Go back 10 Glossary
The dose makes the poison. PARACELSUS, 1540
10-1 RISKS AND HAZARDS
What Is Risk? Risk is the possibility of suffering harm from a hazard that can cause injury, disease, economic loss, or environmental damage. Risk is expressed in terms of probability: a mathematical statement about how likely it is that some event or effect will occur.
Probability often is stated in terms such as "The lifetime probability of developing cancer from exposure to a certain chemical is 1 in 1 million." This means 1 of every 1 million people exposed to the chemical at a specified average daily dosage will develop cancer over a typical lifetime (usually considered 70 years).
How Are Risks Assessed and Managed? Risk assessment involves (1) identifying a real or potential hazard ("What is the hazard?"), (2) determining the probability of its occurrence ("How likely is the event?"), and (3) assessing the severity of its health, environmental, economic, and social impact ("How much damage is it likely to cause?"; Figure 10-l,left).
Figure 10-1 Risk assessment and risk management. These are important, difficult, and controversial processes.
After a risk has been assessed, the next step is risk management, in which people make decisions about (1) how serious it is compared to other risks (comparative risk analysis), (2) how much (if at all) the risk should be reduced, (3) how such risk reduction can be accomplished, and (4) how much money should be devoted to reducing the risk to an acceptable level (Figure la-I, right).
What Are the Major Types of Hazards? Types of hazards we face can be categorized as follows:
. Cultural hazards such as unsafe working conditions, smoking (Spotlight, p. 220), poor diet, drugs, drinking, driving, criminal assault, unsafe sex, and poverty . Chemical hazards from harmful chemicals in the air, water, soil, and food
. Physical hazards such as ionizing radiation, fire, earthquake, volcanic eruption, flood, tornadoes, and hurricanes
. Biological hazards from pathogens (bacteria, viruses, and parasites), pollen and other allergens, and animals such as bees and poisonous snakes
What Determines Whether a Chemical Is Harmful? Dose and Response Toxicity measures how harmful a substance is. Whether a chemical (or other agent such as ionizing radiation) is harmful depends on several factors. One is the dose, the amount of a potentially harmful substance a person has ingested, inhaled, or absorbed through the skin. Whether a chemical is harmful depends on (1) the size of the dose over a certain period of time, (2) how often an exposure occurs, (3) who is exposed (adult or child, for example), (4) how well the body's detoxification systems (liver,lungs, and kidneys) work, and (5) genetic makeup that determines an individual's sensitivity to a particular toxin.
The type and amount of health damage that result from exposure to a chemical or other agent are called the response. An acute effect is an immediate or rapid harmful reaction to an exposure; it can range from dizziness or a rash to death. A chronic effect is a permanent or long-lasting consequence (kidney or liver damage, for example) of exposure to a harmful substance.
What is roughly the diameter of a 30-caliber bullet, can be bought almost anywhere, is highly addictive, and kills about 11,000 people every day, 460 per hour, or 1 person every 8 seconds? It is a cigarette. Cigarette smoking is the single most preventable major cause of suffering and premature death among adults.
The World Health Organization (WHO) estimates that each year tobacco contributes to the premature deaths of at least 4 million people from 25 illnesses including (1) heart disease, (2) lung cancer, (3) other cancers, (4) bronchitis, (5) emphysema, and (6) stroke. The annual death toll from smoking-related diseases is projected to reach 10 million by 2030 (70% of them in developing countries)-an average of about 27,400 preventable deaths per day or 1 death every 3 seconds.
According to a 2002 study by the U.S. Centers for Disease Control and Prevention, smoking prematurely kills about 440,000 Americans per year, an average of 1,180 deaths per day (see figure). This death toll is roughly equivalent to three fully loaded jumbo (400-passenger) jets crashing accidentally every day with no survivors. Smoking causes more deaths each year in the United States than do all illegal drugs, alcohol (the second most harmful legal drug after nicotine), accidents, suicide, and homicide combined (see figure). According to a 1998 study, secondhand smoke (inhaled by nonsmokers) causes 30,000-60,000 premature deaths per year in the United States.
The overwhelming scientific consensus is that the nicotine (and probably the acetaldehyde) inhaled in tobacco smoke is highly addictive. Only 1 in 10 people who try to quit smoking succeed, about the same relapse rate as for recovering alcoholics and those addicted to
According to a 2002 study by the Centers for Disease Control and Prevention, the United States spends about $158 billion a year on (1) medical bills, (2) higher insurance costs, (3) disability, (4) lost earnings and productivity because of illness, and (5) property damage from smoking-caused fires. This is an average of about $7 per pack of cigarettes sold in the United States.
Many health experts urge that a $3-5 federal tax be added to the price of a pack of cigarettes in the United States. Such a tax would mean the users of cigarettes (and other tobacco products), not the rest of society, would pay a much greater share of the health, economic, and social costs associated with their smoking: a user-pays approach.
Other suggestions for reducing the death toll and health effects of smoking in the United States include (1) banning all cigarette advertising, (2) forbidding the sale of cigarettes and other tobacco products to anyone under 21 (with strict penalties for violators), (3) banning all cigarette vending machines, (4) classifying nicotine as an addictive and dangerous drug (and placing its use in tobacco or other products under the jurisdiction of the Food and Drug Administration), (5) eliminating all federal subsidies and tax breaks to U.S. tobacco farmers and tobacco companies, and (6) using cigarette tax income to finance an aggressive anti-tobacco advertising and education program.
Explain why you agree or disagree with imposing a heavy tax on cigarettes and implementing the other six suggestions listed here for reducing the death toll and health effects of smoking.
Annual deaths in the United States from tobacco use and other causes. Smoking is by far the nation's leading cause of preventable death, causing more premature deaths each year than all the other categories in this figure combined. (Data from National Center for Health Statistics) heroin or crack cocaine. A British government study showed that adolescents who smoke more than one cigarette have an 85% chance of becoming smokers. According to a 1999 World Bank study, each day some 80,000-100,000 young people become regular long-term smokers, primarily in developing countries.
What Is a Poison?
Legally, a poison is a chemical that has an LDso of 50 milligrams or less per kilogram of body weight. The LDso is the median lethal dose: the amount of a chemical received in one dose that kills exactly 50% of the animals (usually rats and mice) in a test population within a 14-day period.
