Environmental media, typical concentrations

The RfDs and TDIs are often used to establish regulatory standards. Such standards usually specify a limit on the allowable concentration of a chemical in an environmental medium. The process is not difficult to understand. The RfD and its related estimates of population thresholds is a dose, typically expressed in mg/(kg b.w. day), that is considered to be without significant risk to human populations exposed daily, for a lifetime. Consider mercury, a metal for which an RfD of 0.0003 mg/(kg b.w. day) has been established by the EPA, based on certain forms of kidney toxicity observed in rats (Table 8.4). These are not the only toxic effects of mercury, but they are the ones seen at the lowest doses. Note also that we are dealing with inorganic mercury, not the methylated form that is neurotoxic. 

Intake of lead or other substances in humans is typically indexed as daily intake. However, intakes scaled for different time frames have been employed in different settings over the years, e.g., weekly intakes (FAO/ WHO, 1993) of Pb and other contaminants per body weight. This specifically applies to ingestion, inhalation, and, in some cases, dermal application. Daily lead intake into body compartments is the product of lead concentration in some medium and the mass (diet Pb) or volume (air Pb, water Pb) of lead-containing medium taken in daily. High levels of lead in an environmental medium can be quite toxic when ingested in relatively modest quantities daily. 

After biosynthesis of the polyester and separation of the bacterial biomass from the supernatant, the required recovery process (typically a solid-liquid extractiOTi procedure) can constitute another not negligible cost factor, especially in large-scale production. Here extraction solvents that can easily be recycled will be of interest [53]. In order not to leave the patterns of sustainability in biopolymer production, it will be indispensable to concentrate the development of new extraction processes on such recyclable solvents that are also of environmentally sound nature [54], Typical harmful chlorinated solvents like chloroform must be avoided. A PHB production process embedded in an ethanol production plant has the advantage to utilise the medium chain length alcohol fraction (fusel alcohols) from the distillery step, consisting mainly of iso-pentanol. The application of the fusel alcohols as extracting solvents unites two important points On the one hand, this liquid normally constitutes a surplus product that has little market value. When used as an extraction solvent the costs for alternative solvents are saved. Furthermore, this extraction solvent is less harmful to handle than the classical extraction solvent chloroform [27],... 

Concentration" Compounds at low concentrations in relatively large volumes can be concentrated by extraction onto a solid phase extraction medium and then eluted m a small volume of a strong eluent. This is typically used for concentrating analytes that are present in only trace amounts, such as drug metabolites in serum samples, or environmental contaminants in seawater. 

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