Risk assessment methods concentration-exposure time

SCCPs are persistent and bioaccumulative, and thus concentrations in the environment and biota are expected to increase with continued release to the environment. Standard risk assessment methods comparing effect levels to environmental concentrations may underestimate the risk of persistent and bioaccumulative substances, such as SCCPs. Persistent substances can take decades to reach a maximum steady state concentration in the environment, resulting in an underestimation of the potential exposure to these compounds if steady-state has not been achieved, and releases into the environment continue. Similarly, it can take a long time for persistent and bioaccumulative substances to reach a maximum steady-state concentration within an organism this is supported by the observations of Sochova et al., [62] who noted an increase in toxicity of SCCPs for longer exposure duration with nematodes. The durations of standard toxicity tests may be insufficient to achieve the maximum tissue concentration, resulting in an underestimation of the effect threshold. 

The environmental concentration of a stressor, either measured or estimated, is compared with an effect concentration such as an LC50 (lethal concentration to kill 50% of individuals in a theoretical population in a set period of time) or no observed effect concentration (NOEC) [31, 32]. These are simple ratios of single exposure and effects values and may be used to express hazard or relative safety. This deterministic method uses point estimates to represent one or more factors in a risk assessment and treats them as if they were fixed and precisely known [33]. The calculation of HQs... 

The manner in which a test substance is applied to the skin surface can be a major determinant of its subsequent penetration and absorption. Several factors must be considered when selecting a suitable application procedure, including the nature of the vehicle, the permeant concentration, the amoimt of vehicle applied, the mechanism of application, the exposure time, and the method for removing an applied vehicle (if required). Many of these issues may be intrinsic to the purpose of the study. For example, risk assessment involving the study of the skin permeation of an ingredient in a cormnercially available product should be performed with the material in the formulation as it is marketed or used and with an application regime that mimics as closely as possible the in-use situation (see, for example. Brain et al., 1995 Walters, Brain, Howes, et al., 1997). 

Because one has to consider unpredictably higher concentrations, especially in the case of the last two jobs mentioned, the use of respiratory protection is often necessary. For selection of the most suitable devices, concentration measurements have to be performed. It is important to consider the surrounding environment of workplaces if other persoimel occasionally work in these areas without respiratory protection. If jobs are of short duration, short-term measurements need to be performed to assess the exposure. A risk assessment of short-term exposures on the basis of an 8-h measurement is only possible in exceptional cases. For short-term exposure measurements, not every sampling technique is suitable. In this case, special sampling methods must be used in order to achieve a sufficiently low detection limit during short measurement times, especially for complex mixtures of substances. In many cases of this type thermodesorption has been proved to be the method of choice. 

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