Chemicals vary widely in their toxicity (Table 10-1). Some poisons can cause serious harm or death after a single acute exposure at very low dosages. Others cause such harm only at such huge dosages that it is nearly impossible to get enough into the body. Most chemicals fall between these two extremes.
How Are the Case Reports and Epidemiological Studies Used to Estimate Toxicity?
Two methods that scientists use to get information about the harmful effects of chemicals on human health are as follows:
. Case reports (usually made by physicians) provide information about people suffering some adverse health effect or death after exposure to a chemical. Such information often involves accidental poisonings, drug overdoses, homicides, or suicide attempts. Most case reports are not a reliable source for determining toxicity because the actual dosage and the exposed person's health status often are not known.
However, such reports can provide clues about environmental hazards and suggest the need for laboratory investigations.
. Epidemiological studies in which the health of people exposed to a particular toxic agent (the experimental group) is compared with the health of another group of statistically similar people not exposed to the agent (the control group). The goal is to determine whether the statistical association (if any) between an exposure to a toxic chemical and a health problem is strong, moderate, or weak. Such studies are limited because (1) too few people have been exposed to high enough levels of many toxic agents to detect statistically significant differences, (2) conclusively linking an observed effect with exposure to a particular chemical is very difficult because people are exposed to many different toxic agents throughout their lives, and
(3) they cannot be used to evaluate hazards from new technologies or chemicals to which people have not been exposed.
How Are Laboratory Experiments Used to Estimate Toxicity?
The most widely used method for determining acute toxicity and chronic toxicity is to expose a population of live laboratory animals (especially mice and rats) to measured doses of a specific substance under controlled conditions. Animal tests take 2-5 years and cost $200,000 to $2 million per substance tested.
Table 10-1 Toxicity Ratings and Average Lethal Doses for Humans
|Toxicity Ratings||LD (milligrams per
kilogram of body weight)"
|Average Lethal Doset|
|Super toxic||Less than 0.01||Less than 1 drop|
|Extremely toxic||Less than 5||Less than 7 drops|
|Very toxic||5-50||7 drops to 1 teaspoon|
|Toxic||50-500||1 teaspoon to 1 ounce|
|Moderately toxic||500-5,000||1 ounce to 1 pint|
|Slightly toxic||5,000-15,000||1 pint to 1 quart|
|Essentially nontoxic||15,000 or greater||More than 1 quart|
Nerve gases, botulism toxin, mushroom toxins, dioxin (TCDD)
Potassium cyanide, heroin, atropine, parathion, nicotine
Mercury salts, morphine, codeine
Lead salts, DDT, sodium hydroxide, fluoride, sulfuric acid, caffeine, carbon tetrachloride
Methyl (wood) alcohol, ether, phenobarbitol, amphetamine, kerosene, aspirin
Ethyl alcohol, Lysol, soaps
Water, glycerin, table sugar
*Dosage that kills 50% of individuals exposed.
tAmounts of substances that are liquids at room temperature when given to a lOA-kilogram (155-pound) human.
Animal welfare groups want to limit or ban use of test animals or ensure that experimental animals are treated in the most humane manner possible. More humane methods for carrying out toxicity tests include using (1) bacteria, (2) cell and tissue cultures, and (3) chicken egg membranes. In 1999, scientists developed a cheaper and much more sensitive way to determine toxicity by almost continuous measurement of changes in the electrical properties of individual animal cells. The United States, Japan, and most European countries are gradually replacing older LDso methods with these newer procedures.
These alternatives can greatly decrease the use of animals for testing toxicity. However, scientists point out that some animal testing is needed because the alternative methods cannot adequately mimic the complex biochemical interactions of a live animal.
Acute toxicity tests are run to develop a doseresponse curve, which shows the effects of various dosages of a toxic agent on a group of test organisms (Figure 10-2). Such tests are controlled experiments in which the effects of the chemical on a test group are compared with the responses of a control group of organisms not exposed to the chemical. Care is taken to ensure that organisms in each group are (1) as identical as possible in age, health status, and genetic makeup and (2) exposed to the same environmental conditions.
Fairly high dosages are used to reduce the number of test animals needed, obtain results quickly, and lower costs. Otherwise, tests would have to be run on millions of laboratory animals for many years, and manufacturers could not afford to test most chemicals. For the same reasons, the results of high-dose exposures usually are extrapolated to low-dose levels using mathematical models. Then the low-dose results on the test organisms are extrapolated to humans to estimate LDso values for acute toxicity (Table 10-1).
Figure 10-2 Hypothetical dose-response curves. The linear and nonlinear curves in the left graph show that exposureto any dosage of a chemical or ionizing radiation has a harmful effect that increases with the dosage. The curve on the right shows that a harmful effect occurs only when the dosage exceeds a certain threshold level. There is much uncertainty about which of these models applies to various harmful agents because of the difficulty in estimating the response to very low dosages.
According to the non threshold dose-response model (Figure 10-2, left), any dosage of a toxic chemical or ionizing radiation causes harm that increases with the dosage. With the threshold dose-response model (Figure 10-2, right), a threshold dosage must be reached before any detectable harmful effects occur, presumably because the body can repair the damage caused by low dosages of some substances. Establishing which of these models applies at low dosages is extremely difficult. To be on the safe side, the nonthreshold dose-response model often is assumed.
Some scientists challenge the validity of extrapolating data from test animals to humans because human physiology and metabolism often are different from those of the test animals. Other scientists counter that such tests and models work fairly well (especially for revealing cancer risks) when the correct experimental animal is chosen or a chemical is toxic to several different test animal species.
How Valid Are Estimates of Toxicity?
As we have seen, all methods for estimating toxicity levels and risks have serious limitations. However, they are all we have. Because of this uncertainty, exposure standards for toxic substances and ionizing radiation typically are set at levels 1/100 or even 1/1,000 of the estimated harmful levels.
Despite their many limitations, carefully conducted and evalua'ted toxicity studies are important sources of information for understanding doseresponse effects and estimating and setting exposure standards. However, citizens, lawmakers, and regulatory officials must recognize the huge uncertainties and guesswork involved in all such studies.
10-3 CHEMICAL HAZARDS
What Are Toxic and Hazardous Chemicals?
Toxic chemicals generally are defined as substances fatal to more than 50% of test animals (LDso) at given concentrations. Hazardous chemicals cause harm by (1) being flammable or explosive, (2) irritating or damaging the skin or lungs (strong acidic or alkaline substances such as oven cleaners), (3) interfering with or preventing oxygen uptake and distribution (asphyxiants such as carbon monoxide and hydrogen sulfide), or (4) inducing allergic reactions of the immune system (allergens) .
