Once a chemical is absorbed and distributed through the plasma pool, it attaches itself to the molecular target either directly or after metabolic activation, then a cascade of biochemical and physiological changes occurs, which triggers the morphological, clinical expression of toxicity. Evaluations of human exposure might be performed at any stage of this process but their significant is obviously different.
The best available tools are in the area of biomarkers of exposure is the of measurement of neurotoxic chemicals and their metabolites in biological fluid which provide useful and reliable indicators of exposure Henderson et al. An ideal biomarker of exposure is chemical specific, detectable in trace quantities, inexpensive and quantitatively relatable to prior exposures. Also, the binding of a toxicant to hemoglobin is considered a good biomarkers to measure cumulative internal dose due to repeated exposures, because red blood cells are long-lived approximately 4 months in humans , while adducts to albumin reflect more recent exposure because albumin has a shorter lifetime in blood days Henderson et al.
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Biomarkers of effect should reflect early biochemical modifications that precede structural or functional damage. Thus, knowledge of the mechanism s that led to ultimate toxicity is necessary to development specific and useful biomarkers. Such markers should identify early and reversible biochemical events that may also be predictive of later responses Silbergeld, The oldest and probably still the best example of the application of such strategy to neurotoxic compounds is represented by the measurement of red blood cell AChE following exposure to OPs.
Erythrocyte AChE, in particular, was found to be better correlated with brain or diaphragm activity than plasma ChE Padilla et al.
OPs such leptophos, EPN, cyanofenophos, trichloronate and salithion proved to cause irreversible ataxia in chicken, mice and sheep and their AChE inhibition stands for their acute toxicity, while NTE inhibition is responsible for their paralytic ataxia and can be used as a standard screening method for delayed neuropathy El-Sebae et al.
The precise measurement of ChE erythrocyte or plasma can be useful as a measurement for low level exposure to OPs in epidemiological research Richter et al. Recent studies conducted have shown that lower group mean plasma and erythrocyte ChE level in populations living adjacent to cotton fields which are sprayed regularly with OPs Richter et al.
Also, urinary alkylphosphates are sensitive indicators of OPs exposure, and have been shown to correlate with symptoms. Although there are inherent differences in kinetics of substrate hydrolysis between rodent and human erythrocyte AChE, the kinetics of inhibition in vitro by carbofuran and carbaryl as estimated by the comparative of ki are quite similar between species and may be useful in human risk assessment. Also, optical sensors can be used for detection anticholinesterases by immobilizing fluorescein isothiocyanate FIC -tagged ell organ AChE on quartz fibers and monitoring enzyme activity Rogers et al.
These biosensors detect concentrations of the carbamate insecticides such bendiocarb and methomyl and the OPs echothiophate and paraoxon in the nanomolar to micromolar range.
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On the other hand, malathion, parathion, and dicrotophos were not detected even at millimolar concentrations, but, longer exposure or prior modification of these compounds i. These AChE biosensors are fast, sensitive, reusable, easy to operate and portable, so it show potential adaptability to field use. So, the measurement of blood AChE activity remains an excellent biomarker for exposure and effect of OPs exposure under both acute and chronic conditions.
The sensitivity of human lymphocyte NTE to several OP inhibitors is similar to that of the nervous system enzyme. The measurement of alteration in antioxidants or oxygen free radical scavenging enzymes can be used as biomarkers for exposure and effects. Tests exploring the toxicokinetics include measurements of the chemical or its metabolites in body fluids.
Virtually all pesticide exposure can be assessed in this way, only depending on availability of analytical procedures. Furthermore, the understanding of the mechanism of action and the availability of a biomarker effect allows studies on quantitive relationships between the concentration of the compound or its metabolites in the body fluids and their effect on the target.
Most OPs are activated to their corresponding oxygen analog by an oxidative desulfuration reaction, which is catalyzed by cytochrome P Vasilic et al. Upon phosphorylation of AChE, a portion of the molecule, the leaving group, is released and excreted. Both the parent compound and the oxon can undergo a series of detoxication reactions that are mediated by various A-esterases paraoxonase, carboxyesterase , by P, and by glutathione transeferases.
The leaving group, p -nitrophenol in the case of parathion, which is also generated by hydrolytic cleavage, and alkylphosphates are excreted in the urine and can be quantified as an index of OPs exposure Richter et al. For occupational exposures, pesticides are unusual in that dermal residues are often the most important source of systemic absorption.
brigernanfuddding.gq In general, respiratory exposure in the occupational setting is much less than dermal exposure, with the exception of exposure to aerosols, powders or dust, concentrated vapors, work in enclosed spaces, or pesticides which are gaseous at room temperature or on contact with water especially the fumigants. Dermal absorption of drift may contribute to community exposures, and for evaluation of community exposure, monitoring of respirable residues is important in the research setting WHO, a.
In agricultural communities little studies have been done to evaluate routes of exposure to communities exposed through skin and by aerosol inhalation to drift from adjacent fields, although some studies suggest significant systemic absorption of pesticides, resulting in ChE depression, among persons in such communities under selected conditions Richter et al.
The provision of dermal absorption data is required for the registration of agrochemicals, particularly in USA Scott et al. Sharp , and Maddy reported that exposure of users of pesticides containing active ingredients which have the potential of causing adverse effects, especially chronic effects, has to be accurately measured in order to make meaningful risk assessment and risks mitigation determinations. Analysis of residues on cloth pads that had been worn on various parts of the body may provide an overestimate exposure than in the case when such studies are done on humans or other primates.
