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Michael H. Don g MPH, DrPA, PhD readin gs Epidemiologic Side of Toxicology (6th of 10 Lectures on Toxicologic Epidemiology)

Michael H. Dong MPH, DrPA, PhD readings Epidemiologic Side of Toxicology (6th of 10 Lectures on Toxicologic Epidemiology)

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  • Michael H. DongMPH, DrPA, PhDreadingsEpidemiologic Side of Toxicology(6th of 10 Lectures on Toxicologic Epidemiology)

  • Taken in the early 90s, when desktop computers were still a luxury.

  • Learning Objectives

    Appreciate the importance of the epidemiologic side of toxicology.Study the epidemiologic relevance through three historical events.Learn the impact of epidemiology, which is dynamic, case-dependent, and often tremendous.

  • Performance Objectives

    Able to describe the toxicologic course of the three historical events presented.To characterize the epidemiologic side of these courses and events.To outline the purpose as well as the principles of presenting the three toxicologic events.

  • Three Case Studies for the Epidemiologic Side of Toxicology: 1. Jamaica Ginger Epidemic 2. London Smog of 1952 3. Multistage Model of Carcinogenesis

  • Ginger Paralysis: Syndrome & CauseCharacterized by ataxia, muscular weakness, unsteady gait, flaccid paralysis of the legs.Also known as jake leg, wrist drop, and foot drop.Caused by exposure to TOCP, with delayed onset of 1 to 3 weeks.

  • Ginger Paralysis: History and ToxicityThe syndrome known for a century; and later, as ginger paralysis due to its first major episode in the USA.TOCP is the most toxic isomer of TCP; both, like some other OP, can induce delayed neurotoxicity.The initial effects likely involve the inhibition of neurotoxic esterase.

  • Ginger Paralysis: The Epidemiologic Side20,000 cases reported in the USA in 1930, related to consumption of illegal alcohol contaminated with TOCP.10,000 cases reported in Morocco in 1959, related to consumption of food cooked in oil contaminated with TOCP.The association was initiated by two Oklahoma doctors: Miles and Goldfain.

  • Ginger Paralysis: The Impact of EpidemiologyIt was the first major epidemic that enabled Smith et al. to focus on TOCP as the prime suspect.A large number of toxicology studies were hence launched, along with U.S. EPAs development of a regulatory guideline specifically for testing delayed neurotoxicity.

  • Ginger Paralysis: The Lesson (and Speculation)Despite the Jamaica ginger episode, there were still numerous outbreaks involving cooking oil contaminated with TOCP.Delayed neurotoxicity is extremely specific to chemical structure.It was epidemiologic evidence that advanced the toxicology of TOCP.

  • London Smog: History & the Epidemic4,000 premature deaths, with most being elderly or having preexisting diseases, from breathing heavily polluted air in London in 1952.Smog is a mixture of smoke and fog, now also involving the equally irritating photochemical air pollution.Air pollution is predictable, and was recognized back in the Roman period.

  • London Smog: Toxicity of the Air PollutantsSulfur dioxide is an upper airway irritant.Carbon dioxide is a potent asphyxiant.Nitrogen dioxide causes severe irritation of the innermost parts of the lungs.Ozone is a reactive and toxic form of elemental oxygen.

  • London Smog: Other Pollutants & Newer ProblemsOther pollutants: suspended particulate matter (e.g., black smoke); and volatile organic compounds (e.g., petroleum benzene as an exhaust product).1.6 million people may now be at risk from poor air quality in urban areas throughout the world.There are also areas everywhere filled with traffic-generated pollutants.

  • London Smog: The Impact of EpidemiologyThe 1952 incident led to the passage of the British Clean Air Act of 1956.More epidemiologic studies have since been conducted to cope with air pollution problems and episodes.Also more studies on long-term toxic effects and on photochemical formation.

  • London Smog: The LessonThe older winter smog problem in London and worldwide is now being replaced with summer smogs from photochemical formation.The adverse health effects of the winter or summer air pollutants cannot be investigated using classic toxicology studies alone.

