Final chronic value

Criterion continuous concentration The 4-day average concentration of a toxicant not to be exceeded more than once every 3 yr and defined by the EPA to be the minimum of the final chronic value and the final plant value. 

Final chronic value The concentration of a toxic substance expected to exert a chronic stress on no more than 5% of the genera in an aquatic ecosystem. 

If data on a sufficient number and diversity of organisms are available, a final chronic value for a particular toxicant may be calculated in the same way that final acute values are determined. In practice, however, there are seldom sufficient data to allow a direct graphical estimation of the toxicant concentration that would exert a chronic stress on no more than 5% of the species in the system. In such cases an acute toxicity standard is established on the basis of an adequate amount of short-term toxicity tests, and an average acute/chronic toxicity ratio is then calculated on the basis of a smaller amount of information. The rationale for this procedure is that for a given pollutant the acute/chronic ratio is likely to be more constant between species than is the chronic or sublethal stress level itself Hence less information is required to estimate the acute/chronic ratio. The chronic toxicity standard is established by dividing the acute toxicity standard by the so-called final acute/chronic ratio. The EPA considers this procedure acceptable if acute/chronic ratios are available for at least three species and (a) at least one of the species is a fish, (b) at least one is an invertebrate, and (c) at least one is an acutely sensitive fi eshwater species or saltwater species when the ratio is being used to establish freshwater or marine criteria, respectively. 

An example of the calculation of a final acute/chronic ratio is shown in Table V for the pesticide dieldrin. Chronic values for this pesticide were available for only four species in 1980 when the guidelines for dieldrin were established (EPA, 1980a), and therefore it was necessary to use acute/chronic ratios to estabhsh the final chronic value. Acute toxicity values were available in only three of the four cases where chronic effects were studied, but the three species satisfied the criteria for calculating acute/chronic ratios in both freshwater and salt water. Since the acute/chronic ratios for the three species differed by less than a factor of 2, it was appropriate to calculate the final acute/chronic ratio for dieldrin by taking the geometric mean of the three ratios, which is [(11)(9.1)(6.2)] / = 8.5. Final acute values for dieldrin in freshwater (EPA, 1996b) and salt water are 0.48 and 0.71 ppb, respectively. Hence the final chronic values for dieldrininfreshwaterand saltwater areO.48/8.5 = 0.056 ppb and 0.71/8.5 = 0.084 ppb, respectively. 

Final acute value Final chronic value Final plant value Criterion maximum concentration Criterion continuous concentration... 

Under the present system (EPA, 1996b), the two numbers in the criterion are calculated from the final acute value, the final chronic value, and the final plant value. The three values for dieldrin in freshwater and salt water are shown in Table VII. The criterion maximum concentration is equated to half the final acute value. Division by 2 in this case to some extent corrects for the fact that much of the acute toxicity information is based on observations of lethal effects, whereas the real concern is protection of organisms fi om sublethal stresses. The criterion continuous concentration is the smaller of the final chronic value and the final plant value. 

An analogous equation for the final chronic value may be calculated by simply dividing the equation for the final acute value by the final acute/chronic ratio. However, if there is evidence that there is a difference in the functional dependence of chronic toxicity and acute toxicity on water quality characteristics such as temperature and hardness, then the final chronic equation may be determined independently of the final acute equation. In the case of cadmium, for example, chronic toxicity appears to be less sensitive to water hardness than acute toxicity appears to be, thus a final fi eshwater chronic equation was developed solely from chronic toxicity studies performed with 16 fi eshwater species. The final chronic equation for dissolved cadmium in fi eshwater is... 

Talmage et al. [4] analyzed an extensive TNT toxicity database and derived freshwater final acute and chronic values (FAV and FCV, respectively) of 4.99 and 0.410 pmoles E and the respective criterion maximum concentration (CMC) of 2.50 pmoles E that is, half the FAV. The lowest chronic effect value for fish, of 0.176 pmoles E, was suggested as a better screening benchmark than the calculated FCV until a sufficient chronic toxicity database becomes available [4], All these values are above the LOEC from nine-month life-cycle tests with fathead minnows, of 0.06 pmoles L 1 [76] (Table 4.2), suggesting that the proposed chronic values need to be revised for adequate long-term protection of aquatic life. 

Final and Interim Acute and Chronic Values Derived as Water Quality Criteria (pmoles L 1) for the Protection of Aquatic Life... 

Predicted no-effect concentrations (PNECs) are traditionally used in risk assessments and form the basis of most EQS values. There are 3 basic approaches to the derivation of PNECs and resultant EQS values. The traditional method uses standard toxicity data and applies an AF to the most sensitive endpoint to derive a protective concentration. The SSD approach utilizes all available toxicity data to derive a value that is protective of a given percentage (e.g., 95%) of the species and is increasingly being used by many countries, often with a small AF placed on, for example, a predicted HC5 (hazardous concentration to 5% of species, i.e., the 5th percentile of the SSD) based on chronic data. Finally, model ecosystems such as lentic mesocosms can be used to derive safe values, again usually with a small AF. 

Environmental criteria have been established for many of these, but the utility and applicability of such criteria for indoor environments is controversial for at least four reasons. Eor example, the goals of the threshold limit values often do not include preventing irritation, a primary concern in indoor environments with requirements for close eye work at video display terminals. For most of the pollutant categories, the problem of interactions, commonly termed the multiple contaminants problem , remains inadequately defined. Even for agents that are thought to affect the same receptor, such as aldehydes, alcohols, and ketones, no prediction models are well established. Finally, the definition of representative compounds for measurement is unclear. That is, pollutants must be measurable, but complex mixtures vary in their composition. It is unclear whether the chronic residual odor annoyance from environmental tobacco smoke correlates better with nicotine, particulates, carbon monoxide, or other pollutants. The measure total volatile organic compounds is meanwhile... 

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