(2) Exposure pathway identification: Potential exposed populations may come into
contact with pollutants through multiple pathways. The mechanism of interaction between pollutants and exposed populations varies under different
exposure pathways. Exposure pathway identification should consider a variety
of factors, including pollution sources, discharge pathways, pollutant types, and
potential migration and fate characteristics of pollutants (including persistence,
distribution, migration and transformation), as well as activity patterns of
potential exposed receptors, point and way of contact between exposed
receptors and pollutants (e.g. ingestion and inhalation).
(3) Exposure quantification: The concentration, frequency and duration of exposure needs to be quantified. Exposure concentration refers to the concentration
of pollutants that may come into contact with receptors throughout the exposure
period. Where the exposure occurs over a period of time, the total exposure
divided by the exposure duration is the average exposure per unit of time. As
the average exposure may also be a function of body weight, the exposure
standardized by weight and time is defined as average daily exposure (ADE),
i.e. the dose of exposure to pollutants per unit of time and per unit of weight
[mg/(kgÁd)]. The ADE calculation depends on exposure concentration, exposure rate, exposure frequency, exposure period, body weight, and mean duration of action. Equations for quantifying exposure through different pathways
are listed in Tables 4.1 and 4.2.
4.1.1.4 Risk Characterization
Risk characterization aims to summarize and integrate information from the proceeding steps of health risk assessment to synthesize a quantitative and qualitative
conclusion about risk. It is a bridge between risk assessment and risk management
and the most critical step for the final decision-making. Since carcinogens and
non-carcinogens vary in chemical toxicity, carcinogenic and non-carcinogenic
effects should be considered separately in the assessment. To characterize a
potential non-carcinogenic effect, a comparison should be made between intake and
toxicity. To characterize a potential carcinogenic effect, the probability of cancer
resulting from individual lifelong exposure should be assessed based on intake and
specific stoichiometric response data.
The approach to extrapolation of exposure and dose characterizes risk by the
maximum excess risk or by the number of excess cases. The maximum excess risk
refers to the maximum excess risk of an individual continuously exposed to a
hazardous factor at a certain exposure level during a certain period of time. This
model is the most widely used for risk calculation in recent years. The number of
excess cases refers to the number of excess cases in a given population exposed to a
hazardous factor at a certain exposure level.
Acceptable risk is a criterion for judging whether human health risks associated
with environmental pollution can be tolerated, which takes into account social,
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4 Risk Assessment of Groundwater Contamination Sites
contact with pollutants through multiple pathways. The mechanism of interaction between pollutants and exposed populations varies under different
exposure pathways. Exposure pathway identification should consider a variety
of factors, including pollution sources, discharge pathways, pollutant types, and
potential migration and fate characteristics of pollutants (including persistence,
distribution, migration and transformation), as well as activity patterns of
potential exposed receptors, point and way of contact between exposed
receptors and pollutants (e.g. ingestion and inhalation).
(3) Exposure quantification: The concentration, frequency and duration of exposure needs to be quantified. Exposure concentration refers to the concentration
of pollutants that may come into contact with receptors throughout the exposure
period. Where the exposure occurs over a period of time, the total exposure
divided by the exposure duration is the average exposure per unit of time. As
the average exposure may also be a function of body weight, the exposure
standardized by weight and time is defined as average daily exposure (ADE),
i.e. the dose of exposure to pollutants per unit of time and per unit of weight
[mg/(kgÁd)]. The ADE calculation depends on exposure concentration, exposure rate, exposure frequency, exposure period, body weight, and mean duration of action. Equations for quantifying exposure through different pathways
are listed in Tables 4.1 and 4.2.
4.1.1.4 Risk Characterization
Risk characterization aims to summarize and integrate information from the proceeding steps of health risk assessment to synthesize a quantitative and qualitative
conclusion about risk. It is a bridge between risk assessment and risk management
and the most critical step for the final decision-making. Since carcinogens and
non-carcinogens vary in chemical toxicity, carcinogenic and non-carcinogenic
effects should be considered separately in the assessment. To characterize a
potential non-carcinogenic effect, a comparison should be made between intake and
toxicity. To characterize a potential carcinogenic effect, the probability of cancer
resulting from individual lifelong exposure should be assessed based on intake and
specific stoichiometric response data.
The approach to extrapolation of exposure and dose characterizes risk by the
maximum excess risk or by the number of excess cases. The maximum excess risk
refers to the maximum excess risk of an individual continuously exposed to a
hazardous factor at a certain exposure level during a certain period of time. This
model is the most widely used for risk calculation in recent years. The number of
excess cases refers to the number of excess cases in a given population exposed to a
hazardous factor at a certain exposure level.
Acceptable risk is a criterion for judging whether human health risks associated
with environmental pollution can be tolerated, which takes into account social,
72
4 Risk Assessment of Groundwater Contamination Sites