  • Multistage Model: The Course of CarcinogenesisCarcinogenesis is the biochemical process characterizing the progression of normal cells to neoplastic and later into tumor cells.Multistage model is a quantitative as well as a mechanistic theory used to characterize this biochemical process.Two of the stages basic to the model are presumably initiation and promotion.

  • Multistage Model: The Underlying TheoriesIn addition to being an initiator or a promoter, an agent initially can be a precarcinogen and later be transformed into a harmful ultimate carcinogen.Initiation is usually irreversible, of short duration, and invisible, whereas promotion has the opposite effects.As an outgrowth of the challenge to the single stage and the multicell theories.

  • Multistage Model: The Epidemiologic SideThe single stage model and the multicell model were found to be incompatible with epidemiologic data.Although the multistage model is based on a single cell theory, its development was driven by epidemiologic data that many cancer incidences increased with the 5th or 6th power of age (that also implicating latency period).

  • Multistage Model: The Relevance and ImpactGaining wide acceptance due to the strong evidence that cancer is a single cell in origin.Useful as a quantitative tool in the cohort analysis of tumors induced by chemicals.Found for the large part successful in describing many experimental and epidemiologic data.

  • Multistage Model: The Lesson & Its UtilitiesLeading to the use of more proper mitigation measures; and to the adoption by regulatory agencies for cancer risk assessment.Toxicologists using animal studies, without this epidemiology-based theory, would likely fail to elucidate or make fuller use of the mechanistic process of carcinogenesis.

  • Overview of Next Lectures Human Exposure Assessment I & IIHuman exposure assessment is one of the key components in the health risk assessment.Lecture 7 (Assessment I) will cover the direct measurement methods.Lecture 8 (Assessment II) will focus on the indirect measurement methods.

