ENV330

· Chapter Introduction

· Core Case Study Mercury’s Toxic Effects

· 17.1 Health Hazards and Risk Assessment

· 17.1a Risk and Hazards

· 17.2 Biological Hazards

· 17.2a Infectious Diseases

· 17.2b Viral Diseases and Parasites

· 17.2c Reducing the Incidence of Infectious Diseases

· 17.3 Chemical Hazards

· 17.3a Some Chemicals Can Cause Cancers, Mutations, and Birth Defects

· 17.3b Some Chemicals Can Affect Our Immune and Nervous Systems

· 17.3c Some Chemicals Affect the Endocrine System

· 17.4 Evaluating Risks from Chemical Hazards

· 17.4a Many Factors Determine the Toxicity of Chemicals

· 17.4b Methods for Estimating Toxicity

· 17.4c Are Trace Levels of Toxic Chemicals Harmful?

· 17.4d Why Do We Know So Little about the Harmful Effects of Chemicals?

· 17.4e Pollution Prevention and the Precautionary Principle

· 17.4f Implementing Pollution Prevention

· 17.5 Perceiving and Avoiding Risks

· 17.5a The Greatest Health Risks Come from Poverty, Gender, and Lifestyle Choices

· 17.5b Estimating Risks from Technologies

· 17.5c Most People Do a Poor Job of Evaluating Risks

· 17.5d Guidelines for Evaluating and Reducing Risk

· Tying It All Together Mercury’s Toxic Effects and Sustainability

· Chapter Review

· Critical Thinking

· Doing Environmental Science

· Data Analysis

17.1aRisk and Hazards

A  risk  is the probability of suffering harm from a hazard that can cause injury, disease, death, economic loss, or damage. Scientists often state the probability of a risk in terms such as, “The lifetime probability of developing lung cancer from smoking one pack of cigarettes per day is 1 in 250.” This means that 1 of every 250 people who smoke a pack of cigarettes every day will likely develop lung cancer over a typical lifetime (usually considered to be 70 years). Probability can also be expressed as a percentage, as in a 30% chance of developing a certain type of cancer. The greater the probability of harm, the greater the risk.

Risk assessment  uses statistical methods to estimate how much harm a particular hazard can cause to human health or to the environment. It helps us compare risks and establish priorities for avoiding or managing risks.  Risk management  involves deciding whether and how to reduce a particular risk to a certain level and at what cost. Figure 17.2 summarizes how risks are assessed and managed.

Figure 17.2

Risk assessment and risk management are used to estimate the seriousness of various risks and to help reduce such risks.

Critical Thinking:

1. What is an example of how you have applied this process in your daily living?

Many people take avoidable risks every day. For example, they might drive or ride in a car without a seatbelt or text while driving. They might choose to eat foods that are high in cholesterol or that have too much sugar. They might drink too much alcohol or smoke.

No one can live a risk-free life, but we can reduce exposure to risks. When assessing risks, it is important to understand how serious the risks are and whether the benefits of certain activities outweigh the risks.

Five major types of hazards pose risks to human health:

· Biological hazards from more than 1,400  pathogens , or microorganisms that can cause disease in other organisms. Examples are bacteria, viruses, parasites, protozoa, and fungi.

· Chemical hazards from certain harmful chemicals in the air, water, soil, food, and human-made products (Core Case Study).

· Natural hazards such as fires, earthquakes, volcanic eruptions, floods, tornadoes, and hurricanes.

· Cultural hazards such as unsafe working conditions, criminal assault, and poverty.

· Lifestyle choices such as smoking, making poor food choices, and not getting enough exercise.

Critical Thinking

1. Think of a hazard from each of these categories that you may have faced recently. Which one was the most threatening?

17.2aInfectious Diseases

An  infectious disease  is a disease caused by a pathogen such as a bacterium, virus, or parasite invading the body and multiplying in its cells and tissues.  Bacteria  are single-cell organisms that are found everywhere and that can multiply rapidly on their own. Most bacteria are harmless and some are beneficial. However, those that cause diseases such as strep throat or tuberculosis are harmful.

