POLLUTION LOCATOR|How Exposure Potential is Estimated for Toxic Equivalency Potentials

This website's risk scoring method uses CalTOX, an integrated environmental fate and exposure model, to estimate the total, cumulative chemical dose an individual could receive after a unit amount of a chemical is released to air or water. This model predicts the concentrations of a chemical in seven environmental compartments that result from a continuous release of a pollutant into a generic environment, taking into account transport and transformation processes that affect the pollutant. The model combines this data on environmental concentrations with information about various exposure pathways to estimate the total, cumulative dose taken in by an individual. For most pollutants, 23 distinct exposure pathways are included, ranging from the inhalation of gaseous pollutants to the ingestion of milk produced by cows that eat fodder polluted by dry and wet deposition of a chemical from the atmosphere.

CalTOX models the environment by assuming that it is made up of seven interconnected compartments, each representing a different environmental medium. These compartments are air, plants, ground-surface soil, root-zone soil, vadose zone (unsaturated) soil below the root zone, surface water, and sediments. A unit release of a contaminant to air or water will spread through the seven compartments in a manner that is determined by the chemical's physical and chemical properties and a set of environmental, or landscape, parameters.

Different chemicals will have different fates in the environment. Some may accumulate in the compartment into which they were released (e.g., heavy metals released into surface water tend to accumulate in the sediments). Other chemicals may physically, chemically, or biologically be transformed through processes such as hydrolysis, oxidation, or biodegradation. Certain contaminants may move into another compartment through processes including advection, volatilization, and precipitation.

CalTOX accounts for all of these transport and transformation processes and imposes the condition of conservation of mass. The model uses a set of linear, first-order differential equations to calculate stead-state concentrations of chemicals in each compartment. The transport and transformation processes are modeled mathematically as first-order losses, where the rate of movement or decay is directly proportional to the concentration. CalTOX was run assuming that the region being modeled (the entire United States) constitutes a closed control volume (meaning that chemicals released here are not transferred out of state via transport processes like wind or runoff).

To estimate environmental concentrations, the CalTOX model requires the following input: physical-chemical data and landscape parameters.

Human exposure to toxic chemicals occurs as a result of contact with contaminated food, water, air, or soil. A variety of pathways (such as eating produce or meat, drinking water, breathing indoor air, etc.) may contribute to the dose of a chemical which a person receives. To characterize health risks, it is essential to estimate the cumulative dose of a chemical received via all pathways contributing to three main exposure routes (ingestion, inhalation, and dermal contact). CalTOX sums across 23 distinct exposure pathways to estimate average daily doses.

The total chemical dose a person receives is a function of contaminant concentrations in different media and various exposure factors. These factors include a person's breathing rate, body weight, time spent in various locations (school, work, yard, etc.), time spent engaging in various activities (showering, swimming, resting, etc.), and dietary choices. While these factors vary between individuals, CalTOX was run using point estimates of exposure factors and did not attempt to estimate the variability in chemical doses an actual exposed population might receive. CalTOX's exposure factors are largely derived from the U.S. Environmental Protection Agency Risk Assessment Guidance for Superfund and are consistent with values in EPA's new Exposure Factors Handbook. These values are derived from scientifically reviewed studies which cover a range of populations and exposure scenarios.

To estimate total chemical dose, the CalTOX model requires the following input: chemical concentrations in different environmental media and exposure factors.