    This is the sixth of 10 lectures on toxicologic epidemiology. It has been prepared to provide the students with three toxicologic events for the purpose of illustrating the epidemiologic side of toxicology. The linkage between epidemiology and toxicology was introduced briefly in Lecture 1, and further conceptualized in the second through fourth lectures. It is thought that for some toxicologist and epidemiologist practitioners, the use of historical cases is perhaps the most direct approach to convincing them about the intimate relationship between the two professions. Three historical epidemics were already presented in the last (fifth) lecture praising the toxicologic side of epidemiology.Note that the distinction made in this and the last lecture between the two sides is more a gimmick of drawing the students attention, than a representation of the reality. For each of these six cases and others not discussed, there is some truth that one side might have initiated or dominated the historical events or undertaking, but that would be pretty much the extent of the distinction. The titles of the 10 lectures are: (1) Toxicology and Epidemiology; (2) Public Health and Risk Assessment; (3) Toxicology and Risk Assessment; (4) Epidemiology and Risk Assessment; (5) Toxicologic Side of Epidemiology; (6) Epidemiologic Side of Toxicology; (7) Human Exposure Assessment I; (8) Human Exposure Assessment II; (9) Characterization of Health Risk; and (10) Toxicologic Epidemiology.Dr. Michael H. Dong, born in Hong Kong, holds a DrPA (Doctor of Public Administration) in health policy from the University of Southern California, and a PhD in Environmental Epidemiology from the University of Pittsburgh. He has also earned a BSc in biochemistry from the University of California at Davis/Riverside, a second BSc in forensic science from the California State University at Sacramento, and a MPH in environmental and nutritional sciences from UCLA. Dr. Dong currently is a Diplomate of the American Board of Toxicology (DABT) and a Certified Nutrition Specialist (CNS).Dr. Dong has been working for 11 years as a regulatory toxicologist at the State of California Department of Pesticide Regulation. Previously, he was on active duty for several years as a U.S. Public Health Service Commissioned Corps officer, working at the Center for Drug Research and Evaluation, U.S. Food and Drug Administration. He also served for several years as a U.S. Army Medical Service Corp officer with various assignments working as a nutritional, clinical, or research biochemist. Between the two military services, Michael worked for a year as an occupational toxicologist as well as an environmental epidemiologist at the Occupational Safety and Health Administration, U.S. Department of Labor.Michaels academic and professional trainings, his selected published work in such innovative concepts as physiologically-based pharmacokinetic (PB-PK) modeling and aggregate exposure assessment through Monte Carlo simulation, and the series of 10 lectures on toxicologic epidemiology, all point to his continuing interest in and commitment to promoting global health. In this lecture, students will learn that one critical side of toxicologic pursuits is epidemiology or the undertaking by epidemiologists. As defined in Lecture 1 (Slide 15), epidemiology is the study of factors that modify or influence the distribution of diseases in a (human) population. In many ways, toxicology is a discipline also striving to achieve ultimately much the same goals. The main difference between the two sciences is the study tools and subjects that they employ. Toxicologists tend to rely on controlled experiments, in vitro assays, and laboratory animals to pursue their professional interests. Epidemiologists on the other hand involve themselves more in associating human exposures to outcomes. In addition, the two professions are often different in their focus on the same health problems.Through three historical toxicologic events, students will learn the importance of epidemiology to toxicologic pursuits. They will appreciate that without this side of the input, many toxicologists cannot pursue too far with their professional or academic interests.Students will learn too that the impact of epidemiology on toxicologic events is not only dynamic, case-dependent, but usually also tremendous. They will find that in some instances, epidemiologic information is needed to assess the adverse effect(s) of a host of pollutants each with their acute toxic effects already well studied. Yet in other cases, the same type of evidence can be used to confirm the mechanism of toxicity in question. As with the last lecture, at the end of this lecture, students should be able to outline the epidemiologic incidents and the toxicologic events for each of the three case studies presented here. Each case has had a course of actions or incidents that sets its history. Students should also be able to characterize the epidemiologic information that is deemed critical to the events. Although some epidemiologic information cannot bring a toxicologic event to a closure, at the minimum it would narrow the knowledge gaps that otherwise cannot be completely filled with toxicology studies alone. In addition, students should contrast the various impacts made by the epidemiologic information on these toxicologic events, and be able to