EPA-Expo-Box (A Toolbox for Exposure Assessors)
- Fate & Transport
Sources and Releases
People can be exposed to a variety of potentially harmful substances in the air they breathe, food they eat, water they drink, and products they use, as well as by skin contact with treated or contaminated surfaces. Examples of sources of stressors could be places, objects, activities, or entities that release chemicals (e.g., hazardous waste disposal facility, automobile, pesticide application). Typically, the source is defined as the origin of an agent, or stressor, for the purposes of an exposure assessment.
When performing an exposure assessment, assessors need the concentration of the substance designated as the stressor of concern at the point(s) of contact with a receptor, in this case a human receptor. The phrase "fate and transport" refers to the processes that a substance undergoes between release from a source and contact with a receptor. When a substance is released from a source, it is subject to transport and transformation in the environment. Fate and transport processes "link" the formation or release of substances at a source with the resultant environmental concentrations to which people can be exposed, potentially at considerable distance from the source. Where information is known about the source, environmental concentrations are sometimes estimated through modeling of emissions and fate and transport.
Environmental monitoring can provide information on release rates and environmental concentrations of a stressor, and it can also assist in evaluating source/stressor formation and fate and transport. Monitoring data can be used with environmental fate and transport models to characterize media-specific exposure concentrations with more precision.
Monitoring data are not always available. For some exposure assessments, data on sources and releases include quantitative information about emission rates of chemicals and these data are available in emission inventories maintained by government agencies, in facility-specific records, or via direct measurement at the site of release. Emission rates can also be estimated using emission factors. An emission factor can be used to calculate an unknown emission rate of a substance from a known rate, such as one for the commodity or processes associated with production of the substance, if this relationship is known for a similar source. For example, the emission rate of an air pollutant released during manufacturing of cement can be estimated based on the tons of cement produced each day. Similarly, the emission rate of a disinfection byproduct used at a water treatment facility can be estimated based on the rate (i.e., gallons/day) of water processed at the facility. While this method is not 100 percent accurate, it allows exposure assessors to estimate emission rates of substances from readily available data.
Sources release a substance into a receiving medium (e.g., air, water), but that initial receiving media compartment can subsequently serve as a source by releasing into other media. In other words, environmental media can serve as both sources and receiving media.
Data are collected to characterize the rate of release of agents into the environment from a source of emission such as an incinerator, landfill, industrial or municipal facility, or consumer product. Databases and other resources are available that identify common sources of stressors in the environment and quantify their releases to and from air, water, soil/sediment, food, biota, and consumer products.