What Are Mutagens, Teratogens, and Carcinogens?
Three types 'of potentially toxic agents are
. Mutagens that cause random mutations, or changes, in the DNA molecules found in cells. Most mutations are harmless, probably because all organisms have biochemical repair mechanisms that can correct mistakes or changes in the DNA code.
. Teratogens that cause birth defects while the human embryo is growing and developing during pregnancy, especially during the first 3 months.
. Carcinogens that cause or promote the growth of a malignant (cancerous) tumor, in which certain cells multiply uncontrollably. Many cancerous tumors spread by metastasis when malignant cells break off from tumors and travel in body fluids to other parts of the body. There, they start new tumors, making treatment much more difficult.
Typically, 10-40 years may elapse between the initial exposure to a carcinogen and the appearance of detectable symptoms. Partly because of this time lag, many healthy teenagers and young adults have trouble believing their smoking (Spotlight, p. 220), drinking, eating, and other lifestyle habits today could lead to some form of cancer before they reach age 50.
How Can Chemicals Harm the Immune, Nervous, and Endocrine Systems?
Since the 1970s, a growing body of research on wildlife and laboratory animals and epidemiological studies of humans has indicated that long-term (often low-level) exposure to various toxic chemicals in the environment can disrupt the body's immune, nervous, and endocrine systems.
The immune system consists of specialized cells and tissues that protect the body against disease and harmful substances by forming antibodies that make invading agents harmless. Weakening the human immune system can leave the body wide open to attacks by (1) allergens, (2) infectious bacteria, (3) viruses, and (4) protozoans.
Some chemicals in the environment can harm the human nervous system (brain, spinal cord, and peripheral nerves). For example, many poisons are neurotoxins, which attack nerve cells (neurons).
The endocrine system is a complex network of glands that releases very small amounts of hormones into the bloodstream. In humans and other animals, these chemicals control body functions such as (1) sexual reproduction, (2) growth, (3) development, and behavior. There is concern that human exposure to low levels of synthetic chemicals, known as hormonally active agents (HAAs), can mimic and disrupt the effects of natural hormones (Connections, p. 224).
Why Do We Know So Little About the Harmful Effects of Chemicals?
According to risk assessment expert Joseph v. Rodricks, "Toxicologists know a great deal about a few chemicals, a little about many, and next to nothing about most." The U.s. National Academy of Sciences estimates that only about (1) 10% of at least 75,000 chemicals in commercial use have been thoroughly screened for toxicity, and (2) 2% have been adequately tested to determine whether they are carcinogens, teratogens, or mutagens. Hardly any of the chemicals in commercial use have been screened for damage to the nervous, endocrine, and immune systems.
Currently, federal and state governments do not regulate about 99.5% of the commercially used chemicals in the United States. There are three major reasons for this lack of information and regulation:
. Under existing laws most chemicals are considered innocent until proven guilty.
. Not enough funds, personnel, facilities, and test animals are available to provide such information for more than a small fraction of the many chemicals we encounter in our daily lives.
. Analyzing the combined effects of multiple exposures to various chemicals and the possible interactions of such chemicals is too difficult and expensive. For example, just studying the possible different three-chemical interactions of the 500 most widely used industrial chemicals would take 20.7 million experiments-a physical and financial impossibility.
What Is the Precautionary Approach?
Because of the difficulty and expense of getting information about the harmful effects of chemicals, an increasing number of scientists and health officials are pushing for much greater emphasis on pollution prevention. This strategy greatly reduces (1) the need for statistically uncertain and controversial toxicity studies and exposure standards and (2) the risk from exposure to potentially hazardous chemicals and products and their possible but poorly understood multiple interactions.
This approach is based on the precautionary principle. According to this concept, when we are uncertain about potentially serious harm from chemicals or technologies, decision makers should act to prevent harm to humans and the environment. The principle is based on familiar axioms: "Look before you leap," "better safe than sorry," and" an ounce of prevention is worth a pound of cure."
Under this approach, those proposing to introduce a new chemical or technology would bear the burden of establishing its safety. In other words, new chemicals and technologies would be assumed guilty until proven innocent. Manufacturers and businesses contend that doing this would make it too expensive and almost impossible to introduce any new chemical or technology.
Over the last 25 years, experts from a number of disciplines have been piecing together (1) field studies on wildlife, (2) studies on laboratory animals, and (3) epidemiological studies of human populations. This analysis suggests that a variety of human-made chemicals, known as hormonally active agents (HAAs), can act as hormone or endocrine disrupters.
Numerous studies indicate that very low concentrations of HAAs can disrupt the endocrine system by blocking, turning off, slowing down, or speeding up the roles of natural hormones, especially in the reproductive system and the body's growth and development processes. There is also growing concern about pollutants that can act as thyroid disrupters and cause growth, weight, brain, and behavioral disorders.
Most natural hormones are broken down or excreted. However, many synthetic hormone impostors are stable, fat-soluble compounds whose concentrations can be biomagnified as they move through food chains and webs (Figure 8-11, p. 176). Thus they can pose a special threat to humans and other carnivores dining at the top of food webs.
Numerous wildlife and laboratory studies reveal various possible effects of estrogen mimics and hormone blockers (HAAs). Here are a few of many examples:
. Ranch minks that were fed Lake Michigan fish contaminated with endocrine disrupters such as DDT and PCBs failed to reproduce.
. Exposure to PCBs has reduced penis size in some test animals and in 118 boys born to women who were exposed to a PCB spill in Taiwan in 1979.
. A 1999 study by Michigan State University zoologists found that female rats exposed to PCBs were reluctant to mate, raising the possibility that such contaminants could cause low sex drives in women.
. In 1973, estrogen mimics called PBBs accidentally got into cattle feed in Michigan, and from there into beef. Pregnant women who ate the beef (and whose breast milk had high levels of PBBs) had sons with undersized penises and malformed testicles.
. The past 50 years have seen dramatic increases in testicular and prostate cancer in males almost everywhere.
A 1999 study by a u.s. National Academy of Sciences panel of scientists (1) concluded that far too little is known about the effects of such chemicals to come to a definitive conclusion about their effects on humans and (2) called for greatly increased research to determine whether low levels of various HAAs in the environment pose a threat to the human population.
