Risk Threshold

Another important part of TDH is the idea of a risk threshold. This risk threshold was conceptualized as being similar to that already proposed by earlier theories, such as zero-risk theory (Naatanen and Summala, 1974), and triggers when drivers are near the limit of their preferred range of task difficulty. In this way it can act as 

FIGURE 4.2 Task difficulty homeostasis theory. (From Fuller, R. et al. 2008a. The Conditions for Inappropriate High Speed—A Review of the Research Literature from 1995 to 2006. London Department for Transport. With permission.) 

FIGURE 4.3 Risk allostasis theory. (From Handbook of Traffic Psychology, Fuller, R., Driver control theory From task difficulty homeostasis to risk allostasis, pp. 13-26, Waltham, MA Academic Press, Copyright 2011, with permission from Elsevier.) 

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Substance Maximum 96 h acceptable concentrations Bg/L" Minimum risk threshold, Bg/L... 

In the presence of copper or zinc in concentrations of 1 mg/L or more, the minimum risk threshold should be lower by a factor of 10. 

In oyster growing areas, the minimum risk threshold should be lower. 

Skorupa, J.P. and H.M. Ohlendorf. 1991. Contaminants in drainage water and avian risk thresholds. Pages 345-368 in A. Dinar and D. Zilberman (eds.). The Economics and Management of Water and Drainage in Agriculture. Kluwer Acad. Publ., Boston. 

Numerous clinical and epidemiological studies have demonstrated the inverse and independent association between HDL cholesterol and the risk of fatal and nonfatal coronary heart disease events. As a consequence, many national and international guidelines endorse the screening for low HDL cholesterol and define 1.05 mmol/1 (40 mg/dl) as the cardiovascular risk threshold value [18]. 

Atmospheric PAH depositions are usually from very dispersed sources but they cover significant amounts of land surface. PAH concentrations from these sources are typically quite low in soil and they are adsorbed strongly to soil particles. Consequently, there is minimal leaching into the soil below and the adsorbed PAHs tend to resist biodegradation, volatilization, and/or photolysis. If low concentrations of HMW PAHs, such as benzoMpyrene, need to be reduced below some established risk threshold (often determined in a site-specific manner by the regulatory officials), bioremediation of these low concentrations will likely be a desired alternative because of the scale and magnitude of the problem. 

Recommendation 4-2. The Army should define the quantity and characteristics of anticipated nonprocess waste streams and evaluate options for the final disposition of each type of waste. On-site processes should be developed and demonstrated for treating all agent-contaminated wastes. Methods of representative sampling and characterization of wastes should be developed to verify that treated wastes are free of agent to below appropriate risk thresholds. 

F. Flury and F. Zemik indicate that 200 ppm can be fatal within five to ten minutes, while 270 ppm are immediately fatal.420 These are not, of course, the results of experiments on human beings, but rather extrapolations, in which lower risk thresholds have been determined on... 

A relationship between the serum concentration of Lp(a) and coronary heart disease has been postulated by several investigators (A9, B30). Kostner et al. (K20), in a case-control study of patients who had had myocardial infarction, suggested that plasma apo(a) concentration above 30 mg/100 ml represented a 1.75-fold relative risk and above 50 mg/100 ml represented a 2.3-fold relative risk for myocardial infarction. They suggested the risk threshold might be 30 mg/100 ml. 

It is always open to question as to what extent the risk threshold for chemical noxae may be exceeded due to altered technical working procedures and individual conditions despite compliance with the legal regulations. The recruitment and monitoring criteria prescribed by the employers liability insurance association are of great importance in this respect. 

A three-step approach was followed in the Rhine study (Heise et al., 2004), in which the hazards of substances of concern and of areas of concern could be determined with higher certainty than the areas which posed environmental risks to downstream sites. Description of the latter required the combined information from critical erosion thresholds, indications that resuspension took place and the extent by which particle bound contaminant concentrations exceeded risk thresholds. Addressing... 

The notion of de minimis risk is similar to that of ALARP. A risk threshold is deemed to exist for all activities. Any activity whose risk falls below that threshold value can be ignored— no action needs to be taken to manage this de minimis risk. The term is borrowed from common law, where it is used in the expression of the doctrine de minimis non curat lex, or, the law does not concern itself with trifles. In other words, there is no need to worry about low-risk simations. Once more, however, an inherent circularity becomes apparent for a risk to be de minimis it must be low, but no prescriptive guidance as to the meaning of the word low is provided. 

Communities where increased environmental lead exposures and various toxic endpoints have been explicitly incorporated into a dose—response framework currently highlight health risk characterization using case-specific evidence. Data generated in such communities typically first permit determination of distributions of risk group PbB values, specifically prevalences and incidences of PbB above an accepted health risk threshold. These PbB statistics are then incorporated into health risks using dose—toxic response relationships. 