conceptualize the difference in these impacts. Insofar as the same type or the same level of epidemiologic information is not always needed or readily attainable, the importance and relevance of epidemiologic information to a toxicologic event necessarily varies with the situation involved. Finally, students should be able to appreciate the purpose for which the three case studies were presented in this lecture. There are numerous toxicologic events in history. The case studies were purposely selected to represent certain important concepts and principles which were highlighted in the slides labeled The Lesson (that to be learned). Like the three case studies selected in the last lecture, the three selected for illustration here are all too familiar to many epidemiology or toxicology students. These are the ginger jake paralysis epidemic, the London smog of 1952, and the multistage model of carcinogenesis.The ginger jake paralysis episode was a result of delayed neourotoxicity induced by tri-ortho-cresyl phosphate (TOCP), which is one of the organophosphates (OP) generally recognized as having the ability to selectively inhibit the acetylcholinesterase (AChE) enzyme activity in mammals and hence to cause fatal hyperactivity of their acetylcholine neurotransmitter. The London smog of 1952 was a predictable environmental disaster involving fatal respiratory stress caused by breathing heavily air pollutants each with their acute toxic effects already well studied. It was this 1952 incident that had brought the issue of air pollution to global attention. Carcinogenesis is the biochemical as well as mechanistic process that characterizes the transformation of normal cells to neoplastic cells and the further development of neoplastic cells into a tumor. The focus with this case is to demonstrate how epidemiologic information can be used to confirm the multistage development of certain forms of human cancer. Numerous toxicologic events occurred in history. The above three case studies were selected primarily because they represent a wide spectrum of use of epidemiologic studies, even though these cases all occurred within a period of three decades running from the 1930s to the 1950s.Like the nutritional disease pellagra discussed in the last lecture, in the United States ginger jake paralysis is a disease of the past. This paralysis, also nicknamed jake leg, is the common type of organophosphate-induced delayed neuropathy (OPIDN). It is characterized by muscular weakness, unsteady gait, ataxia, and flaccid paralysis of the limbs, particularly the legs. Historically, the first organophosphate (OP) used as an insecticide was the extremely potent acetylcholinesterase (AChE) enzyme inhibitor tetraethylpyrophosphate (TEPP) developed in Germany in 1942. Other 200 some OP esters available today include the more commonly used pesticides malathion and diazinon, and the widely used liquid plasticizer tri-ortho-cresyl phosphate (TOCP).All OP are generally considered as capable of inhibiting the AChE enzyme in mammals and insects. When AChE is severely inhibited, the acetylcholine neurotransmitter will not be hydrolyzed but accumulated to cause mild or fatal effects related to excessive nerve response. Some such adverse neurologic effects have a delayed onset and, as stated above, are referred to as OPIDN. Through its metabolic products, and hence presumably leading to a delayed (1 to 3 weeks) type onset, TOCP can inhibit AChE and was proven to be responsible for the ginger jake syndrome. One of its neurotoxic metabolites seen to be definitely associated with the jake leg syndrome is tri-ortho-tolyl phosphate. The delayed peripheral paralysis that follows the early syndrome of nausea, vomiting, and diarrhea is typically evidenced by wrist drop and foot drop. As stated in Ecobichon (1996), the syndrome like ginger jake paralysis has been known for a century. It is a delayed neurotoxicity, especially that induced by organophosphates (OP). The initial symptoms are characterized by muscle weakness in the arms and legs, eventually leading to clumsy, shuffling gait, hyperreflexia, and even a permanent damage to the pyramidal tracts. The syndrome is later known as ginger jake paralysis due to its first major epidemic resulting from the consumption of an illegal alcohol Jamaica ginger jake contaminated with the OP ester tri-ortho-cresyl phosphate (TOCP).The biochemistry and toxicity of TOCP (available as a colorless liquid) were studied extensively soon after the syndrome was associated with the ginger jake episode. TOCP is apparently the most toxic isomer of tricresylphosphate (TCP), the latter also being available as an industrial chemical with numerous commercial applications. TCP is relatively nontoxic and has been widely used as a lubricant, gasoline additive, plasticizer, and flame retardant.TCP as well as TOCP, together with some OP, can induce a delayed neuropathy after a single dose. The initial effects of these OP do not seem to involve the inhibition of acetylcholinesterase, but rather the inhibition of the neurotoxic esterase (NTE) enzyme (Johnson, 1975, 1976). The exact role of NTE inhibition in the initiation of delayed neuropathy remains unknown, although its enzymatic activity is known to be highest in nervous tissue (see, e.g., Hodgson et al., 1998). Early in 1930, American newspapers in the south and midwest began to report on a strange