Some health scientists believe we should begin sharply reducing the use of potential hormone disrupters now because they meet the two requirements of the precautionary principle: great scientific uncertainty and a reasonable suspicion of harm.
Other researchers disagree. They point out that (1) the hormonal effects of synthetic hormone imposters are much weaker than those of naturally occurring hormones and (2) current levels of exposure to such chemicals are not high enough to pose any real danger to humans.
Do you consider the possible threat from hormone disrupters a problem that could affect you or any child you might have? Explain. If so, what should be done about this potential problem?
10-4 BIOLOGICAL HAZARDS: DISEASE IN DEVELOPED AND DEVELOPING COUNTRIES
What Are Nontransmissible Diseases?
A nontransmissible disease is not caused by living organisms and does not spread from one person to another. Examples are (1) cardiovascular (heart and blood vessel) disorders, (2) most cancers, (3) diabetes, (4) asthma, (5) emphysema, and (6) malnutrition. Such diseases typically (1) have multiple (and often unknown) causes and (2) tend to develop slowly and progressively.
What Are Transmissible Diseases?
A transmissible disease is caused by a living organism (such as a bacterium, virus, protozoa, or parasite) and can be spread from one person to another (Figure 10-3). These infectious agents, called pathogens, are spread by air, water, food, body fluids, some insects, and other nonhuman carriers called vectors.
Antibiotics have greatly reduced the incidence of infectious disease caused by rapidly producing bacteria. However, widespread use and misuse have increased the genetic resistance of many disease-causing bacteria to antibiotics (Spotlight, p. 226).
Some great news is that since 1900, and especially since 1950, we have greatly reduced the incidence of infectious diseases and the death rates from such diseases. Some bad news is that worldwide, infectious diseases cause about one of every fom deaths each year-mostly in developing countries.
According to the World Health Organization (WHO), the world's seven deadliest infectious diseases are (1) acute respiratory infections, mostly pneumonia and flu (caused by bacteria and viruses and killing about 3.9 million people per year), (2) acquired immune deficiency syndrome (AIDS, a viral disease, killing about 3 million per year, most of them young adults), (3) diarrheal diseases (caused by bacteria and viruses, killing about 2.1 million per year, about 1.9 million of them children under age 5), (4) tuberculosis (TB, a bacterial disease, with about 1.6 million deaths per year; Case Study, p. 228), (5) malaria (caused by parasitic protozoa, with about 1.1 million deaths per year, (6) hepatitis B (a viral disease, killing about 1 million people annually), and (7) measles (a viral disease, which kills about 800,000 people annually, most of them children under age 5).
Figure 10-3 Examples of pathogens or agents that can cause transmissible diseases. A micron is onemillionth of a meter.
How Rapidly Are Viral Diseases Spreading?
Viral diseases include (1) influenza or flu (transmitted by the bodily fluids or airborne emissions of an infected person), (2) Ebola (transmitted by the blood or other body fluids of an infected person), (3) West Nile virus (transmitted by the bite of a common mosquito that becomes infected by feeding on birds that carry the virus), (4) rabies (transmitted by dogs, coyotes, raccoons, skunks, and bats), and (5) HIV (transmitted by unsafe sex, sharing of needles by drug users, infected mothers to offspring before or during birth, and exposure to infected blood).
Health officials worry about the emergence of new viral diseases such those caused by the Ebola and West Nile viruses. However, they recognize the greatest virus health threat to humans is the emergence of new, very virulent strains of influenza. Flu viruses move through the air and are highly contagious. During 1918 and 1919, a flu epidemic.infected more than half the world's population and killed 20-30 million people (including about 500,000 in the United States). Today, flu kills about 1 million people per year (20,000 of them in the United States).
Globally, the spread of acquired immune deficiency syndrome (AIDS), caused by the human immunodeficiency virus (HIV), is a growing threat. The virus itself is not deadly, but it kills immune cells and leaves the body defenseless against infectious bacteria and other viruses. HIV can be transmitted (1) during unprotected sexual activity, (2) from one intravenous drug user to another through shared needles, (3) from an infected mother to an infant before or Cluring birth, or through breast-feeding after birth, and (4) by exposure to infected blood.
According to the WHO, by the end of 2001 some 40 million people (two-thirds of them in sub-Saharan Africa) were infected with HIY. During 2001, 5.4 million people (80% of them in Africa and Asia) were newly infected with HIV-an average of 15,300 new infections per day. In seven sub-Saharan African countries 20% or more of adults are infected with HIY.
Within 7-10 years, at least half of those with HIV develop AIDS. This long incubation period means that infected people often spread the virus for several years without knowing they are infected. So far, no cure for AIDS exists, although drugs may help some infected people live longer (if they can afford the treatment, which can cost up to $15,000 per year).
According to the United Nations, by the end of 2001 about 24.8 million people (420,000 people in the United States) had died of AIDS-related diseases. According to the WHO, between 2000 and 2010 AIDS probably will kill as many people as all the wars in the 20th century combined.
How Are Viral Diseases Treated?
Once a viral infection starts, it is much harder to fight than infections by bacteria and protozoans. Only a few antiviral drugs exist because most drugs that will kill a virus also harm the cells of its host. Treating viral infections (such as colds, flu, and most mild coughs and sore throats) with antibiotics is useless and increases genetic resistance in disease-causing bacteria (Spotlight, p. 226).
Medicine's only effective weapons against viruses are vaccines that stimulate the body's immune system to produce antibodies to ward off viral infections. Immunization with vaccines has helped reduce the spread of viral diseases such as (1) smallpox, (2) polio, (3) rabies, (4) influenza, (5) measles, and (6) hepatitis B.
Growing evidence indicates we may be falling behind in our war against infectious bacterial diseases because bacteria are among the earth's ultimate survivors. When a colony of bacteria is dosed with an antibiotic such as penicillin, most of the bacteria are killed.
However, a few have mutant genes that make them immune to the drug (see figure, right, top). Through natural selection (p. 43), a single mutant can pass such traits on to most of its offspring, which can amount to 16,777,216 in only 24 hours.
Even worse, bacteria can become genetically resistant to antibiotics they have never been exposed to. When a resistant and a nonresistant bacterium touch one another (say, on a hospital bedsheet or in a human stomach), they can exchange a small loop of DNA called a plasmid, thereby transferring genetic resistance from one organism to another (see figure, right, bottom).