A  virus  is a pathogen that invades a cell and takes over its genetic machinery to copy itself and spread throughout the body. Viruses can cause diseases such as flu and acquired immunodeficiency syndrome (AIDS). A  parasite  is an organism that lives on or inside another organism and feeds on it. Parasites range in size from one-celled organisms called protozoa to worms that are visible to the naked eye. They can cause an infectious disease such as malaria.

A  transmissible disease  is an infectious disease that can be transmitted from one person to another. Some transmissible diseases are bacterial diseases such as tuberculosis, many ear infections, and gonorrhea. Others are viral diseases such as the common cold, flu, and AIDS. Transmissible diseases can be spread through air, water, and food. They can also be transmitted by insects such as mosquitoes and ticks and by body fluids such as feces, urine, blood, semen, and droplets sprayed by sneezing and coughing.

A  nontransmissible disease  is caused by something other than a living organism and does not spread from one person to another. Nontransmissible diseases include cardiovascular (heart and blood vessel) diseases, most cancers, asthma, and diabetes.

In 1900, infectious disease was the leading cause of death in the world. Since then, and especially since 1950, the incidences of infectious diseases and the death rates from them have dropped significantly. This has been achieved mostly by a combination of improved sanitation, better health care, the use of antibiotics to treat bacterial diseases, and the development of vaccines to prevent the spread of some viral diseases. According to the World Health Organization (WHO), during the last decade vaccines have saved more than 10 million lives.

Despite the declining risk of harm from infectious diseases, they remain serious health threats, especially in less-developed countries. A large-scale outbreak of an infectious disease in an area or a country is called an epidemic. A global epidemic, like tuberculosis (see Case Study that follows) or AIDS is called a pandemic. Figure 17.3 shows the annual death tolls from the world’s seven deadliest infectious diseases.

Figure 17.3

Leading causes of death by infectious diseases in the world.

Data Analysis:

1. How many people die from all seven of these infectious diseases every year? Every day?

(Compiled by the authors using data from the World Health Organization and the U.S. Centers for Disease Control and Prevention)

Case Study

The Global Threat from Tuberculosis

Tuberculosis (TB) is an ancient and highly contagious bacterial infection that destroys lung tissue. Many TB-infected people do not appear to be sick and most of them do not know they are infected. Left untreated, each person with active TB typically infects a number of other people. Without treatment, about half of the people with active TB die from bacterial destruction of their lung tissue (Figure 17.4).

Figure 17.4

Colorized red areas in this chest X-ray show where TB bacteria have destroyed tissue in both lungs.

Puwadol Jaturawutthichai/ Shutterstock.com

In 2017, there were about 10 million new cases of TB and 1.7 million people died from TB, according to the WHO. Several factors account for the spread of TB since 1990. One is a lack of TB screening and control programs, especially in less-developed countries where more than 90% of the new cases occur. However, researchers are developing new and easier ways to detect TB and to monitor its effects (Individuals Matter 17.1).

A second problem is that most strains of the TB bacterium have developed genetic resistance to the majority of the effective antibiotics (Science Focus 17.1). In addition, population growth, urbanization, and air travel have greatly increased person-to-person contacts. A person with active TB might infect several people during a single bus or plane ride. TB is spreading faster in areas where large numbers of poor people crowd together, especially in the rapidly growing slums of less-developed countries.

Slowing the spread of the disease requires early identification and treatment of people with active TB, especially those with a chronic cough, which is the primary way in which the disease is spread from person to person. However, because many people do not show symptoms of TB, they are unaware that they are infected and can infect other people. Treatment with a combination of four inexpensive drugs can cure 90% of individuals with active TB, but to be effective, the drugs must be taken every day for 6–9 months and these drugs can have serious side effects. Symptoms often disappear after a few weeks of treatment, so many patients think they are cured and stop taking the drugs. This can allow TB to recur, possibly in drug-resistant forms, and to spread to others.

