The expanding field of environmental health encompasses various scientific disciplines in order to investigate the physiological impact of environmental conditions on human health. Since the late 1960s, information concerning the affect of environmental factors on health has proliferated. This substantial gain of knowledge comes from many scientific disciplines, including biology, chemistry, physics, mathematics, and engineering sciences. Increasing sophistication of laboratory techniques, diagnostic equipment, and in-the-field surveillance has greatly contributed to the prevention and control of human health problems by enabling environmental factors to be analyzed in minute quantities. OCCUPATIONS AND AGENCIES Occupations in environmental health include medicine, ecological research, pollution sciences and engineering, and regulatory enforcement through governmental organizations such as the \TEnvironmental Protection Agency\t (EPA), \TFood and Drug Administration\t (FDA), \TOccupational Safety and Health Administration\t (OSHA), \Tpublic health\t Service, and state and local environmental agencies. Public health departments routinely inspect meat, fish, and poultry packaging industries, restaurants, markets, and hospitals, and enforce regulations designed to prevent the contamination of food, beverages, drugs, and cosmetics. The Environmental Protection Agency is empowered to protect the public health by controlling air and water pollution and by regulating the production, use, and disposal of toxic chemicals. The Food and Drug Administration is responsible for overseeing the wholesomeness of products consumed in the United States and for the banning and control of harmful chemicals in food, cosmetics, and drugs. The Occupational Safety and Health Administration supervises work environments to protect workers from exposure to unsafe industrial chemicals, machinery, and operating procedures. Most cities have sanitary engineering departments responsible for garbage collection and sewage treatment. Communities frequently chlorinate drinking water to kill pathogens, ever since John L. Leal demonstrated (1908) the efficacy of chlorinated waters in Jersey City, N.J. Research currently focuses on environmental pollution (see \Tpollution, environmental\t) and its consequent harm. Adverse health effects resulting from environmental contaminants, or pollution, are an important concern of most environmental health research. When industrialization was in its infancy, the impact of pollution was localized and minimal. With rapid worldwide industrialization, however, using \Tpetrochemicals\t as an energy base, toxic pollution has increased to global proportions. The stress placed on our planet by pollution has far-reaching effects and constitutes a grave crisis. COMMUNICABLE DISEASES History Environmental health originated in the control of communicable diseases during the past century. The most spectacular achievement of the biological sciences in the last hundred years has been the identification of pathogenic organisms as the cause of disease. The germ theory of disease, pioneered by Louis \TPasteur\t and others, led to the general concept that disease could be prevented by vaccination and controlled by interrupting the transmission of pathogenic organisms to humans. Three 19th-century scientists contributed greatly to understanding the causes and prevention of \Tinfectious disease\t: Peter Ludwig Panum, who researched (1847) measles; John Snow, who published (1849) findings on the mode of cholera communication; and William Budd, who wrote (1873) about the spread of typhoid fever from contaminated water supplies in Wales and England. Certain animals, such as mosquitoes, flies, fleas, lice, rats, mice, and snails, were found to harbor pathogenic organisms and parasites that can cause human diseases. As a result of this major discovery, populations of these host animals, known as vectors, were controlled through eradication and better sanitation, reducing the prevalence of disease. The recognition that disease could also be transmitted by fecal contamination of water and food led to the development of sewage treatment, systematic garbage collection, the protection and treatment of drinking water supplies, and the establishment of public health enforcement agencies. Results of Control The prevention and control of communicable disease in industrial societies began to show dramatic results during the 1920s. Incidences of such major epidemic diseases as \Tbubonic plague\t, \Tdiphtheria\t, \Tmalaria\t, \Tscarlet fever\t, \Tsmallpox\t, and \Twhooping cough\t dropped radically. Conversely, human life expectancy in developed nations began to rise from less than 40 to more than 75 years. Through the intensive efforts of the World Health Organization (\TWho\t) and the cooperation of all nations, endemic smallpox appears to have been eradicated; the last case of endemic smallpox occurred on Oct. 26, 1977, in Somalia. This achievement marks the first time in human history that a communicable disease has been eliminated. The smallpox virus is apparently extinct except for laboratory cultures. Ironically, smallpox outbreaks associated with laboratory accidents have occurred since 1977. As a result, the \TWho\t insisted that all existing cultures be destroyed, except for those in four tightly guarded places, including the Center for Disease Control (CDC) in Atlanta, Ga. These remaining cultures are available for research purposes and also for producing vaccines should mutations of smallpox-type viruses arise. Legionnaires' Disease Developed nations have instituted public health and enforcement systems to reduce the occurrence of disease. In the United States, many diagnosed diseases must be reported to state officials and to the CDC, where information is compiled and disease incidence is surveyed. Teams of experts are thus available and easily mobilized to investigate disease, as was necessary during the mysterious pneumonialike outbreak at an American Legionnaires Convention (1976) in Philadelphia. After several months of intensive investigation, a particularly elusive and previously unknown bacterium was found to be the cause of the infection now known as \TLegionnaires' disease\t. This bacterium as well as a group of similar bacteria now isolated are believed to be responsible for as many as 5 to 10 percent of deaths diagnosed as pneumonia. Nosocomial Diseases Hospitals cooperate in environmental health concerns by hiring and training specialists to curb the rising incidence of \Tnosocomial\t infections, which result from exposure to pathogens during a hospital stay. The cross-contamination of sick patients is prevalent, as is transmittal of an infection from the hospital staff to patients. Because \Tdrug\t therapy is widely employed, hospitals have been shown to be the breeding grounds of antibiotic-resistant organisms. At present, hospitals are placing great emphasis on the identification and control of these organisms and on tighter control of the spread of infection. CHRONIC AND DEGENERATIVE DISEASE Although the prevalence of diseases caused by pathogens and parasites has declined substantially, chronic illnesses and degenerative disorders have become the leading causes of death in developed nations. In the United States, the only epidemic disease still prevalent is gonorrhea, having doubled in annual cases from 1969 to 1976. The four leading causes of death are, in order, \Theart\t \Tdisease\t (cardiovascular disease), \TCancer\t, central nervous disorders (cerebrovascular disease), and accidents. The first three of these are chronic and degenerative disorders. The greatest percentage of accidental deaths involve the automobile, which is also responsible for up to 90 percent of the air pollution in most cities. FACTORS CAUSING DISEASE A major difficulty in the control and prevention of chronic and degenerative illnesses such as cancer, \Trespiratory system disorders\t, and heart, liver, and kidney disorders is that the symptoms are caused by complex factors. Differing forms of cancer have myriad probable causes, each exerting influences over the course of time. Most chronic illnesses are not caused by any single etiologic agent, for example, a pathogenic organism. Smallpox could be eradicated because its virus was isolated and a vaccine produced; a vaccine for heart disease, however, appears unlikely. More important, with an infection by a biological agent, the onset of disease symptoms is usually a matter of hours, days, or weeks. On the other hand, the onset of cancer from exposure to carcinogens, which are chemical agents, may take years, even decades. Epidemiologic research is complicated by such factors as the length of exposure (whether in inhalation, ingestion, or skin contact), the stress of the individual, and the toxicity of the agents involved. Some agents become toxic when present in the human body in conjunction with other agents. Other agents that are already toxic become even more powerful in the presence of other chemical compounds. For example, deaths from cardiac and respiratory failures have resulted from the effects of combining sleeping pills and alcohol. THE CHEMICAL ENVIRONMENT Each year, hundreds of new chemical substances are created. Because many of these chemicals differ significantly from natural biochemicals, detailed studies have been designed to discover potential harmful effects. Often, when synthetic compounds are introduced into the environment, their movement through ecosystems is difficult to detect. Many compounds are not biodegradable and