new paralytic illness that eventually had affected some 20,000 people. This first major epidemic was soon linked to the consumption of Jamaica ginger commonly referred to as jake by those using the fluid extract as an alcohol substitute during the prohibition years. The association was initiated by two Oklahoma doctors named E. Miles and W.H. Goldfain (Parascandola, 1994).The alcoholic extracts of Jamaica ginger was not considered as the culprit at the time, because it had been used in the United States since the 19th century, long before the advent of the Prohibition (of the 1930s). Working closely with colleagues in other bureaus and laboratories, Dr. Maurice Smith of the U.S. Public Health Service eventually and shortly confirmed that tri-ortho-cresyl phosphate (TOCP) was the compound responsible for the jake leg syndrome. Because of its relatively low acute toxicity, TOCP almost escaped blame as the poison causing the epidemic. Yet fortunately or unfortunately, TOCP was firmly identified as a constituent of the adulterated jake. And the same delayed paralysis symptoms were seen in rabbits and some other species dosed with a 2.5% solution of TOCP in 80% alcohol (Smith and Elvove, 1930).Another 10,000 people in Morocco were also poisoned with TOCP in 1959. The victims had eaten food cooked in olive oil adulterated with lubricating oil; and the latter happened to be contaminated with TOCP (Manahan, 1989).The two major incidents of ginger jake or ginger jake like paralysis described in the last slide had a great impact on the toxicology of tri-ortho-cresyl phosphate (TOCP). Together, the adulteration of Jamaica ginger and of cooking oil with TOCP had affected nearly half million of people.More than a century ago, TOCP was already found to be capable of inducing some form of polyneuritis (Lorot, 1899, as cited by Ecobichon, 1982). Its ability to induce delayed polyneuropathy was not confirmed, however, until after the two outbreaks had occurred. The ginger extract samples collected from the victims and from the production sources provided strong evidence linking the delayed neurotoxicity to TOCP. It was this epidemiologic evidence that enabled Maurice Smith and his colleagues at the U.S. National Institutes of Health to focus their attention on TOCP as the prime suspect. After all, not every test species in their animal studies developed the paralysis syndrome when given the suspected gingers or the TOCP solution.The concern over this type of adverse effects has persisted to this date. For example, the U.S. Environmental Protection Agency (U.S. EPA, 1996) subsequently developed a regulatory guideline specifically for testing delayed neurotoxicity of organophosphates from acute and 28-day exposure. A large number of toxicology studies also have been conducted attempting to reveal the actual mechanism by which TOCP or other organophosphorus esters cause the ginger jake like paralysis (see, e.g., Abou-Donia, 1985; Ecobichon, 1982). The purpose of presenting this first case study here is at least three-fold. Although as mentioned in Slide 8, tri-ortho-cresyl phosphate (TOCP) has relatively low acute toxicity and widely used in industry, the compound is a potent neurotoxin if abused. Ironically, even after the Jamaica ginger epidemic in the United States was over, there were still numerous outbreaks involving TOCP contained in mineral oils and fat substitutes that were deliberately used for cooking (Ecobichon, 1982).Available toxicologic investigations and epidemiologic studies all have pointed to the fact that only the ortho isomer of tricresylphosphate is a strong inducer of delayed polyneuropathy in humans and some mammals. These observations suggest that, like some other adverse effects (such as those of thalidomide discussed in the last lecture), delayed neurotoxicity is extremely specific to chemical structure. Because TOCP is a strong inducer of delayed neuropathy, it is not a relatively potent inhibitor of the acetylcholinesterase enzyme. This fact was also confirmed by the numerous epidemiologic studies that were conducted to investigate the TOCP-related poisonings.Finally, this case study shows that, like in many other incidents, it was epidemiologic evidence that had facilitated as well as advanced the toxicology of TOCP. It had been estimated that the infamous London smog of 1952, that lasting about a week from December 4 to 10, resulted in nearly 4,000 premature deaths. Most of these excess deaths were elderly and those with preexisting cardiac and respiratory diseases, who apparently were unable to cope with the added stress imposed by breathing heavily polluted air. As noted by Costa and Amdur (1996), it is ironic that 16 years earlier the prediction had been made that if an incident like that in the Meuse Valley (in Belgium in 1930) occurred in London, some 3,200 deaths would result.The term smog is now more commonly used to characterize the irritating photochemical air pollution caused by the effects of bright sunlight on chemicals in vehicle emissions (Hodgson et al., 1998). Initially it was referred to as the mixture of smoke (with particle diameter of 0.05 - 1.0 m) and fog that provided such limited visibility as < 3 feet to the victims of the 1952 incident. According to Elsom (1996), such a limited visibility was in part a result of the vast amounts of suspended particulates and sulfur dioxide from