Development of Data Used in Risk Assessment
This appendix provides additional information on the data needed to estimate different elements in the risk-characterization steps of emission characterization, transport and fate, exposure assessment, and assessment of toxicity.
The best approach to characterizing emissions is to measure the flux from each manufacturing, storage, use, or disposal facility. However, such flux measurements are generally not available, because sources are not uniform across geography or time, because they are so large (e.g., a several-square-block manufacturing site) that no point for measuring flux is apparent, or because flux measurements are so difficult and expensive, and require such detailed knowledge of local meteorology, as to be impractical. Therefore, most emission data are calculated or estimated from industry-wide averages applied to such things as "emission factors," process rates, quantities of chemical present at given locations, or numbers of individual components. Some information that might be needed to estimate and characterize emissions from a facility is provided in Table L-1. (Not all information is needed for all calculation methods.)
Transport And Fate
Atmospheric-chemistry models are used to determine where emitted chemicals are transported and their characteristics when deposited. Several kinds of information are needed to estimate the transport and fate of pollutants:
To evaluate human exposure for risk-assessment purposes, information is needed on the following:
These are described in more depth below.
For the contaminant, the minimum data need include measured or estimated concentrations at the point of human contact for a specified duration. For air, concentration data are generated by sampling air and simultaneously or sequentially measuring the toxicant trapped at a given air flow rate and for a given period monitored. Beyond those generalities, analytical methods vary widely in specifics and in the key dimensions of accuracy (agreement with true value), precision (spread in data), and limit of detection. Errors can be large, particularly in trace analysis, so concerns are warranted about the quality of concentration data used in risk assessments. The following cautions are pertinent:
For the exposed population, the nature of the harm must be defined. It is important to assess the various degrees of exposure and the numbers within each identifiable set of the population, such as sets defined by age or health status. In the absence of personal monitoring data, geographic, behavioral (e.g., activity-pattern), and demographic considerations will often allow estimation of the exposure, although the estimated exposure might not be directly related to an individual's exposure.
Because exposure to a specific chemical is rarely confined to a single route (although one route might dominate), the total exposure must be calculated by summing air (inhalation), dermal, and dietary (food and water) intakes. For example, pollutants that begin as "air pollutants" can generate substantial exposures through other media if they can move from air to water, soil, or vegetation.
A case in point is that of chlorinated hydrocarbons (polychlorinated biphenyls, toxaphene, DDT, etc.) in the Arctic; the mechanism was long-range transport in the air, but the exposure of indigenous peoples in the region is through the diet and results from the uptake of chemicals deposited in the food chain.
Assessment Of Toxicity
A risk analysis must include an assessment of the toxicity of a chemical, i.e., of the potential hazard the public health. Such analysis can be based on a combination of experimental toxicity and human data. Clearly, information on the incidence of disease associated with known exposures to toxicants is the most useful for human risk assessment. It is also the least available, however, because it depends on the occurrence of some unplanned or unforeseen event (e.g., an accident or malfunction in a manufacturing facility) or it is collected for a narrowly defined population (e.g., a workforce) exposed at magnitudes and for durations well beyond what the general population experiences. For ethical (and also sometimes legal) reasons, controlled dose-response studies in humans are rare.
The human data that might be available for risk assessment are in three broad categories:
The opinions of medical experts on the findings and the applicability of the results to the general population are also important in determining the usefulness of clinical evidence for risk assessment.
In addition, the method of exposure (nature and composition of toxic agent, routes of exposure, media and means of exposure, time of exposure, and doses) and statistical evaluation (e.g., point and range estimates, measures of association and significance, and dose-response and time-response relations) should be described.
EPA (U.S. Environmental Protection Agency). 1991. Procedures for Establishing Emissions for Early Reduction Compliance Extensions. Vol. 1. EPA-450/3-91-012a. U.S. Environmental Protection Agency, Washington, D.C.
Keith, L.H., G. Choudhary, and C. Rappe. 1983. Chlorinated Dioxins and Dibenzofurans in the Total Environment. Woburn, Mass.: Ann Arbor Science.