The incredible genetic adaptability of bacteria is one reason the world faces a potentially serious rise in the incidence of some infectious bacterial diseases once controlled by antibiotics. Other factors also playa key role, including (1) spread of bacteria (some beneficial and some harmful) around the globe by human travel and the trade of goods, (2) overuse of antibiotics by doctors, often at the insistence of their patients, (3) failure of many patients to take all of their prescribed antibiotics, which promotes bacterial resistance, (4) availability of antibiotics in many countries without prescriptions, (5) overuse of pesticides, which increases populations of pesticideresistant insects and other carriers of bacterial diseases, and (6) widespread use of antibiotics in the livestock and dairy industries to control disease in animals and to promote animal growth.
The result of these factors acting together is that every major diseasecausing bacterium now has strains that resist at least one of the roughly 160 antibiotics we use to treat bacterial infections. In 2000, officials at the u.s. Centers for Disease Control and Prevention estimated that
(1) about 2.2 million people (most with a weakened immune system) a year get sick, and (2) at least 88,000 die from infectious diseases they pick up in U.S. hospitals, nursing homes, or home health-care settings. Patients and their loved ones can reduce such infections by asking any doctor or health-care worker coming into their room, "Did you wash your hands? Or "Did you change your gloves?"
What role, if any, have you played in the increase in genetic resistance of bacteria to widely used antibiotics? List three ways to reduce this threat.
Case Study: Malaria, a Protozoal Disease About 40% of the world's people live in tropical and subtropical regions in which malaria is present (Figure 10-4). Currently, an estimated 300-500 million people are infected with malaria parasites worldwide, and 270-500 million new cases are reported each year.
Malaria's symptoms come and go and include (1) fever and chills, (2) anemia, (3) an enlarged spleen, (4) severe abdominal pain and headaches, (5) extreme weakness, and (6) greater susceptibility to other diseases. The disease kills about 1.1 million people each year, more than half of them children under age 5.
Figure 10-4 Worldwide distribution of malaria. About 40% of the world's current population lives in areas in which malaria is present, with the disease killing at least 1.1 million people a year.
Antibiotics attack harmless and harmful microbes. Drug resistance that develops in harmless bacteria may be transferred to harmful bacteria. One bacterium attaches itself to another, and a channel is opened between them in a process called conjugation. A copy of the genes that make the microbe resistant can then be passed from one microbe to the other.
Development of genetic resistance in strains of bacteria exposed to repeated doses of antibiotics.
Malaria is caused by four species of protozoa of the genus Plasmodium. Most cases of the disease are transmitted when an uninfected female of anyone of 60 species of Anopheles mosquito (1) bites an infected person, (2) ingests blood that contains the parasite, and (3) later bites an uninfected person (Figure 10-5). When this happens, Plasmodium parasites (1) move out of the mosquito and into the human's bloodstream, (2) multiply in the liver, and (3) enter blood cells to continue multiplying. Malaria also can be transmitted by blood transfusions or by sharing needles.
The malaria cycle repeats itself until immunity develops, treatment is given, or the victim dies. Over the course of human history, malarial protozoa probably have killed more people than all the wars ever fought.
Figure 10-5 The life cycle of malaria. This life cycle of Plasmodium circulates from mosquito to human and back to mosquito. Although various mosquito species carry diseases (such as malaria, yellow fever, encephalitis, and dengue fever to humans and heartworm to dogs), mosquitoes also play important ecological roles. Their eggs are a major food source for fish, various insects, and frogs and other amphibians, and adult mosquitoes are an important source of food for bats, spiders, and many insect and bird species.
During the 1950s and 1960s, the spread of malaria was sharply curtailed by (1) draining swamplands and marshes, (2) spraying breeding areas with insecticides, and (3) using drugs to kill the parasites in the bloodstream. Since 1970, malaria has come roaring back. Most species of the malaria-carrying Anopheles mosquito have become genetically resistant to most insecticides. Worse, the Plasmodium parasites have become genetically resistant to common antimalarial drugs.
Researchers are working to develop new antimalarial drugs, vaccines, and biological controls for Anopheles mosquitoes. However, such approaches receive too little funding and have proved more difficult than originally thought.
According to health experts, prevention is the best approach to slowing the spread of malaria. Methods include (1) increasing' water flow in irrigation systems to prevent mosquito larvae from developing (an expensive and wasteful use of water), (2) using mosquito nets dipped in a nontoxic insecticide (permethrin) in windows and doors of homes, (3) cultivating fish that feed on mosquito larvae (biological control), (4) clearing vegetation around houses, (5) planting trees that soak up water in low-lying marsh areas where mosquitoes thrive (a method that can degrade or destroy ecologically important wetlands), (6) using zinc and vitamin A supplements to boost children's resistance to malaria, and (7) greatly increased public education.
How Can We Reduce the Incidence of Infectious Diseases?
The WHO estimates that only 2% of the world's global research and development funds are devoted to infectious diseases in developing countries, even though more people worldwide suffer and die from these diseases than from all others combined. Figure 10-6 lists measures health scientists and public health officials suggest for preventing or reducing the incidence of infectious diseases that affect humanity. They also call for increased emphasis on preventive health care in developing countries (Solutions, p. 230).
Since 1990, one of the world's most underreported stories has been the rapid spread of tuberculosis (TB), a highly infectious bacterial disease that kills about 1.6 million people and infects about 8 million people per year (see figure).
Major reasons for the recent increase in TB are (1) poor TB screening and control programs (especially in developing countries, where about 95% of the new cases occur), (2) development of strains of the tuberculosis bacterium that are genetically resistant to almost all effective antibiotics (typically leading to mortality rates of more than 50%), (3) population growth and increased urbanization (which increase contacts among people), (4) poverty, and (5) the spread of AIDS, which greatly weakens the immune system and allows TB bacteria to multiply.
Slowing the spread of the disease involves early identification and treatment of people with active TB, usually those with a chronic cough. Treatment with a combination of four inexpensive drugs can cure 90% of those with active TB. However, to be successful the drugs must be taken every day for 6-8 months. Because the symptoms disappear after a few weeks, many patients think they are cured and stop taking the drugs. This allows the disease to recur in a hard-totreat form. It then spreads to other people, and drug-resistant strains of TB bacteria develop.
According to the WHO, a worldwide campaign to help control TB would cost about $360 million to help save at least 20 million lives during the next decade.