millions of domestic chimneys serving inefficient coal-burning household fires, together with emissions from thousands of wasteful industrial plants being spewed into the foggy, stagnant atmosphere of the River Thames basin.The effects of poor quality of urban air were recognized as early as some 2,000 years ago, by the Roman philosopher Seneca, who wrote As soon as I had gotten out of the heavy air of Rome . . ., I felt an alteration to my disposition. (Miller and Miller, 1993, as cited by Costa and Amdur, 1996).The major air pollutants of the 1952 incident were primairly sulfur dioxide (SO2), along with carbon monoxide (CO), nitrogen oxides (NOx), and ozone (O3). These pollutants accounted for about 98% of air pollution then (and now as well) worldwide (see e.g., Costa and Amdur, 1996; Elsom, 1996).SO2 is an intermediate in the formation of sulfuric acid. This irritant affects predominantly the upper airway, and is a common air pollutant produced by the combustion of pyrite (FeS2) available in coal and fuel oil. These sources add millions of tons of SO2 to the global atmosphere each year. Acid rain results if the pH of the deposition is lowered by this air pollutant.CO is a potent chemical asphyxiant, in that it can readily bind to hemoglobin to prevent oxygenation of the blood for systemic transport. The two most common oxides of nitrogen are nitric oxide (NO) and nitrogen dioxide (NO2), the latter of which can be readily converted from the former through the so-called photochemical smog formation (2NO + O2 2NO2). This conversion consists of complex chain reactions involving light energy and unstable reactive intermediate species. Inhalation of NO2 causes severe irritation of the innermost parts of the lungs resulting in primarily pulmonary edema and fatal bronchiolities fibrosa obliterans (Manahan, 1989). O3 is a reactive and toxic form of elemental oxygen. The production of pollutant atmospheric ozone occurs readily under the conditions of photochemical smog formation discussed above.In addition to the 4 major air pollutants (sulfur dioxide, nitrogen oxide, carbon monoxide, and ozone) discussed in the last slide, suspended particulate matter (PM10) and volatile organic compounds (VOC) are also pollutants commonly studied by inhalation toxicologists and air pollution epidemiologists.PM10 is a generic term for fine particles that are light enough to be suspended in air for hours to days. These solid and liquid particles commonly have effective diameters < 1 m, but maybe up to 5 or 10 m, and hence are typically within the inhalable range. Black smoke, that produced in the incomplete combustion of coal or oil, is one component of PM10. VOC as a group are also important because of their role as precursors, with oxides of nitrogen, in the photochemical formation of summer smog pollution. One VOC component of interest is , primarily for its direct toxic effects, petroleum benzene emitted to air as an exhaust product or through evaporative losses.Data on air pollution levels collected by the Global Environmental Monitoring System indicate that as many as 1.6 billion people may be at risk from poor air quality in urban areas throughout the world (reported by the Hong Kong Government, Environmental Protection Department, as cited by Elsom, 1996). Also pointed out by Elsom (1996), a professor of climatology at the Oxford Brookes University, is that congested aeras likewise suffer high levels of traffic-generated pollutants, such as carbon monoxide, VOC, and nitrogen dioxide. Professor Elsom has provided an extensive review of the seriousness and causes of air pollution problems encountered by many cities worldwide.The London smog of 1952 shortly prompted the British government to introduce the Clean Air Act of 1956, which permitted local authorities in Greater London to designate areas within which only smokeless fuels could be burned.Initially most studies on adverse health effects of air pollution had a focus on short-term changes in incidence associated with places where coal had been widely used for domestic heating and industrial purposes. According to Maynard and Waller (1991), epidemiologic studies (e.g., Martin and Bradley, 1960) subsequent to those for the London smog of 1952 continued to show a fair correlation between smaller increases in daily deaths in Greater London and more modest episodes of high pollution like smoke and sulfur dioxide. Further epidemiologic studies related to major air pollution episodes were summarized in Maynard and Waller (1991), Costa and Amdur (1996), Elsom (1996).Since the conduct of these acute epidemiologic studies, especially those on the 1952 incident, chronic effects of the major air pollutants have become an equally important subject matter to inhalation toxicologists. This series of acute epidemiologic and chronic toxicologic studies eventually led to a better understanding of the formation of photochemical (summer) smog. Summer smogs are major air pollution problems currently encountered by many traffic-congested megacities like Los Angles, Mexico City, and London. Winter and summer smogs are weather-specific. Unfavorable meteorological conditions are in fact fundamental to the adverse effects of air pollutants, which act as an agglomerate. The once predominant sulfur dioxide in the London winter smog of 1952 is now being replaced gradually by the fine particulates, nitrogen oxides, and