There are several ways one can quantify human health risk characterization for humans at risk through lead exposure. The first and simplest examines the prevalences or incidences of blood lead levels above some health risk threshold, with frequencies of exceedance identifying those at more risk (compared to those with PbB values below the risk threshold). Expressions of health risk in terms of elevated PbB occurrences (e.g., 10 jig/dl) do not simultaneously provide quantitative estimates of organ- or system-specific toxic harm, such as actual loss of IQ points or increases in SBP or DBP. A health risk threshold indexed in terms of a PbB level, however, represents the synthesis of numerous empirical dose—toxic response relationships, as developed and discussed in previous chapters. 

Estimations of exposure frequencies above some health risk threshold in terms of PbB can be done using empirical data or through lead exposure modeling approaches, such as the U.S. EPA lEUBK model for childhood lead exposure simulations. Both approaches have their advantages and limitations, and a comparative look at the topic was provided by Mushak (1998). 

A number of studies of Bunker HiU site children in terms of PbB screening programs began in the 1980s and have continued. Much of this information appeared in the previous chapter. This chapter confines itself to PbB frequencies at selected health risk thresholds that have been widely accepted, most frequently PbB concentrations >10 pg/dl. These various PbB screening results have been summarized in Tables 24.1—24.4. 

Six illustrative years of the total described for annual community screenings 1988—1998 are depicted 1988, 1990, 1992, 1994, 1996, and 1998. From 1988 onward, there was an increasingly higher fraction of remediated yards of the total of residential yards available for cleanup. Among 2- and 3-year-old children, in particular, the impact of increased remediation with time on elevated PbB frequencies is shown in Table 24.1 to be evident by 1992. For these ages, there was an approximate 50% decline in PbB exceedance rates for the risk threshold, from the low 60s to the low 30s. By 1998, there were three- to fivefold declines in those frequencies. 

The observed percentages of PbB measurements in these children at or exceeding 10 and 15 p.g/dl as the health risk thresholds were 23% and 12%, respectively. The corresponding geometric mean PbB in this cohort was... 

Predicted PbB levels for these children ranged considerably across two selections in the model parameters for apportioning dust and sod. The selection which included community soil Pb (the 40 30 30 distribution in Table 24.4) gave a closer estimate (17%) to the measured frequency for the percentage frequency of PbB estimates at or above the health risk threshold 10 ig/dl. For frequency percentages >15 pg/dl, the estimate using the same 40 30 30 distribution underestimated the measured frequency by twofold, 6% versus 12%. Use of the lEUBK model s default Pb apportionment figure for dust and sod, by contrast, markedly exceeded the measured frequencies for >10 and >15 p.g/dl, 37% and 19%, respectively. 

The Lanphear et al. s (1998) analysis of 12 pooled studies of dust and soil Pb relationships to yoimg children s PbB value was described in the previous chapter in terms of modeled estimates of PbB mean values, given changes in interior dust Pb loadings (fig/ft ) and exterior (soil plus exterior surface Pb) Pb concentrations (ppm). Here, the analysis is extended to estimates of percentage of PbB exceeding two health risk thresholds, >10 and sl5 p.g/dl. These health risk threshold exceedances parallel the approach used in the case study for the smelter Superfund site in the Silver Valley of Idaho. 

Table 24.5 summarizes the percent probability estimates for the two PbB thresholds for children <3 years of age. For example, at a dust Pb loading of 100 M-g/ft and a yard soil concentration of 500 ppm, the frequency of PbB values >10 JLg/dl is 43%. A peak value is encountered at this dust loading and 2,000 ppm exterior Pb, 54%. The frequency for the higher PbB risk threshold, >15 jig/dl, at the 100p,g/ft dust loading and 500 ppm exterior soU/surface Pb level is approximately half that of the lower threshold figure, 18%. 

The principal objective of technical safety assessments is to demonstrate that the risk associated with plant operations is As Low as Reasonably Practicable (ALARP). First, the assessed frequency of major accidents has to be better than the applied tolerable risk threshold. Thereafter, there must be a balance between the cost of further risk reduction, and the assessed frequency of major accidents causing fatalities. 

The return of risk as a variable of import did not stop with the risk threshold however. Rather, in 2008 TDH evolved into RAT. This new form of the theory is summarized in Figure 4.3. 

Boltze U. and de Freitas M. H. (1996) Changes in atmospheric pressure associated with dangerous emissions from gas generating disposal sites. The explosion risk threshold concept. Proceedings of the Institution of Civil Engineers-Geotechnical Engineering, 119,177-181. 

A performance assessment has been made to calculate the incremental activity concentrations equivalent to the risk threshold specified in the de-licensing criterion. It is also necessary to be able to determine the background . 

The ability to detect environmental contaminants at very low levels had important regulatory implications under the 1958 Delaney Clause of the Federal Food, Drug and Cosmetic Act (FFDCA), which prohibited the presence of carcinogenic substances as additives in processed foods (47), Pesticide residues were included under this rule. The Delaney Clause, based upon the theory that no risk threshold exists for chemical carcinogens, did not permit balancing benefits of food additives with potentially low carcinogenic risk when ingested... 

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