The current global tuberculosis epidemic. This easily transmitted disease is spreading rapidly and now kills about 1.6 million people a year, mostly in developing countries. (Data from World Health Organization
Before you read this report, were you aware of the serious global TB epidemic, primarily in developing countries? Why do you think this important story has gotten so little media attention compared to other diseases that cause many fewer deaths per year?
Figure 10-6 Solutions: ways to prevent or reduce the incidence of infectious diseases.
10-5 RISK ANALYSIS
How Can We Estimate Risks?
Risk analysis involves (1) identifying hazards and evaluating their associated risks (risk assessment; Figure 10-1, left), (2) ranking risks (comparative risk analysis), (3) determining options and making decisions about reducing or eliminating risks (risk management; Figure 10-1, right), and (4) informing decision makers and the public about risks (risk communication).
Statistical probabilities based on past experience, animal testing and other tests, and epidemiological studies are used to estimate risks from older technologies and chemicals. To evaluate new technologies and products, risk evaluators use more uncertain statistical probabilities, based on models rather than actual experience and testing. .
The left side of Figure 10-7 (p. 231) illustrates comparative risk analysis, summarizing the greatest ecological and health risks identified by a panel of scientists acting as advisers to the U.s. Environmental Protection Agency. Note the difference between the comparison of relative risk by scientists (Figure 10-7, left) and the general public (Figure 10-7, right). These differences result largely from failure of professional risk evaluators to educate the public about the nature of risks and their relative importance. Some risk experts contend that much of our risk education is based on often misleading media reports on the latest risk scare (based mainly on frontier science, p. 16) that do not put such risks in perspective.
Once a risk assessment has been completed, decision makers must decide what level of risk is acceptable. Figure 10-8 (p. 232) shows four methods used to determine the acceptability of a risk. The most widely used method is benefit-cost analysis, which attempts to determine whether the estimated short- and long-term benefits of using a particular technology or chemical outweigh the estimated short- and long-term risks or costs.
What Are the Greatest Risks People Face?
The greatest risks many people face today are rarely dramatic enough to make the daily news. In terms of the number of premature deaths per year (Figure 10-9, p. 232) and reduced life span (Figure 10-10, p. 233), the greatest risk by far is poverty.
After the health risks associated with poverty, the greatest risks of premature death are mostly the result of voluntary choices people make about their lifestyles (Figures 10-9 and 10-10).
By far the best ways to reduce one's risk of premature death and serious health risks are to (1) not smoke, (2) avoid excess sunlight (which ages skin and causes skin cancer), (3) not drink alcohol or drink only in moderation (no more than two drinks in a single day), (4) reduce consumption of foods containing cholesterol and saturated fats, (5) eat a variety of fruits and vegetables, (6) exercise regularly, (7) lose excess weight, and (8) for those who can afford a car, drive as safely as possible in a vehicle with the best available safety equipment.
How Can We Estimate Risks for Technological Systems?
The more complex a technological system and the more people needed to design and run it, the more difficult it is to estimate the risks. The overall reliability of any technological system (expressed as a percentage) is the product of two factors:
System reliability (%) = Technology reliability x Human reliability x 100
With careful design, quality control, maintenance, and monitoring, a highly complex system such as a nuclear power plant or space shuttle can achieve a high degree of technology reliability. However, human reliability usually is much lower than technology reliability and almost impossible to predict; to err is human.
Suppose the technology reliability of a nuclear power plant is 95% (0.95) and human reliability is 75% (0.75). Then the overall system reliability is 71% (0.95 X 0.75 X 100 = 71%). Even if we could make the technology 100% reliable (1.0), the overall system reliability would still be only 75% (1.0 X 0.75 X 100 = 75%). The crucial dependence of even the most carefully designed systems on unpredictable human reliability helps explain essentially "impossible" tragedies such as the (1) Chernobyl (Case Study, p. 125) nuclear power plant accident and (2) explosion of the space shuttle Challenger.
One way to make a system more foolproof or failsafe is to move more of the potentially fallible elements from the human side to the technical side. However, (1) chance events such as a lightning bolt can knock out an automatic control system, (2) no machine or computer program can completely replace human judgment, (3) the parts in any automated control system are manufactured, assembled, tested; certified, and maintained by fallible human beings, and (4) computer software programs used to monitor and control complex systems can also contain human error or can be deliberately modified by computer viruses to malfunction.
With adequate funding, the health of people in developing countries and the poor in developed countries can be improved dramatically, quickly, and cheaply by providing the following forms of mostly preventive health care:
. Better nutrition, prenatal care, and birth assistance for pregnant women.
. Better nutrition for children.
. Greatly improved postnatal care (including promotion of breastfeeding) to reduce infant mortality. Breast-fed babies get natural immunity to many diseases from antibodies in their mothers' milk.
. Immunization against the world's five largest preventable infectious diseases: tetanus, measles, diphtheria, typhoid fever, and polio. Some good news is that since 1971, the percentage of children in developing countries immunized against these diseases has increased from 10% to 84%, saving about 10 million lives a year.
. Oral rehydration therapy for victims of diarrheal diseases, which cause about one-fourth of all deaths of children under age 5. A simple solution of boiled water, salt, and sugar or rice, at a cost of only a few cents per person, can prevent death from dehydration.
. Careful and selective use of antibiotics for infections (Spotlight, p. 226).
. Clean drinking water and sanitation facilities for the one-third of the world's population that lacks them.
In 2001, the WHO began promoting a do-it-yourself technique that uses sunlight to disinfect water. The process is simple: (1) fill a transparent plastic bottle with contaminated water, and (2) lay it horizontally on a flat black surface (which absorbs more heat and kills more pathogens). The heat and ultraviolet rays of the sun kill most illnesscausing microorganisms in polluted water. This method is especially useful in tropical countries where sunlight is intense.
According to the WHO, extending such primary health care to all the world's people would cost an additional $10 billion per year, about 4% of what the world spends every year on cigarettes or devotes every 4 days to military spending. The cost of this program is about $1 per child. In 2002, Bill and Melinda Gates created a fund of more than $24 billion to help fight diseases that affect the world's poorest people.
1. Do you believe developed countries should foot at least half the bill for implementing such proposals? What economic and environmental benefits would this provide for developed countries?
2. How many dollars per year of your taxes would you be willing to spend for such a preventive health program in developing countries?