carbon monoxide from vehicle emissions. Even before the winter smogs returned in the late 1980s, London began experiencing severe summer smogs from photochemical formation, partly due to the increased levels of ozone occurring since the mid-1970s. As another example, Los Angeles experienced the worst air quality in the USA in 1993, also due to high levels of ozone present in this traffic-dense megacity (Elsom, 1996). Ozone is a strong oxidizing agent and reacts with a range of cellular components. As a photochemical pollutant, ozone (O3) reacts with nitric oxide (NO) to form nitrogen dioxide (NO2) and oxygen (O2): O3 + NO NO2 + O2.From this second case study, it is clear that the adverse health effects of the air pollutants cannot be investigated through classic toxicology studies alone. As concluded by Elsom (1996), The polluted urban atmospheres that most of us breathe are cocktails of many different pollutants. Although the adverse effects of each of the major air pollutants can be investigated effectively using one or more laboratory animal species, it is almost impractical to purposely place the animal subjects in a controlled environment (e.g., a chamber) where a collection of air pollutants would exert their effects exactly as if when they were present in the urban (ambient) atmospheres.Carcinogenesis per se is referred to as the biochemical process characterizing the transformation of normal cells to neoplastic cells which may eventually progress into tumor cells. The multistage model of carcinogenesis is a mechanistic as well as a biomathematical theory used to characterize this biochemical process. The mechanistic process of carcinogenesis has been studied rigorously by toxicologists specialized in molecular biology and the like, beginning at the time when these fields were still in their infancy.Tumor as a human disease, on the other hand, has been recognized for centuries, dating back at least to the description of breast cancer in the Edwin Smith Papyrus (that Ancient Egyptian medical treatise first deciphered by the American Egyptologist Edwin Smith). The notion that exposure to chemicals can induce carcinogenesis is likewise ancient. In fact, cancer epidemiologists are often proud of citing the classic observation made in 1775 by the eminent English physician Percivall Pott, that an unusually high incidence of scrotal cancer was found in patients with a history of employment as chimney sweeps.The overall biochemical and cellular processes for the induction of cancer are thought to be quite complex, in some cases likely involving several stages. Two major carcinogenic stages are generally recognized: an initiation step followed by a promotional process. Chemical carcinogens are often mutagenic, suggesting that they are likely to alter the macromolecule DNA in a manner such that the affected cell would continue to replicate itself and eventually develop into some cancerous tissues.In addition to the initiation and promotion steps mentioned in the last slide, Miller and Miller (1979) also suggested two additional stages for the induction of cancer by most chemicals or agents. Some agents are initially unharmful precarcinogens that later can be biotransformed into harmful ultimate carcinogens that are usually mutagenic and electrophilic, those tending to bind to cellular macromolecultes (DNA, RNA, proteins, etc.).In accordance with the classic Millers model, the early phase of the multistage process is generally described as initiation or transformation. The initiated cell(s) appears not to be visible as tumor cell(s) during this apparently irreversible phase involving a short(er) duration. This phase is presumed to exist, however, in that a subsequent phase known as promotion or progression can cause an activated (and only activated) cell to develop into tumor cells. Promotion per se is thought to be a highly complex long process, for which several (sub)stages are involved and the early ones are usually reversible.The classic system for the demonstration of this multistage process is the induction of skin tumor in mice. This system was based partly on Deelmans induction scheme (1927), and in part on the somatic cell mutation theory of cancer advanced by Bauer (1928). More recent data showed that the classic two-event model is valid for sites other than animal skin. Dong (1984) also noted that although the classic initiation-promotion model has much to contribute, the multistage theory of carcinogenesis was actually an outgrowth of the challenge to both the single stage model and the multicell theory.The single stage model proposed by Iversen and Arley (1950) is perhaps the earliest quantitative theory of carcinogenesis. At about that time, some investigators were intrigued by the fact (see, e.g., Dong, 1984; Lilienfeld and Lilienfeld, 1980) that many cancer incidences in the adult human population increased proportionally with the 5th or 6th power of age, a variable supposedly implicating exposure duration. The single stage model is unable to account for the power law relationship, however, given that it assumes a single transformation rate throughout the course of carcinogenesis. Fisher and Hollomon (1951) attempted to explain this relationship with a multicell theory, which assumes that a certain number and concentration of different cells must assemble in a single tissue in order for a tumor to