Water Reuse: Public Health Protection and Risk Assessment Notes

Overview of Public Health Protection and Risk Assessment

• The necessity of quantifying health risks linked to exposure from environmental and occupational toxicants has led to the development of an interdisciplinary methodology known as health risk analysis.

• Health risk analysis serves as a critical tool for comparing risks to human health resulting from exposure to microbiological, natural, and anthropogenic constituents found in water and wastewater.

• While health risk analysis is considered to be in its infancy within the specific field of water reuse, scientific knowledge in this area has been accumulating rapidly in recent years.

• Key objectives provided by risk assessment include:   1. A characterization of the types of health effects expected.   2. An estimate of the probability (or risk) of these health effects occurring.   3. An estimate of the total number of cases expected to exhibit these health effects.   4. A suggested acceptable concentration for specific hazards in air, water, or food.

• The outputs generated from risk assessments are vital for making informed regulatory decisions concerning:   - Worker exposures.   - Plant emissions and effluents.   - Ambient air and water exposures.   - Chemical residues in food products.   - Waste disposal sites.   - Consumer products.   - Naturally occurring contaminants.

• Risk assessment and risk management are fundamental components of contemporary regulatory activities performed by federal and state agencies.

Terminology and Definitions

• Acute toxicity: A toxic effect that occurs shortly after a single exposure event.

• Anthropogenic: Human-induced or resulting from human activities; often used in the context of chemical compounds, biological emissions, or environmental changes produced by humans.

• Cancer potency: Represented by the upper $95$ percent confidence limit slope of the dose-response relationship as the dose approaches zero.

• Chemical carcinogen: A chemical agent shown to produce tumors in experimental animals or humans.

• Chemical noncarcinogen: A chemical capable of producing adverse health effects other than tumors in humans or experimental animals.

• Chronic toxicity: A toxic effect resulting from exposure over a long duration, typically defined as one year or more.

• Drinking water equivalent level (DWEL): The concentration of a chemical in water at which no adverse noncancer health effect is anticipated over a lifetime, assuming an adult weight of $70\,kg$ and a daily consumption of $2\,L$.

• Ecological risk assessment: An evaluation of available toxicological and ecological information to estimate the probability of an undesired ecological event.

• Ecotoxicology: The study of the fate and effects of toxic substances on entire ecosystems.

• Exposure: Contact with a physical or chemical agent through ingestion, inhalation, or dermal routes.

• Hazard: The intrinsic capacity of a substance to cause harm.

• Health risk assessment: An evaluation of the potential for adverse health effects resulting from actual or potential exposure to chemicals.

• Maximum contaminant level (MCL): Enforceable drinking water standards for public water supplies, set as close to the MCLG as feasible using best available technology and considering cost.

• Maximum contaminant level goal (MCLG): A nonenforceable health goal set at a level where no known or anticipated adverse health effects occur, including an adequate safety margin.

• Reference dose (RfD): An estimate of a daily dose to the human population (including sensitive subgroups) likely to be without appreciable risk of deleterious effects over a lifetime. It is usually expressed in units of $mg/kg\cdot d$.

• Risk: The probability that an organism exposed to a specific hazard will experience an adverse response.

• Risk analysis: A framework consisting of three elements: risk assessment, risk management, and risk communication.

• Risk assessment: The qualitative or quantitative characterization and estimation of potential adverse health effects associated with exposure to hazardous materials or situations.

• Risk communication: The interactive exchange of information and opinions regarding risk and risk management among assessors, managers, consumers, and interested parties.

• Risk management: The process of evaluating and potentially controlling sources of exposure and risk by weighing decision attributes and developing alternatives.

• Vector: An organism, such as a tick or mosquito, that carries disease-causing microorganisms from one host to another.

The Three Elements of Risk Analysis

• Risk Assessment: Involves the qualitative or quantitative estimation of potential adverse health effects for individuals or populations exposed to hazardous situations.

• Risk Management: Involves examining policy alternatives in light of risk assessment results and selecting/implementing control options, including regulatory measures.

• Risk Communication: Involves the exchange of information regarding the nature, magnitude, significance, and control of risks among stakeholders.   - Stakeholders/Interested parties include: government agencies, industry groups, corporations, unions, the media, scientists, professional organizations, special interest groups, communities, and individual citizens.

