EPA-Expo-Box (A Toolbox for Exposure Assessors)
Aggregate and Cumulative
Cumulative and aggregate exposure assessments are referred to as “combined exposure assessments” because cumulative assessments estimate exposure to multiple stressors by multiple routes and aggregate assessments estimate exposures to a single stressor from multiple sources and by multiple routes. Cumulative assessments focused on the combined effects of multiple stressors more realistically depict real-world exposure, but also introduce a layer of complexity not found in traditional exposure assessments, which evaluate stressors individually.
Although exposure assessments conducted by EPA are usually focused on exposures to chemical, non-chemical stressors can be considered in a cumulative assessment – and can sometimes be very important. Biological threats (such as mold or other microorganisms), physical threats (e.g., noise, temperature), can cause direct effects on those exposed (such as hearing loss). They can modify the exposures or level of effects.
(U.S. EPA, 1992) Guidelines for Exposure Assessment
Whether a cumulative or aggregate approach (or some combination of the two) is used will vary according to the scope and objective of the assessment. The aggregate exposure assessment approach is commonly used when receptors can be exposed to a single contaminant in various ways; for example, residues of the same pesticide could be found on multiple foods, in water, and/or in products used in and around the home. A receptor might have the opportunity to take up the contaminant via dermal contact, inhalation, ingestion, and other routes.
On the other hand, cumulative exposure assessments are conducted for contaminants that produce toxic responses by the same mode of action. For example, cumulative exposure could occur for a population in a specific location (e.g., a city center, near an industrial site, a rural setting) that is exposed to a variety of stressors present in that particular area (e.g., noise, air toxics, fugitive dust) that might produce a variety of adverse impacts via different modes of action.
Cumulative exposure assessments can be conducted for chemical stressors and also for non-chemical stressors, including “current physical and mental health status and past exposure histories…and social factors such as community property values, sources of income, level of income, and standard of living” (U.S. EPA, 2003). Examples of non-chemical stressors include threats that can be quantified in an exposure assessment, such as biological threats (e.g., mold or other microorganisms) and physical threats such as noise, odor, and temperature. These types of stressors could result in direct impacts on those exposed – for example, noise at a high volume could lead directly to hearing loss or impairment. Other non-chemical stressors might be less easily quantified via traditional exposure assessment methods, but could still directly or indirectly affect exposure. For example, an individual’s activity patterns during a period of chemical exposure might increase that person’s susceptibility to the toxic effects linked to chemical exposure. Any non-chemical stressors might increase the vulnerability of a population to exposure or the effects of exposure to chemical stressors.
Cumulative exposures are of particular interest when conducting community-based assessments, which are intended to encompass exposure to stressors (both chemical and non-chemical) potentially affecting a community, which can be defined as “a group of individuals in the same geographical area and/or with the same demographic attributes considered to be key factors in assessing human exposure” (Zartarian and Schultz, 2010). EPA has developed a range of tools, guidelines, and other resources (including grant programs) intended for use by community groups or other organizations interested in evaluating cumulative exposure and risk for a community. Cumulative, community-based assessments are a cornerstone of assessments of environmental justice issues because they can characterize exposures or risks that disproportionately and unfairly affect certain communities. (See EPA’s Environmental Justice website for more information.) Hence, methods and tools used for community-based assessments, cumulative exposure analyses, and assessments of environmental justice are fundamentally intertwined.
Listed below are characteristics typical of aggregate and cumulative assessments, but these are not necessarily components of all aggregate or cumulative assessments.
|Aggregate exposure assessments often include a summation of all potential exposure pathways for a single stressor. This is a conservative, health-protective approach that assumes that a single receptor will be exposed to one stressor through all possible exposure pathways.
Cumulative exposures to multiple stressors, on the other hand, generally are not additive unless they act via the same mode of action. In addition to chemical stressors, cumulative exposure assessments might also take into account non-chemical stressors. Assessments that consider risk from multiple chemicals separately and do not consider how chemical interactions affect risk are not cumulative assessments (U.S. EPA, 2003). It may also be necessary to determine how the various stressors interact to amplify or attenuate a response (i.e., synergistic, antagonistic, or additive).
|Both aggregate and cumulative assessments can use either a deterministic or a probabilistic approach, but a probabilistic approach allows one to more fully evaluate exposure and resulting risk across the entire population, not just the exposure of a single, high-end individual.
Higher-tier or more refined exposure assessments for combined exposures are more common for aggregate exposure scenarios than for cumulative scenarios due to the complexities of determining an exposure estimate that encompasses information about multiple stressors and multiple pathways.
Aggregate and cumulative exposure assessments can include both deterministic and probabilistic components. See the Deterministic and Probabilistic Module in this tool set for more information and resources. Screening-level assessments for combined exposures are carried out, but these assessments are typically more resource-intensive, and often more complex, than screening-level assessments for single stressors via a single pathway. See the Screening and Refined Module of this tool set for more information and resources.
Tools available for aggregate chemical exposure assessments include exposure to agricultural and residential pesticides (e.g., CALENDEX, CARES, LifeLine) and exposure to chemicals in the environment (e.g., SHEDS, TRIM). Cumulative exposures can be assessed using similar tools and several are available that incorporate varying exposure time periods (e.g., APEX, CALENDEX). Fewer tools are available for non-chemical stressors, although some guidelines and resources have been developed by EPA and other groups.
|An aggregate approach produces a summation of all potential exposure pathways for a single chemical stressor. This approach does not consider stressors beyond the single chemical of interest. For example, this is commonly used in the regulation of pesticides. EPA conducts risk assessments for active ingredients in pesticides by evaluating all of the potential pathways of exposure for pesticide residues to determine the potential risk from aggregate exposure.
Cumulative assessments address the overall impact on human health of multiple stressors (chemical and non-chemical) that often act by the same mechanism of toxicity. The presence of multiple stressors, however, does not necessarily mean that all of the stressors will cause or contribute to an adverse effect. Cumulative exposure assessment considers multiple chemicals and multiple pathways of exposure, but it is not always the simple sum of multiple, aggregate exposure assessments or even quantitative in nature at all. Some cumulative exposure assessments yield qualitative results that can be helpful for informing long-term planning decisions. Incorporating the effects of exposure to non-chemical stressors often results in a qualitative description of exposure or risk because numeric values for these factors are often not available.
Identification and characterization of VULNERABILITY in terms of SUSCEPTIBILITY/sensitivity in populations is limited when using aggregate approaches, but possible with cumulative approaches, provided a relationship exists between these factors and changes in risk U.S. EPA, 2003). Research to improve the ability of cumulative exposure assessments to identify areas of high exposure and high vulnerability is underway.
The following resources provide information for conducting aggregate and cumulative exposure assessments.