What Are the Limitations of Risk Analysis?
Here are some of the key questions involved in evaluating risk analysis:
. How reliable are risk assessment data and models?
. Who profits from allowing certain levels of harmful chemicals into the environment, and who suffers? Who decides this?
. Should estimates emphasize short-term risks, or should more weight be put on long-term risks? Who should make this decision?
. Should the primary goal of risk analysis be to (1) determine how much risk is acceptable (the current approach) or (2) figure out how to do the least damage (a prevention approach)? . .
. Who should do a particular risk analysis, and who should review the results? A government agency? Independent scientists? The public?
. Should cumulative effects of various risks be considered, or should risks be considered separately, as is usually done? Suppose a pesticide is found to have an annual risk of killing 1 person in 1 million from cancer, the current EPA limit. Cumulatively, however, effects from 40 such pesticides might kill 40, or 400, of every 1 million people. Is this acceptable and to whom is it acceptable?
. How widespread is each risk?
. Should risk levels be higher for workers (as is almost always the case) than for the general public? What say should workers and their families have in this decision? For example, work-related illnesses and injuries kill about 80,000 workers each year in the United States-about twice the country's annual death toll from automobile accidents (Spotlight, p. 220). The situation is much worse in developing countries, with more than 1 million work-related deaths occurring worldwide each year.
. How much risk is acceptable and to whom is it acceptable? According to the National Academy of Sciences, exposure to toxic chemicals is responsible for 2-4% of the 521,000 cancer deaths in the United States; this amounts to 10,400-20,800 premature cancer deaths per year.
Proponents contend that risk analysis is a useful way to (1) organize and analyze available scientific information, (2) identify significant hazards, (3) focus on areas that warrant more research, (4) help regulators decide how money for reducing risks should be allocated, and (5) stimulate people to make more informed decisions about health and environmental goals and priorities.
However, critics point out that results of risk analysis are very uncertain. For example, a recent study documented the significant uncertainties involved in even simple risk analysis. Eleven European governments established 11 different teams of their best scientists and engineers (including those from private companies) to assess the hazards and risks from a small plant storing only one hazardous chemical (ammonia). The 11 teams, consisting of world-class experts analyzing this very simple system, disagreed with one another on fundamental points and varied in their assessments of the hazards by a factor of 25,000. Such inherent uncertainty explains why regulators setting human exposure levels for toxic substances usually divide the best results by 100 to 1,000 to provide the public with a margin of safety.
According to critics, the main decision-making tool we should rely on is not to find out how much risk is acceptable, which is mostly a political decision. Instead, it should be to find out the least damaging reasonable alternatives by asking, "Which alternative will bring sufficient benefits and minimize damage to humans and to the earth?" To these critics, the emphasis should be on alternative assessment, not risk assessment.
Figure 10-7 Comparative risk analysis of the most serious ecological and health problems according to scientists acting as advisers to the U.S. Environmental Protection Agency (left column). Risks in each of these categories are not listed in rank order. The right side of this figure represents polls showing how U.S. citizens rank the ecological and health risks they perceive as the most serious. Why do you think there is such a great difference between the ranking by risk experts and by the general public? (Data from Science Advisory Board, Reducing Risks, Washington, D.C.: Environmental Protection Agency, 1990)
Figure 10-8 Methods for determining the acceptability of a risk after a risk assessment has been made. Cost-benefit analysis is the most widely used method.
Figure 10-9 Number of deaths per year in the world from various causes. Numbers in parentheses give these deaths in terms of the number of fully loaded jumbo (400-passenger) jets crashing accidentally every day of the year with no survivors.
Figure 10-10 Comparison of risks people face, expressed in terms of shorter average life span. After poverty, the greatest risks people face result mostly from voluntary choices they make about their lifestyles. These are only generalized relative estimates.(Data from Bernard L. Cohen)
Individual response to some of these risks can vary with factors such as (1) genetic variation, (2) family medical history, (3) emotional makeup, (4) stress, and (5) social ties and support.
How Should Risks Be Managed?
Risk management includes the administrative, political, and economic actions taken to decide whether and how to reduce a particular societal risk to a certain level and at what cost.
Risk management involves answering the following questions:
. How reliable is the risk analysis for each risk? . Which risks should have the highest priority? . How much risk is acceptable (Figure 1O-8)?
. How much it will cost to reduce each risk to an acceptable level?
. How should limited funds be spent to provide the greatest benefit?
. How will the risk management plan be monitored, enforced, and communicated to the public?
Each step in this process involves making value judgments and weighing trade-offs to find some reasonable compromise among conflicting political, economic, health, and environmental interests.
How Well Do We Perceive Risks?
How much risk is acceptable? Studies indicate that if the chance of death from a chemical or activity is less than 1 in 100,000, most people are not likely to be worried enough to change their ways.
However, most of us do poorly in assessing the relative risks from the hazards that surround us (Figures 10-7, 10-9, and 10-10). Also, many people deny or shrug off the high risk of death (or injury) from voluntary activities they enjoy, such as (1) motorcycling (1 death in 50 participants), (2) smoking (1 in 300 participants by age 65 for a pack-a-day smoker), (3) hang gliding (1 in 1,250), and (4) driving (1 in 3,000 without a seatbelt and 1 in 6,000 with a seatbelt).
Yet some of these same people may be terrified about the possibility of dying from (1) a commercial airplane crash (1 in 1 million), (2) being killed by a handgun (1 in 2 million), (3) being struck by lightning (1 in 4 million), (4) a train crash (1 in 20 million), (5) snakebite (1 in 36 million), (6) shark attack (1 in 300 million), or (7) exposure to trichloroethylene (TCE) in drinking water at the trace levels allowed by the EPA (1 in 2 billion).
Being bombarded with news about people killed or harmed by various hazards distorts our sense of risk. However, the most important good news each year is that about 99.1% of the people on the earth did not die. Despite the greatly increased use of synthetic chemicals in food production and processing, the general health and average life expectancy of people in the United States (and most developed countries) have increased during the past 50 years.
Our perceptions of risk and our responses to perceived risks often have little to do with how risky the experts say something is (Figure 10-7). The public generally sees a technology or a product as being riskier than experts do when the following conditions exist:
. It is new or complex rather than familiar. Examples include genetic engineering or nuclear power, as opposed to large dams or coal-fired power plants.
. It is perceived as mostly involuntary. Examples include nuclear power plants or food additives, as opposed to driving or smoking.
