PRINCIPLES OF TOXICOLOGY - Biology East Borneo

480 EXAMPLE OF RISK ASSESSMENT APPLICATIONSUse of the USEPA soil screening levels is one example of a first tier risk assessment. The USEPAhas calculated soil concentrations of chemicals called soil screening levels (SSLs) as a preliminarymeans of assessing human health risks from exposure to chemicals in soil. The exposure assumptionsused are quite conservative in that they assume that an individual ingests soil on a daily basis for 30years and that the chemical concentration remains constant over the 30-year exposure period.Concentrations of a chemical less that the SSL are generally acknowledged to be associated withacceptably low levels of human health risk. Thus, if the concentration of a chemical in soil is lowerthan the SSL, the risk assessment process is often concluded at this initial step.The risk assessor should be cautious in applying risk-based screening levels for soil, water, or air,since exposure pathways used in the calculation of the screening level may not address all pathwaysof exposure relevant to a site or exposure scenario. For example, the USEPA soil screening levelsconsider possible residential exposure to soil via incidental soil ingestion. Before using the SSLs, therisk assessor should examine whether the exposure assumptions used in calculating the SSLs areapplicable to the specific site of interest.The decision to use a higher, more complex risk assessment approach is often governed by the needfor more accurate estimates of risk from environmental or occupational exposure. More complex riskassessments are inevitably more costly. Higher tiers of the risk assessment process generally includethe collection of more detailed and refined exposure data. For example, it may be important to monitorexposure to airborne contaminants in the workplace using personal monitoring devices rather than useair samples collected near a point of release. This allows estimates of chemical exposure to beindividualized to workers with specific tasks or work habits rather than assume that all workers areexposed to levels of airborne chemicals near the source. While collection and analysis of this additionaldata would likely be more costly, it nonetheless allows for better estimates of worker exposure.Higher tiers of the risk assessment process may also require further investigation of the toxicity ofa chemical. This is particularly true of potential carcinogens, since extrapolation of cancer data fromhigh dose animal studies to low levels of exposure in humans is an area of great uncertainty in thecancer risk assessment process. Further animal studies regarding the mechanism of carcinogenic actionand the applicability of the mechanism to humans may provide much needed information to increasethe accuracy of the risk assessment. For example, elucidation of a receptor-mediated mechanism ofaction for a potential carcinogen may indicate the existence of a threshold for the carcinogenic response.Because current risk assessment methods default to the position that there is no threshold for thecarcinogenic response, this type of information would significantly affect the determination of cancerrisk at low, environmentally relevant exposures. Few would argue that more complex risk assessmentmethods and additional basic research will provide more accurate characterization of human healthrisks. However, the added cost of this improved accuracy may not be justifiable except in situationswhere the health or economic impact of the regulatory decision is great.19.2 RISK ASSESSMENT EXAMPLESThis chapter describes short examples of human health risk assessments of chemical exposure. In itsbroadest sense, risk assessment may address the effects of any hazardous agent on living things. Wehave restricted our few examples to characterization of risks posed by chemical exposure in humans.Thus, for the purpose of this chapter, we use the term “risk assessment” to describe human health risksposed by chemical exposure.Brief summaries of risk assessments for persons exposed to lead, petroleum hydrocarbons, arsenic,and antimony are included as diverse examples of risk assessments for persons exposed to chemicalsin occupational or residential settings. These examples illustrate some of the more complex riskassessment problems and how they may be addressed. The lead study presents an example of a novelbiokinetic approach in risk assessment where human data regarding the absorption and distribution oflead in the body are integrated into the risk assessment process. The examples of arsenic and petroleumhydrocarbons closely parallel the risk assessment steps described above. The case of antimony trioxide