Health Risk Assessment Framework

• According to the National Research Council (NRC, 1983), health risk assessment is divided into four major steps:   1. Hazard Identification: Determining if exposure to an agent can increase the incidence of health conditions (e.g., cancer, birth defects). In water reuse, hazards are primarily chemical and microbial constituents.   2. Dose-Response Assessment: Characterizing the relationship between the dose of an administered agent and the incidence of adverse health effects.   3. Exposure Assessment: Measuring or estimating the intensity, frequency, and duration of human exposure to an agent.   4. Risk Characterization: The final integration of the previous steps to estimate risk.

Hazard Identification

• Chemical Constituents: Involves characterizing the evidence of causation. Because human data are rarely definitive, researchers look at laboratory animal systems. If an agent induces cancer in test animals, it is typically taken as evidence of potential risk in humans. In vitro (test tube) tests and structural similarities to known hazards are also considered.

• Microbial Constituents: Identifies microorganisms or microbial toxins of concern using scientific literature, clinical studies, epidemiological surveillance, animal studies, and expert opinions.

Dose-Response Assessment

• Chemical Extrapolation: Requires extrapolating from high doses (used in labs) to low doses (typical human exposure) and from animals to humans. These methods must be justified due to statistical and biological uncertainties.

• Noncarcinogenic Effects: Based on the assumption that thresholds exist for effects like cellular necrosis (localized death of living tissue). RfDs are used as reference points; doses lower than the RfD are generally not associated with health risks.

• Microbial Dose-Response: Describes the likelihood, severity, or duration of effects (infection or illness). If data are missing, expert elicitation regarding factors like infectivity is used.

Quantitative Chemical Risk Assessment

• Incremental Lifetime Risk: The U.S. EPA defines the incremental lifetime risk for cancer (above background) as:   $\text{Incremental lifetime risk} = \text{CDI} \times \text{PF}$

• Chronic Daily Intake (CDI): Calculated over a $70$-year lifetime:   $\text{CDI} = \frac{\text{average daily dose, mg/d}}{\text{body weight, kg}}$

• Total Dose: Generally defined as:   $\text{Total dose, mg/kg}\cdot\text{d} = \text{constituent concentration} \times \text{intake rate} \times \text{exposure duration} \times \text{absorption factor}$

• Standard EPA Values:   - Adult weight: $70\,kg$   - Child weight: $10\,kg$   - Adult water ingestion: $2\,L/d$   - Child water ingestion: $1\,L/d$

• Potency Factor (PF): Also called the slope factor, it is the slope of the dose-response curve at very low doses. The U.S. EPA uses the linear multi-stage model for this.

• Integrated Risk Information System (IRIS): An EPA database containing toxicity data, such as Potency Factors for various chemicals.

Comparative Chemical Toxicity (Table 5-8)

• Arsenic (inorganic): Oral PF = $1.5 \times 10^0\,(mg/kg\cdot d)^{-1}$; Inhalation PF = $3.0 \times 10^{-2}\,(\mu g/kg\cdot d)^{-1}$.

• Benzene: Oral PF = $1.5 \times 10^{-2}$ to $5.5 \times 10^{-2}\,(mg/kg\cdot d)^{-1}$.

• Chloroform: Oral PF = $6.1 \times 10^{-3}\,(mg/kg\cdot d)^{-1}$.

• N-Nitrosodimethylamine (NDMA): Oral PF = $5.1 \times 10^1\,(mg/kg\cdot d)^{-1}$.

• Comparison: Arsenic's oral potency is approximately $245$ times that of chloroform.

Example 5-2: Risk Assessment for NDMA in Drinking Water

• Scenario: Estimate incremental lifetime risk for an adult drinking water with $2.0\,\mu g/L$ of NDMA and determine the concentration needed to limit risk to $1 \text{ in } 100,000$.