. It is viewed as unnecessary rather than as beneficial or necessary. Examples might include using chlorofluorocarbon (CFe) propellants in aerosol spray cans or using food additives that increase sales appeal, as opposed to cars or aspirin.
. Its use involves a large, well-publicized death toll from a single catastrophic accident rather than the same or an even larger death toll spread out over a longer time. Examples might include a severe nuclear power plant accident, an industrial explosion, or a plane crash, as opposed to coal-burning power plants, automobiles, or smoking (Figure 10-9).
. Its use involves unfair distribution of the risks. Citizens are outraged when government officials decide to put a hazardous waste landfill or incinerator in or near their neighborhood, even when the decision is based on risk analysis. This decision is usually seen as politics, not science. Residents will not be satisfied by estimates that the lifetime risks of cancer death from the facility are not greater than, say, 1 in 100,000. Living near the facility means they, not the vast majority of people living farther away, have a much higher risk of dying from cancer by having this risk involuntarily imposed on them.
. The people affected are not involved in the decisionmaking process from start to finish.
. Its use does not involve a sincere search for and evaluation of alternatives. People who believe their lives and the lives of their families are being threatened want to know (1) what the alternatives are and (2) which alternative causes the least harm to them and the earth.
Better education and communication about the nature of risks will help bring the public's perceptions of various risks closer to those of professional risk evaluators. However, such education will not eliminate the emotional, cultural, and ethical factors that decision makers must take into account in determining the acceptability of a particular risk and evaluating the possible alternatives.
The burden of proof imposed on individuals, companies, and institutions should be to show that pollution prevention options have been thoroughly examined, evaluated, and used before lesser options are chosen. JOEL HIRSCHORN
2. What are risk and probability? Distinguish between risk assessment and risk management.
3. What human activity kills the largest number of people each year? List six ways to help reduce the harmful effects of smoking.
4. List two (a) cultural hazards, (b) chemical hazards, (c) physical hazards, and (d) biological hazards.
5. What is toxicity? Distinguish between dose and response for a potentially harmful substance. List five factors that determine whether a chemical is harmful.
6. What is a poison? What is an LDso? List three methods used to determine toxicity, and list the limitations of each method. Describe how laboratory tests are used to determine toxicity. What is a dose-response curve? Distinguish between a non threshold dose-response curve and a threshold dose-response curve.
7. Distinguish between toxic chemicals and hazardous chemicals. Distinguish among mutagens, teratogens, and carcinogens.
8. Distinguish among the immune system, nervous system, and endocrine system, and give an example of something that causes harm to each system. Why is there concern about human exposure to low levels of synthetic chemicals, known as hormonally active agents (HAAs)?
9. About what percentage of the 75,000 chemicals in commercial use in the United States have been screened to (a) assess toxicity, (b) determine whether they are carcinogens, teratogens, or mutagens, and (c) determine whether. they damage the nervous, endocrine, or immune systems?
10. List three reasons for the lack of information about the potentially harmful effects of most chemicals in commercial use. What is the precautionary principle, and how can it be used to help protect the public from harmful chemicals?
11. Distinguish between non transmissible and transmissible diseases, and give two examples of each type. Distinguish between bacterial and viral disease, and give two examples of each type. What are the seven deadliest infectious diseases in order of the number of deaths they cause each year? List five reasons for the increase in TB infections in recent years.
12. How do infectious bact ria become resistant to antibiotics? List six factors that have led to increased genetic resistance by disease-causing bacteria to antibiotics.
13. What is the best way to treat (a) a bacterial disease and (b) a viral disease?
14. Distinguish between HIV and AIDS. List four ways that HIV can be transmitted. About how many people in the world are (a) infected with HIV and (b) have died of AIDS?
15. What causes malaria? About how many people die from malaria each year? List seven ways to help prevent this protozoal infectious disease.
16. List ten ways to prevent or reduce the incidence of infectious diseases throughout the world. List seven major ways to improve health care in developing countries.
17. What is risk analysis? What are its major limitations?
18. List five of the greatest risks people face in terms of (a) number of premature deaths per year and (b) reduced life span. List eight ways to reduce your risk of premature death and serious health problems. How can we estimate the risks from technological systems?
19. What is risk management? What six questions do risk managers try to answer? About what percentage of the people on the earth die each year? List seven reasons why people perceive that certain risks are greater than experts say they are.
1. Do you think chemicals should be regulated based on their effects on the nervous, immune, and endocrine systems? Explain.
2. Should we have zero pollution levels for all hazardous chemicals? Explain.
3. Do you believe health and safety standards in the workplace should be strengthened and enforced more vigorously, even if this causes a loss of jobs when companies transfer operations to countries with weaker standards? Explain.
4. Evaluate the following statements:
a. We should not get so worked up about exposure to toxic chemicals because almost any chemical can cause some harm at a large enough dosage.
b. We should not worry so much about exposure to toxic chemicals because through genetic adaptation we can develop immunity to such chemicals.
c. We should not worry so much about exposure to toxic chemicals because we can use genetic engineering to reduce or eliminate such problems.
5. What do you believe are the three major things we should do to reduce the development of genetically resistant infectious bacteria?
6. How can changes in the age structure of a human population increase the spread of infectious diseases? How can the spread of infectious diseases affect the age structure of human populations?
7. How would you answer each of the questions raised about risk analysis (pp. 230-231) and risk management (p. 233)? Explain each of your answers.
8. What are the five major risks you face from your lifestyle, where you live, and what you do for a living? Which of these risks are voluntary, and which are involuntary? List the five most important things you can do to reduce these risks. Which of these things do you actually plan to do?
Try to find the following articles:
1. Frank, c., A. D. Fix, C. A. Pena, and G. T. Strickland. 2002. Mapping Lyme disease incidence for diagnostic and preventive decisions, Maryland. Emerging Infectious Diseases 8: 427. Keywords: "Lyme disease" and "mapping." GIS mapping technology was used to illustrate the distribution of Lyme disease in Maryland. This information could then be used by doctors and state health departments to assist in diagnosis and distribution of vaccine.
2. Thiele, L. P. 2000. Limiting risks: Environmental ethics as a policy primer. Policy Studies Journal 20: 540. Keywords: "risk" and "environmental ethics." This article explores
the ethical aspects of evaluating involuntary humancaused environmental risks in a technologically driven society.
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