Risk assessment estimates

The United States EPA has used several databases in their estimates for benzene exposure and risk. (Environmental Protection Agency, 5.0 Benzene, 5.1. Chemical and Physical Properties, EPA, 1988) The data utilized by flie EPA to assess the risk included the study by Rinsky et al. in 198U where the duration of exposure was at least 24 years and exposure levels are between 10 to 100 ppm (8 hour TWA) with a statistically significant increase incidence of leukemia. The study of Ott et al. in 1978 showed levels of anywhere from 2 to 25 ppm (8 hour TWA) with increased incidence of leukemia and Wong et al. 1983, where the exposure was at least 6 monflis, levels were from 1 ppm to 50 ppm, and there was a statistically significant increase in the incidence of leukemia, lymphatic and hematopoietic cancers. 

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Human health risk assessment estimates the likelihood of health problems occurring if no cleanup action were ttikcn at the site. To cstiimite the baseline risk at a site, the following four-step process should be taken to detennine Uie possible human risk which will then detennine what sort of corrective action should be employed ... 

If linear (dose) models without thresholds are to be used for carcinogen (or other) risk assessment, estimation of exposure at specified levels becomes irrelevant to risk assessment or, at least, its use is nonintuitive. For example, a carcinogen risk analysis may be based on a linear, nonthreshold health effects model. The total health risk would thus be proportional to the long-term exposure summed for all affected people for the identified period, and exposure of many people at low concentrations would be equivalent to exposure of a few to high concentrations. The atmospheric dispersion that reduces concentrations would also lead to exposure of more people therefore, increments... 

For MTBE, ERA has not formally established health effects and reference dose. A preliminary estimate of the threshold reference concentration was developed for this risk assessment. Estimated concentrations for the MEI location were 10-15% of the reference concentration. No further evaluation was done for MTBE. 

Burmaster DE and von Stackelberg K (1991) Using Monte Carlo simulations in public health risk assessment Estimating and presenting full distributions of risk. Journal of Exposure Analysis and Environmental Epidemiology 1 491-512. 

This experimental strategy provides a multitiered approach with Phases I and II the prime tiers. Use of this multitiered approach provides data sufficient for risk assessment estimates, yet is done in a time, materials and cost effective manner. 

By considering exposure as well as hazard, the process of risk assessment estimates the potential for the inherent hazards of a material actually to be realised during its life cycle. It should be carried out if a proposed solvent/preparation is subject to specific water pollution controls or has been classified as dangerous to the environment. The results will lend support to discussions with pollution authorities. 

The next part of the procedure involves risk assessment. This includes a deterrnination of the accident probabiUty and the consequence of the accident and is done for each of the scenarios identified in the previous step. The probabiUty is deterrnined using a number of statistical models generally used to represent failures. The consequence is deterrnined using mostiy fundamentally based models, called source models, to describe how material is ejected from process equipment. These source models are coupled with a suitable dispersion model and/or an explosion model to estimate the area affected and predict the damage. The consequence is thus determined. 

The analysis of a risk—that is, its estimation—leads to the assessment of that risk and the decision-making processes of selecting the appropriate level of risk reduction. In most studies this is an iterative process of risk analysis and risk assessment until the risk is reduced to some specified level. The subjec t of acceptable or tolerable levels of risk that coiild be applied to decision making on risks is a complex subject which will not oe addressed in this section. 

There was much to do. What should we do first My colleagues and I used what we now call quantitative risk assessment (QRA) to help us decide priorities—which risks to reduce first which to leave alone for the time being. Our calculations were crude but within a couple of decades the technique had advanced so much that sophisticated computerized programs for estimating risk were available. 

THERP (NUREG/CR-1278), is used to estimate HEPs for a risk assessment. It provides error probabilities for generic tasks and describes the process used to modify these rates depending on the specific performance shaping factors (PSFs) involved in the task... 