are of particular concern, especially if they can accumulate in biological organisms, including humans. Since the beginning of the century, the environment has been progressively polluted by chemicals. Two examples of biologically persistent chemicals that have been indiscriminately introduced into the environment in vast quantities are the insecticide \TDDT\t (dichlorodiphenyltrichloroethane) and a group of compounds known as polychlorinated biphenyls (\TPCB\t). \TDDT\t \TDDT\t is probably the best-known synthetic compound, and its environmental effect is of great magnitude. The actual \TDDT\t activity is not fully understood, even though it has been studied for over thirty years. \TDDT\t was originally hailed as the great savior of crops from insect pests and has been extremely important in controlling the spread of malaria by mosquitoes. However, because \TDDT\t and its related compounds can dissolve into fatty animal tissue, \TDDT\t has bio-accumulated through food chains and has manifested pronounced effects in fish, reptilian, and avian metabolism and reproductive systems. Many animal species have suffered substantial reductions of populations from \TDDT\t contamination, including the bald eagle, the national bird of the United States. \TPCB\t Materials containing \TPCB\t were used primarily in electrical transformers because of their unique capacity to conduct electricity and because of their fire resistance. Unfortunately, \TPCB\t compounds have also been shown to be highly carcinogenic. Massive amounts of \TPCB\t have been introduced into the environment. More than 1,500,000 kg (one-third of the nearly 1 billion lb) of \TPCB\t-containing material manufactured since the 1920s has been discharged into waterways or leaked from improper disposal sites. Little information is currently available concerning the bio-accumulation effects of \TPCB\t, which is, like \TDDT\t, soluble in fatty tissue. \TDDT\t and \TPCB\t are present worldwide, having been found in ocean sediments and polar ice. The problem of tracing these compounds and thousands of others (including breakdown products which sometimes are even more harmful) is immense and will occupy the investigations of environmental health researchers for years to come. EFFECTS OF AIR POLLUTION The dangers of air pollution have been well documented. Air pollution kills, especially when prolonged weather inversions prevent adequate dispersion of petrochemical combustion products. During Dec. 5-8, 1952, a killer smog was responsible for an estimated 3,500-4,000 deaths in London. In 1948, a stagnant air mass sent over 10 percent of the population of Donora, Pa., to hospitals. Most urban communities show increased mortality of the aged, very young, and those afflicted with respiratory illnesses during heavy smog episodes. Lung Damage The major forms of air pollution--carbon monoxide, particulates, sulfur oxides, nitrous oxides, and photochemical oxidants--have a wide range of effects on human respiratory tracts. Disorders include \Tasthma\t, \Tbronchitis\t, \Temphysema\t, and lung cancer. Air pollution results primarily from automobile exhaust, fossil fuel power plants, industrial processes, and forest fires; cigarette smoking has also been strongly linked with respiratory illnesses. The extensive damage of respiratory, circulatory, and lymphatic systems by chronic tobacco inhalation makes cigarette \Tsmoking\t a major public health concern. EFFECTS OF WATER POLLUTION Mercury Poisoning One of the most significant harms to human health as a result of water pollution occurred when the people of Minimata Bay in Japan were exposed to mercury-contaminated shellfish and fish caught near the shore. The mercury had been discharged into the water from a local factory. The pollution was not detected until an alarming number of children were born with physical abnormalities. Public Water A wide variety of toxic compounds is present in public drinking water supplies, although mostly in minute amounts. Almost nothing is known about the long-term health effects of these contaminants. Water that is chlorinated in order to eliminate bacterial contaminants may actually increase the toxicity of chemical contaminants. The EPA is extending a list of known and suspected toxic substances and has set limits for the presence of many of these contaminants in drinking water. EFFECTS OF FOOD AND DRUG CONTAMINATION Delaney Amendment \Tfood additives\t are chemicals widely used to enhance the flavor, color, texture, consistency, and wholesomeness of foods. Chemicals are also used in the preparation, mixing, curing, firming, anticaking, and preservation of foods. The Delaney clause of the 1958 amendment to the Food, Drug and Cosmetic Act states that no additive may exist in any food if it produces cancer when fed to a