EPA-Expo-Box (A Toolbox for Exposure Assessors)
Screening-Level and Refined
Several examples of screening-level exposure assessments are publicly available as they are typically prepared as a first step when evaluating risk. Below are some examples of specific assessments and programs that employ screening-level or tiered approaches for evaluating risk.
Programs Using a Tiered Approach to Estimate Exposure and Risk
Screening-level assessments are appropriate particularly when an agency with limited resources is seeking to prioritize a large number of substances for further evaluation. Evaluation of air toxics by EPA through the National Air Toxics Assessments (NATA), the OPPT's New Chemicals Program, and the Canadian Domestic Substances List (DSL) are examples of a how a tiered approach is used to screen a large number of substances to determine which substances should be examined with a more refined assessment.
EPA’s National-Scale Air Toxics Assessment (NATA) is a screening tool to prioritize pollutants, emission sources, and locations of interest for further study to gain a better understanding of risks. Since 1996 four NATA assessments have been completed to characterize at a national level the chronic cancer risk estimates and noncancer hazards from inhaling air toxics. These assessments use general information about sources (e.g., type of source, point estimates of emissions data) to develop conservative risk estimates that allow air pollution experts to focus their limited resources on areas and/or populations with the highest potential health risks.
EPA’s Office of Pollution Prevention and Toxics (OPPT) implements the New Chemicals program as required by the Toxic Substances Control Act to evaluate the potential human health and ecological risks from new chemicals introduced in commerce. A new chemical’s potential for exposure is evaluated against a set of screening-level criteria including production volume, potential occupational exposure, and releases to air and surface water. Similarly, the potential hazard of the chemical and the risk (hazard x exposure) are evaluated against screening-level guidelines. EPA evaluates the results of these screening-level assessments to determine if a chemical can be freely introduced into commerce or if the manufacturer must meet certain conditions to limit potential exposure.
The Canadian Domestic Substances List (DSL) Program uses a tiered approach to evaluate risk for the 23,000 substances required for consideration under the Canadian Environmental Protection Act (CEPA) of 1999. Substances are initially evaluated for prioritization and are recommended for no further action or for a screening-level assessment. Based on the results of the screening assessment a recommendation for “no further action” might be made if predicted exposure to the substance is estimated not to pose a risk to the environment or human health. Alternatively, the substance might be added to a priority substance list to assess the risks more comprehensively (i.e., using a refined assessment) or added to the list of toxic substances that should be considered for regulatory or other controls.
Environment Canada and Health Canada initiated a Screening Assessment Pilot Project in 2001 that is currently evaluating 123 substances that are expected to meet the categorization criteria (e.g., high potential for exposure) from the pool of 23,000 substances mandated for evaluation. Of this group 91 were recommended for screening-level assessments and approximately 30 substances have screening-level assessments that are complete or underway. Exposure estimates for these assessments are based on:
- Known environmental concentrations of the substance;
- Predicted environmental concentrations of the substance from releases during production, processing, uses and disposal; and
- Environmental fate evaluated on the basis of intrinsic physical/chemical properties, environmental mobility, and persistence.
The assessments used a deterministic approach and readily available data.