A fairly detailed risk analysis of fires was in the Clinch River Breeder Reactor (CRBR) Risk Assessment Study, 1977. In this study, FMEA was used to identify important fire locations for a wide variety of combustibles, including cables, oil, and sodium. The resulting estimate of the frequency of fire-induced core melt, 5E-7 per reactor-year, is substantially below the estimates discussed above. 

The Pickering A Risk Assessment (PARA) (Ontario Hydro, 1995) is also a level 3 PSA for 1 of the 4 units at Pickering. A difference between PARA and DPSE is that sequences beyond the design basis were modeled using the MAAP-CANDU codes with best estimate assumptions. Other parts of the analysis used licensing-type conservative assumptions. 

Exposure assessment, step three, allows a risk assessor to estimate the significance of the effects induced by high doses of a chemical in experimental animals in a human situation. Exposure assessment is, in fact, a prerequisite for quantitative risk assessment because it allows a comparison between effects induced by high dose with those induced by low doses, and also allows... 

Risk is often defined as the likelihood of a certain event times a measure of the severity of its consequences. Most risk assessment studies concentrate on estimating the likelihood of certain events. They often concern the release of chemicals, or accidents in engineering projects and the project outcome. In thi.s section, the subject of accidents is not covered. Risk assessment (RA), as a technique, has been adopted by various national governments, by EU, and by OECD.-... 

Start by doing a risk assessment and identify those things on which continuity of business depends power, water, labor, materials, components, services, etc. Determine what could cause a termination of supply and estimate the probability of occurrence. For those with a relatively high probability (1 in 100) find ways to reduce the probability. For those with lower probability (1 in 10000) determine the action needed to minimize the effect. The FMEA technique works for this as well as for products and processes. 

Figure 1 shows part of a solvent phase polypropylene plant. The plant consists of three process lines, denoted A, B, and C. During a risk assessment review, a scenario was identified that involved a release of reactor contents from a location near the west end of the A line. Estimates are needed of the blast overpressures that would occur if the resulting cloud of vapor, mist, and power ignites. 

To estimate tlie potential iiupaet on tlie publie or tlie environment of aeeidents of different types, the likely emergeney zone must be studied. For example, a liazardous gas leak, fire, or explosion may eause a toxie cloud to spread over a great distance. The minimum atmospheric dispersion model. Vtirious models can be used tlie more difficult models produce more realistic results, but tlie simpler and faster models may provide adequate data for planning purposes. A more tliorough discussion of atmospheric dispersion is presented in Part 111 - Healtli Risk Assessment. 

Since 1970 tlie field of healtli risk assessment Itas received widespread attention witliin both tlie scientific and regulatoiy committees. It has also attracted tlie attention of the public. Properly conducted risk assessments have received fairly broad acceptance, in part because they put into perspective the terms to. ic, Itazard, and risk. Toxicity is an inlierent property of all substances. It states tliat all chemical and physical agents can produce adverse healtli effects at some dose or under specific exposure conditions. In contrast, exposure to a chemical tliat lias tlie capacity to produce a particular type of adverse effect, represents a health hazard. Risk, however, is tlie probability or likelihood tliat an adverse outcome will occur in a person or a group tliat is exposed to a particular concentration or dose of the hazardous agent. Tlierefore, risk can be generally a function of exposure and dose. Consequently, healtli risk assessment is defined as tlie process or procedure used to estimate tlie likelihood that... 

A risk estimate indicates Uie likelihood of occurrence of the different types of health or enviroinnental effects in exposed populations. Risk assessment should include both liuimn health and environmental evaluations (i.c., impacts on ecosystems). Ecological impacts include actual or potential effects on plants and animals (other than domesticated species). The number produced from the risk characleriznlion, representing the probability of adi crse... 

Health risk assessment is defined as Uie process or procedure used to estimate Uie likelihood that humans or ecological systems will be adversely affected by a chemical or physical agent under a specific set of conditions. 

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