develop. However, the concentration dependence relationship as conceived in the multicell model was likewise found to be incompatible with epidemiological data.Another new theory, which appears to prevail to this date, was then proposed by Muller (1951) and Nordling (1953) to account for the aforesaid age-dependence relationship. The Muller-Nordling model is basically a single cell theory. This version assumes that only a single cell will give rise to a tumor after the cell has suffered (exponentially) a number k of sequential, heritable changes a, each at a different occurrence rate, and hence is referred to as the multistage model. According to Armitage and Doll (see, e.g., 1954), the multistage model asserts that the background incidence rate B(t) = (a1a2 . . . ak)tk-1, where t is the age or time at which the tumor becomes visible.The Armitage and Doll version of the multistage model (see last slide) was later found to be quite useful as a quantitative tool. For example, a cohort analysis based on the multistage approach indicated that coke oven emissions do not appear to have a late stage effect of lung cancer in steelworkers (Dong, 1984; Dong et al., 1988). The number of carcinogenic stages involved was estimated to be 4 in that analysis, although there was only moderate to weak indication that the coal tar in the emissions acts as an initiator. These findings are compatible with those derived from other empirical data, such as that by Day and Brown (1980) showing that the benzo()pyrene fraction in the cigarette smoke condensate also affects predominately an early carcinogenic stage. One key substance contained in coal tar is also benzo()pyrene.Like the single stage and multicell models described in the last slide, the multistage theory is not completely free of defects. For example, Moolgavkar (1978) has argued that the multistage theory can legitimately be applied only to cohort data for tumors that are not sensitive to environmental influences.Nonetheless, it has been shown that mathematical models based on the general multistage theory are for the large part successful in describing many experimental and epidemiological data (Day and Brown, 1980; Whittemore and Keller, 1978). The multistage theory is gaining wide acceptance in part because there had been strong evidence that cancer is a single cell in origin (Fialkow, 1974; Gartler, 1974).One utility of the multistage model is that more proper measures could be used to mitigate the exposure in question, if the carcinogen were identifiable as having an early or late stage effect. It should be pointed out that the number of stages need not be > 2 when the theory is actually applied. Day and Brown (1980) noted that certain carcinogens appear to affect only the early or late stages, whereas some others would have an effect on both or several stages. In the United States, the multistage model is also used extensively in quantitative cancer risk assessment by several federal health regulatory agencies (see, e.g., Federal Register, 1996, 1997). As summarized in the last slide, it was a series of epidemiologic observations that had driven the development of the multistage theory. This series of epidemiologic studies suggested that for many forms of human cancer, the incidence rates R(t) at age t may be written as: R(t) = p(tk), where k is a constant being about 5 or 6 and p is a population-specific proportionality constant. Although the multistage model rests on the single cell theory assuming that only a single cell will give rise to a tumor, initially it had a close relationship with the classic initiation-promotion system of skin tumor in mice.The important lesson here is that toxicologists using animal studies alone without a quantitative theory would likely fail to elucidate or make fuller use of the mechanistic process of carcinogenesis, at least for some forms of human cancer. Again, the multistage model was made possible due to a series of epidemiologic studies that brought out the above power law relationship. Human exposure assessment is one of the key components in the health risk assessment (RA) paradigm formulated by the National Research Council (1983, 1994). Its importance and relevance were brought up repeatedly in the previous lectures. In these next two lectures, students will be given a detailed description and an expanded discussion on the techniques and methods used for this important RA component.Lecture 7 will focus on the (presumably more) direct measurements used in human exposure assessments, primarily those based on the use of some form of biological monitoring. With biological monitoring of exposure, the health risk can be assessed through the evaluation of the internal dose in individuals. If the internal dose could be measured accurately and precisely, then such a quantity would account for the exposure from all practical sources.Since the internal dose cannot be measured in every case or for all human exposures, measurements of the external dose become necessary in RA. External doses can be extrapolated from assumed contact rates where applicable, and the agents ambient (environmental) concentrations in air, food, water, soil, and clothing. In some cases, the exposure will be measured even more indirectly using survey results, employment histories, medical records, and the like. These indirect measurement methods will be the topics of Lecture 8.