Voluntary Exposure()• Sometimes referred to as exposure to personal pollutants.– Tobacco• 30% of cancers tied to smoking– Alcohol and other drugs• ½ of all deaths in automobiles accidents tied to alcohol use by drivers• Violent crimes, overdoses, chronic alcoholism General Effect s of Pollutants• Almost every part of the human body is affected by one pollutant or another.Concept of Dose and Response• Five centuries ago, the physician and alchemist Paracelsus wrote that “everythingis poisonous, yet nothing is poisonous.”• For Example– Selenium required in small amounts by living things– May be toxic in high concentrationsConcept of Dose and Response• The effect of a chemical on an individual depends on the dose.– Dose response– Dose dependency can be represented by a generalized dose response curve.Concept of Dose and Response• Doses that are beneficial, harmful, or lethal may differ widely for different organisms and are difficult to characterize.• E.g. fluoride and dental health– Fluorine forms fluoride compounds that prevent tooth decay and promote healthy bone structure.– Toxic effects are noticed at concentrations of 6-7 ppmDose-Response Curve• How individuals will response to a chemical not know.• Instead predictions made about how a percentage of the population will respond to a specific dose.• Dose at which 50% of the population dies– Lethal dose 50, LD-50Dose-Response Curve• The ED-50 (effective dose 50%) is the dose that causes an effect in 50% of thepopulation of observed subjects.– E.g. ED-50 of aspirin would be the dose that relieves headaches in 50% of the people.Dose-Response Curve• The TD-50 (toxic dose 50%) is defined as the dose that is toxic to 50% of thepopulation.– Often used to indicate responses such as reduced enzyme activity, decreasedreproductive success, or onset of specific symptoms.Dose-Response Curve• For a particular chemical, there may be a whole family of dose–response curves.– Which dose is of interest depends on what is being evaluated.– Killing insects vs. pesticide residue– Overlap between the therapeutic dose (ED) and the toxic dose (TD)– Measure of the relative safety of a particular drug is the therapeutic index• Defined as the ratio of the LD-50 to the ED-50.• The greater the therapeutic index, the safer the drug.Threshold effects• Threshold is a level below which no effect occurs and above which effects begin to occur.– If a threshold exists, then a concentration below the threshold is safe.– If there is no threshold dose, then even the smallest amount has some negative toxic effect.• A problem in evaluating thresholds for toxic pollutants is that it is difficult to account for synergistic effects.Ecological Gradients• Changes in vegetation with distance from a toxic source define the ecological gradient.– Weedy species adapted to harsh conditions may be closer– Farther away trees and shrubsTolerance• The ability to resist or withstand stress resulting from exposure to a pollutant orharmful condition.– Result from behavioral, physiological, or genetic adaptation.• Behavioral tolerance- change in behavior– Learning to avoid trapsTolerance• Physiological tolerance- the body of an individual adjusts to tolerate a higher level of pollutant.– Many mechanisms including detoxification• the toxic chemical is converted to a nontoxic form– Internal transport of the toxin to a part of the body where it is not harmful, such as fat cells.Tolerance• Genetic tolerance- (adaptation) when some individuals in a population are naturally more resistant to a toxin than others.– Strains of mosquitoes resistance to DDT– Antibiotic resistanceAcute and Chronic Effects• Acute effect is one that occurs soon after exposure.– Usually to large amounts of a pollutant• Chronic effect takes place over a long period– Often as a result of exposure to low levels of pollutantRisk Assessment• The process of determining potential adverse environmental health effects to people exposed to pollutants and potentially toxic materials.Risk Assessment• Such an assessment generally includes four steps:– Identification of the hazard.– Dose–response assessment.– Exposure assessment.– Risk characterization.• Risk assessment is difficult, costly, and controversial.• Risk management integrates the assessment of risk with technical, legal, political, social, and economic issues.
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