• Step 1: Calculate CDI:   $\text{CDI} = \frac{(2.0\,\mu g/L) \times (2\,L/d) \times (1\,mg / 10^{3}\,\mu g)}{70\,kg} = 0.57 \times 10^{-4}\,mg/kg\cdot d$

• Step 2: Calculate Risk:   $\text{Incremental lifetime risk} = (0.57 \times 10^{-4}\,mg/kg\cdot d) \times (5.1 \times 10^{1}\,(mg/kg\cdot d)^{-1}) = 0.29 \times 10^{-2}$   - This equates to an estimated probability of $2.9$ cancer cases per $1000$ persons.

• Step 3: Target Concentration for $10^{-5}$ Risk:   - Estimate required CDI:     $10^{-5} = (\text{CDI}) \times (5.1 \times 10^{1})$     $\text{CDI} = 1.96 \times 10^{-7}\,mg/kg\cdot d$   - Rearrange dose equation for concentration (C):     $\frac{(C\,\mu g/L) \times (2\,L/d) \times (1\,mg / 10^{3}\,\mu g)}{70\,kg} = 1.96 \times 10^{-7}$     $C = 0.0069\,\mu g/L$

Microbial Dose-Response Models

• Models relate the probability of infection to the mean dose ingested. Common models include:   1. Single-hit exponential   2. Multistage   3. Linear multistage   4. Multi-hit   5. Beta-Poisson   6. Probit

• Beta-Poisson Model Formula:   $P_{inf}(d, \alpha, \beta) = 1 - (1 + \frac{d}{\beta})^{-\alpha}$   - $P_{inf}$ = Probability of infection as a function of dose, $\alpha$, and $\beta$.   - $d$ = Mean ingested dose.   - $\alpha, \beta$ = Slope parameters (where $\beta \ge 1$ and $\alpha \le \beta$).

Example 5-1: Application of Beta-Poisson Model

• Scenario: Contaminated water contains $1200\text{ organisms}/100\,mL$ of Campylobacter jejuni. Estimate infection probability for an individual ingesting $250\,mL$. Parameters: $\alpha = 0.145$, $\beta = 7.589$.

• Step 1: Calculate Dose:   $\text{Dose} = (1200\text{ org}/100\,mL) \times (250\,mL) = 3000\text{ organisms}$

• Step 2: Estimate Probability:   $P_{inf} = 1 - (1 + \frac{3000}{7.589})^{-0.145} = 0.58$   - Ingestion of $3000\,C. jejuni$ cells results in a $58$ percent infection rate.

Exposure Assessment Factors

• Exposure assessment identifies control options and predicts the effects of technologies on exposure.

• Microbial specific factors:   - Contamination levels in water.   - Consumption patterns related to: ethnicity, socioeconomic status, seasonality, age (demographics), regional differences, and consumer behavior.   - Environmental conditions and treatment system reliability.

• Visual Examples:   - Children playing in parks irrigated with reclaimed water (Southern California).   - Children in "Seseragi" (Japanese artificial streams) using water treated with microfiltration, reverse osmosis, and low-dose chlorination ($\sim 0.1\,mg/L$).

Indicator Organisms and Emerging Contaminants

• Indicator Organisms: Monitoring every microbe is impractical. Coliform bacteria have traditionally been used; their presence suggests fecal pathogens (e.g., viruses) might be present, while their absence suggests the water is safe.

• Emerging Contaminants: Chemicals and microbes only recently identified in water and currently under regulatory consideration.   - Endocrine Disrupting Compounds (EDCs): Substances that mimic, block, or inhibit natural hormones. Sources include pharmaceuticals, personal care products, household chemicals, pesticides, industrial chemicals, and disinfection byproducts.   - Pharmaceutically Active Compounds (PhACs): Specifically synthesized for medical use, such as antibiotics, anti-inflammatories, and antidepressants.

Static and Dynamic Models in Microbial Risk Assessment (MRA)

• Static Models: Assume the number of susceptible individuals is not time-varying. These shift perspective toward the individual.

• Dynamic Models: Account for time-varying susceptibility and person-to-person transmission, shifting the perspective to the entire population.

• Parameters Comparison:   - Both Models: Pathogen concentration, volume ingested, proportion of population exposed, frequency of exposure, dose-response parameters.   - Dynamic Only: Duration of incubation, duration of infectiousness, duration of disease, duration of protection, probability of symptomatic response, person-to-person transmission potential, background concentration levels.