human or laboratory animal in any concentration or if it can be shown to be a carcinogen by an appropriate test. This law has given rise to a substantial chemical testing industry. Because the burden of proof of safety rests with the manufacturer, much debate has ensued over the methodology used to analyze toxicity. Several areas of controversy include the accuracy of testing techniques, whether animal studies are useful in testing for human drugs, and the safety of dosage levels. Contaminants The contamination of plant and animal foods is another concern in environmental health. The effects of petrochemical \Lfertilizer\ls, \Lherbicide\ls, \Tpesticides\t, and \Lsteroid\ls and other growth-inducing hormonal residues are uncertain. In 1973, more than 10,000 Michigan farm residents were exposed to polybrominated biphenyls (PBBs), a group of fat-soluble, biologically persistent compounds, similar to PCBs, used as flame retardants. A shipping accident substituted several hundred pounds of PBB for magnesium oxide, a nutrient in animal feed. Exposed cattle suffered weight loss, decreased milk production, and death; cows gave birth to dead and malformed calves. Farmers, their families, and other people who drank milk and ate meat from these contaminated herds also had symptoms resulting from PBB toxicity. The long-term human health effects of this accident are unknown, and results of continuing research may not be conclusive for years. Another disaster occurred in the early 1960s, when malformed children were born to mothers who were taking the prescribed drug \Tthalidomide\t. Since that time, side effects of medicines intended to improve health have been rigorously examined. RADIATION The nuclear age has brought a great form of energy to humans; however, nuclear research, including the creation of plutonium and other radioactive isotopes as well as their improper use and disposal, is of grave public concern. Emissions from radioactive decay are known to cause cancer and genetic damage. Plutonium is the most toxic chemical created, and inhalation of 1/10,000 of a gram can induce lung cancer. The use of plutonium to generate electricity from nuclear power plants is of particular concern, especially in light of the 1979 crisis at the Three-Mile Island nuclear power plant near Harrisburg, Pa., during which radioactive material was released into water and the air. Disposal of radioactive wastes and their safe transport is another concern (see \Tnuclear energy\t). Atmospheric atomic testing has spread worldwide a radioactive substance, strontium-90, that accumulates in human bones. Strontium strongly mimics the biochemical pathways of calcium, and decay emissions of strontium that have accumulated in human bones may possibly induce leukemia. Close surveillance of survivors of the two atomic explosions in Japan and the inadvertent victims of fallout from atmospheric testing in the Marshall Islands has shown high incidences of \Tleukemia\t and skin disorders. The significance of the genetic damage is still uncertain because such studies require long and precise epidemiological research. DANGERS IN THE WORKING ENVIRONMENT Many of the early studies of chemically induced diseases were conducted on workers exposed to certain manufacturing processes. Several specific diseases have been identified in relation to the inhalation of asbestos, silicon, talc, and coal dust. Most occupational diseases fall into two time-concentration patterns--those of quick recoveries from a relatively high dose over a short period, and those of cumulative effects (with a slow onset) of a relatively low dose over a long period of time. Ammonia and carbon monoxide are typical examples of the former, whereas inhalation of benzene and carbon tetrachloride has been shown to be carcinogenic over a long-term exposure. The work environment has also been studied for the effects of noise on human health. Most notably, loud chronic noise may lead to temporary or permanent hearing loss. Other effects of noise pollution are the loss of intelligible speech, the disruption of sleep patterns, and increased stress of the individual. Michael C. Cote Bibliography: Blumenthal, D. S., Introduction to Environmental Health (1985); Eckholm, E. P., The Picture of Health (1977); Koren, Herman, Environmental Health and Safety (1974); McKinney, J. D., Environmental Health Chemistry (1981); Meselson, M. S., Chemicals and Cancer (1980); More, J. W., The Changing Environment (1986); Newton, David F., Elements of Environmental Health (1974); Ricci, P. R., ed., Health and Environmental Risk Assessment (1985); World Health Organization, Health Hazards of the Human Environment (1972). See also: \Tchemical and biological warfare\t; \Tdiseases, occupational\t; \Tnervous system, diseases of the\t; \Ttoxicology\t.