Exposure, chemical aggregate

Cement and Concrete Concrete is an aggregate of inert reinforcing particles in an amorphous matrix of hardened cement paste. Concrete made of portland cement has limited resistance to acids and bases and will fail mechanically following absorption of crystalforming solutions such as brines and various organics. Concretes made of corrosion-resistant cements (such as calcium aluminate) can be selected for specific chemical exposures. 

Thus, it can be said that in practically all activities there are risks to health related to the presence of chemical agents. To evaluate their severity we should consider whether the exposure occurs continuously or at irregular intervals, the possible entry routes to the body, the concentration changes with time and place, as well as the aggregation state of the contaminant in liquid, dust, mist, fume, or vapor form. 

Aggregate the hazard for multiple chemical agents and exposure pathways as a hazard index, where appropriate. 

Anthraquinone leuco dyes are widely known as vat dyes.10 Vat dyes possess extensively conjugated aromatic systems containing two or more carbonyl groups, e.g., anthraquinone, indigoid chromophores. The colored form of vat dyes are insoluble in water. The dyes are applied by a process whereby the dye is converted to the reduced form (leuco dye) which is soluble in water and can penetrate into a cellulosic fiber. On exposure to the atmosphere the leuco form is oxidized to the original quinoid form which then precipitates as an aggregate. Vat dyes generally have excellent chemical and photochemical stability. 

Rubber process dust is dust arising in the stages of mbber manufacture where ingredients are handled, weighed, added to or mixed with uncured material or synthetic elastomers. The aggregated maximum exposure limit in the UK is 6 mg/m3 (EH40/2000). The exception is chemicals for which specific exposure limits exist. 

The cumulative risk index (CRI), also referred to as the aggregate risk index (ARI) has been suggested by the US-EPA (Whalan and Pettigrew 1997) to combine MOEs for chemicals with different AFs. The risk index (RI) of a chemical is the MOE divided by the AF or simply the reference dose divided by exposure, and is the reciprocal of the HQ ... 

Although causal evidence in humans of the link between chemical exposure and cancer is quite limited, one might expect that greater aggregate exposure to chemicals in an increasingly industrial society would cause higher aggregate cancer rates over time. 

Until very recently the risks associated with different types of chemicals such as food additives, pesticides, environmental contaminants and natural constituents of food were assessed and managed separately. However, a particular substance might fall into two or more of these categories and so the opportunity for simultaneous exposure might be overlooked. Furthermore, exposure to a chemical could occur through diet, drinking water, air pollution or dermal absorption. Aggregate exposure assessment aims to take all of the possible sources and routes of exposure into account in a realistic manner and thereby obtain a better overall estimate of risk. Initiatives have been set up in both the... 

Micro-exposure event modelling combined with probabilistic modelling provides a great opportunity to assess real aggregate exposures in the real world. However, the method is highly dependent on the availability of complete and accurate information about levels of contamination and human behaviour. Whilst some of this is available, particularly in the context of pesticides, for other chemicals it may be a long time before this approach can be employed. 

Exposure assessments characterize the water, diet, and herbicide handling exposure pathways for atrazine and simazine (Sielken et al, 1996, 1998). For each exposure pathway, the chemical-specific doses (mg/kg/day) from each relevant route (ingestion, inhalation, and dermal) are summed. The total chemical-specific dose for each exposure pathway is characterized separately, and then these doses are aggregated by summing over the multiple exposure pathways. The pathway-specific and aggregate assessments are performed separately for atrazine and simazine. In addition, because atrazine and simazine are assumed to have a common mechanism of toxicity, a cumulative exposure assessment is performed combining the doses of atrazine and simazine. 

Cumulative risk assessments evaluate the health risk for aggregate exposures accumulated over time and for multiple contaminants or stressors. In some contexts (e.g. USEPA pesticide risk assessments), cumulative refers specifically to combined exposures to chemicals that share a common mechanism of toxicity (see http // www.epa.gov/oppsrrdl/cumulative/). Populations may be defined by their location relative to sources, their activities and customs, and their susceptibility to exposures. In this context, populations can include different ethnic groups, different communities, or different age groups. Cumulative risk is a very important concept in understanding environmental health risks to children in different settings, particularly in underdeveloped countries where children may be facing multiple stressors. 

Aggregate risk assessment The evaluation of risk from exposure to a given chemical from multiple routes of exposure. 

Several studies since then have supported this suggestion, and now it is widely accepted that conformational change/structural perturbation is a prerequisite for amyloid formation. Structural perturbation involves destabilization of the native state, thus forming nonnative states or partially unfolded intermediates (kinetic or thermodynamic intermediates), which are prone to aggregation. Mild to harsh conditions such as low pH, exposure to elevated temperatures, exposure to hydrophobic surfaces and partial denaturation using urea and guanidinium chloride are used to achieve nonnative states. Stabilizers of intermediate states such as trimethylamine N-oxide (TMAO) are also used for amyloidogenesis. However, natively unfolded proteins, such as a-synuclein, tau protein and yeast prion, require some structural stabilization for the formation of partially folded intermediates that are competent for fibril formation. Conditions for partial structural consolidation include low pH, presence of sodium dodecyl sulfate (SDS), temperature or chemical chaperones. 

The Food Quality Protection Act (FQPA) in the USA and the Pest Control Products Act (PCPA) in Canada mandate that potential risks to infants and small children be specifically addressed. When assessing the food use of a pesticide, in order to assure that there is a reasonable certainty that no harm will result to infants and children from aggregate exposure to the pesticide s chemical residues , the FQPA and PCPA in the case of threshold effects call for an additional tenfold margin of safety for the pesticide chemical residue and other sources of exposure to be applied to estimating risks to infants and children. A different margin of safety may be used only if, on the basis of reliable data, such a margin will be safe for infants and children. In Canada, these same requirements also apply to non-food-use pesticides. 

This chapter describes and illustrates probabilistic approaches to aggregate and cumulative assessments of exposure, dose and risk. Aggregate assessments account for multiple sources (e.g. food, water, residence and occupation) and multiple routes (ingestion, dermal and inhalation) of exposure for a single pesticide. Cumulative assessments combine exposures for chemicals that share a... 

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