The measurement of dialkyl phosphate metabolites in urine provides a sensitive biological monitoring method suitable for use in the assessment of occupational exposure to many OPs.
Metabolites were detected in people with exposure to OPs at levels below those that cause a decrease in ChE activity. OPs exposure among children living in two Seattle metropolitan communities were assessed by measuring urinary metabolites, and identified possible exposure risk factors through a potential interview. Concentrations of dialkyl phosphate compounds, the common metabolites of OPs, were significantly higher in children whose parents reported to pesticide use in the garden Lu et al.
Although it is intuitive that pesticides which bioaccumulate and biomagnify are of special concern to those species that consume them, the relative contribution of these processes to toxicity is dependent on trophic level in the food web, life stage, physiological conditions favoring lipid mobilization e.
Also Due to the lipophilic nature of DDT and its principle metabolite, DDE, these compounds have been found in diverse human samples of serum, adipose tissue, and breast milk Woodruff et al. The half-life of DDT in human adipose tissue is approximately 7. The ratio between the levels of DDT in adipose tissue and blood was to 1. Risk characterization is the last step of the risk assessment process. This step evaluates assessments of human health and ecological effects, identifies human sub-populations or ecological species potentially at risk, and delineates areas of uncertainty, limitations, and assumptions made in the risk assessment.
Pesticides occupy a rather unique position among the many chemicals that man encounters daily, as they are deliberately added to the environment. Ideally their injurious action would be highly specific for undesirable target pests. However, most of pesticides are not highly selective to many nontarget species, including humans, and other desirable forms of life that coinhabit the environment.
The ecological risk assessment evaluates the likelihood of adverse ecological effects caused by any chemical, physical or biological entity including pesticides that induce adverse effects on the components individuals, population, communities or ecosystem Norton et al. Pesticides may affect the non-target organisms by direct contact or through translocation from the sites of application through the various media. The extent to which translocation within the environment occurs will depend to a large degree on the physicochemical properties of the pesticides Murphy, Factors that affecting the risk assessment of pesticides include the application rate and time, sorption processes in the soil, uptake by crops, volatilization, biotic and abiotic transformation, mineralization, factors influencing the biodegradation of active ingredient in soils, mobility and leaching, and drinking water quality aspects Pawlizki, The "Framework for Ecological Risk Assessment" which is developed for risk assessment of ecological effects is illustrated in Fig.
This framework is conceptually similar to the approach used for human health risk assessment, but it is distinctive in its emphasis in two areas.
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First, ecological risk assessment can consider effects beyond those on individuals of a single species and may examine a population community, or ecosystem. Second, there is no single set of ecological values to be protected that can be generally applied. The nature of environment exposures to chemicals and the population variability in response make it difficult to determine population risk from traditional epidemiological studies Spear, This has given rise to attempt to predict risk from environmental transformation and transport, exposure mechanisms, and biological response probabilities.
For readily degradable chemicals emitted at intervals, the ecological risk can be related to the time taken for the chemical to fall to a level causing no effect on most individuals, e. The deleterious effects of endocrine-disrupting chemicals in the environment on the reproductive success of wildlife population have been documented Colborn et al.
These deleterious health effects have been observed in the presence of numerous man-made chemicals. DDT applied in a mosquito control.
Such small quantities that may be present in mud and surface waters are taken up by plankton and other food sources for phytophagous fish. The phytophagous fish are eaten by carnivorous fish. These fish may migrate and be ingested by birds in Arctic climates, such as falcons and eagles, in sufficient quantities to contribute doses of the insecticide or its metabolites that can affect avian reproduction. The reproductive parameters such as thinned eggshells and lowered the thickness index of American kestrels Faclo sparverius after exposure to kelthane at dietary concentration have been affected Clark et al.
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Also, insecticides may affect bird populations by reducing the insect prey base available to the birds, while herbicides affect various bird populations through a variety of pathways, including a direct reduction of the food base of granivorous species, reducing invertebrate abundance by removing the plants that invertebrates depend on as food or habitat, and reducing nesting cover O'Conner, For human health, a number of factors contribute to a wide range of risks, including endogenous factors such as genetic predisposition, age embryo, fetus and children and gender and exogenous factors which include diet, disease conditions, climate and past exposures.
Endocrine disruption of the developing brain can permanently alter the behavior, whereas similar exposures of fully differentiated brain could be without effect Kavlock et al. There are specific critical periods of sensitivity to endocrine disruption which vary for different organs and species.
The unique changes in physiology during development may increase sensitivity to endocrine-disrupting agents. Also, adult males and females are affected by endocrine disruptors, and the physiologic states in the adult e. Infants and children are growing and developing. Their metabolic rates are more rapid than those of adults. Also, there are differences in their ability to activate, detoxify, and excrete xenobiotic compounds. It is not clear why this occurs, but it could be due to an age-related decrease in percutaneous absorption or an age-related difference in the inflammatory cascade.
Newborns, on the other hand, especially preterm neonates, have immature epidermal barriers, which can lead to potential problems with percutaneous absorption of toxins Kravchenko, I. All these differences can affect the toxicity of pesticide to infants and children, and for these reason the toxicity of pesticides is frequently differed in children and adults. The quantitive differences between children and adults are usually less than a factor of approximately fold NAS, Many OPs appear to be better inhibitors of ChE, suggesting that this enzyme may be a more sensitive indicator of exposure.