific target organ affected (e.g., summing only the HQs for chemicals that affect the liver).

Carcinogenic Risk

Carcinogenic risk is expressed as an Excess Lifetime Cancer Risk, representing the incremental probability of an individual developing cancer over a lifetime due to the exposure.

$\text{Risk} = CDI \times SF$

Regulatory agencies typically consider an excess cancer risk within the range of $10^{-6}$ (one in a million) to $10^{-4}$ (one in ten thousand) to be acceptable, depending on site-specific factors. Risks greater than $10^{-4}$ generally require remediation.

Worked Calculation: Risk Assessment

Problem: An adult resident (BW = 80 kg) drinks groundwater containing 0.05 mg/L of a chemical daily. The contact rate is 2.5 L/day, the exposure frequency is 350 days/year, and the exposure duration is 26 years. The chemical has an RfD of 0.01 mg/kg-day and an SF of 0.05 (mg/kg-day)⁻¹. Calculate the Hazard Quotient and the Carcinogenic Risk.

Step 1: Calculate CDI for Non-Carcinogenic Risk $AT_{non-cancer} = 26 \text{ years} \times 365 \text{ days/year} = 9,490 \text{ days}$ $CDI_{nc} = \frac{0.05 \text{ mg/L} \times 2.5 \text{ L/day} \times 350 \text{ days/yr} \times 26 \text{ yrs}}{80 \text{ kg} \times 9,490 \text{ days}}$ $CDI_{nc} = \frac{1137.5}{759,200} = 0.0015 \text{ mg/kg-day}$

Step 2: Calculate Hazard Quotient (HQ) $HQ = \frac{CDI_{nc}}{RfD} = \frac{0.0015}{0.01} = 0.15$ Since $HQ < 1.0$, the non-carcinogenic risk is acceptable.

Step 3: Calculate CDI for Carcinogenic Risk $AT_{cancer} = 70 \text{ years} \times 365 \text{ days/year} = 25,550 \text{ days}$ $CDI_{c} = \frac{0.05 \times 2.5 \times 350 \times 26}{80 \times 25,550}$ $CDI_{c} = \frac{1137.5}{2,044,000} = 0.000556 \text{ mg/kg-day}$

Step 4: Calculate Carcinogenic Risk $\text{Risk} = CDI_c \times SF = 0.000556 \times 0.05 = 2.78 \times 10^{-5}$ This risk ($2.78 \times 10^{-5}$) falls within the generally acceptable regulatory range of $10^{-6}$ to $10^{-4}$.

Ecological Risk Assessment

While Human Health Risk Assessment (HHRA) focuses on individuals, Ecological Risk Assessment (ERA) focuses on populations, communities, or ecosystems.

• Ecological Receptors: The specific species or habitats being evaluated (e.g., benthic invertebrates, piscivorous birds).
• Exposure Pathways: Similar to HHRA but tailored to the ecology (e.g., root uptake by plants, bioaccumulation through the food web).
• Toxicity Reference Values (TRVs): The ecological equivalent of an RfD. HQs are calculated by comparing the estimated environmental concentration or dose to the TRV.