A deadly form of tuberculosis, known as multidrug-resistant TB, is on the rise. About 480,000 new cases occur every year, according to the WHO. Fewer than half of those cases are cured each year, and only with the best available medical care costing more than $500,000 per person on average. This form of TB kills about 150,000 people every year. Because this disease cannot be treated effectively with antibiotics, victims must be isolated from the rest of society, some permanently, and they pose a threat to health workers.

Since 1993, TB infection rates have been declining in the United States. In 2017, there were 9,098 new cases of TB in the United States, according to the Centers for Disease Control and Prevention (CDC).

Science Focus 17.1

Genetic Resistance to Antibiotics and Antifungals

Antibiotics are chemicals that can kill bacteria. They have played an important role in the increase in life expectancy since 1950 in the United States and in many other countries.

In 2014, the WHO issued a report warning that the age of antibiotics may be ending because many disease-causing bacteria are becoming genetically resistant to the antibiotics that have long been used to kill the bacteria. The WHO considers antibiotic resistance one of the biggest threats of this century and the World Economic Forum calls it a “potential disaster” for the global economy and human health.

One reason for this antibiotic resistance is the astounding reproductive rate of bacteria. Some bacteria can grow from a population of 1 to well over 16 million in 24 hours. As a result, they can quickly become genetically resistant to an increasing number of antibiotics through natural selection (see Figure 4.14). They pass such genetic resistance to their offspring and research indicates that some bacteria can transfer such resistance to others of the same strain as well as to different strains of bacteria.

Another major factor in the rise of such genetic resistance, also called antibiotic resistance, is the widespread use of antibiotics on livestock raised in feedlots (see Figure 12.10) and concentrated animal feeding operations (CAFOs, see Figure 12.11). Antibiotics are used to control disease and to promote growth among dairy and beef cattle, poultry, and hogs that are raised in large numbers in crowded conditions. The U.S. Food and Drug Administration (FDA) has estimated that about 80% of all antibiotics used in the United States are added to the feed of healthy livestock. According to the CDC, about 20% of antibiotic-resistant illness in humans is linked to food, especially food from livestock treated with antibiotics.

Another factor that can promote genetic resistance is the overuse of antibiotics for colds, flu, and sore throats, many of which are caused by viruses that do not respond to treatment with antibiotics. In many countries, antibiotics are available without a prescription, which promotes their excessive and unnecessary use. Another factor is the spread of bacteria around the globe by human travel and international trade. The growing use of antibacterial hand soaps and other antibacterial cleansers could also be promoting antibiotic resistance in bacteria. Such cleaners do not work any better than thorough hand washing, according to the FDA. Research by scientists Paul Dawson and Brian Sheldon indicates that three major sources of infectious bacteria are lemon slices on the rims of water glasses, menus, and hot-air hand dryers in restrooms that blow bacteria into the bathroom air.

Every major disease-causing bacterium has developed strains that resist at least 1 of the roughly 200 antibiotics. According to the CDC, antibiotic resistance causes over 2 million illness and 23,000 deaths in the United States each year. Furthermore, bacteria called superbugs that resist all but a few antibiotics are emerging. In 2018, researchers at the Washington School of Medicine estimated that infectious diseases from superbugs kill 162,000 Americans a year. In addition, 1 of every 25 U.S. hospital patients picks up such an infection while in the hospital. A 2-year British government study led by economist Jim O’Neil estimated that globally, antibiotic-resistant superbugs kill at least 700,000 people per year and by 2050, could kill as many as 10 million people a year.

For example, a bacterium known as methicillin-resistant Staphylococcus aureus, commonly known as MRSA (or “mersa”), has become resistant to most common antibiotics. MRSA can cause severe pneumonia, a vicious rash, and a quick death if it gets into the bloodstream.

MRSA can be found in hospitals, nursing homes, schools, gyms, and college dormitories. It can be spread through skin contact, unsanitary use of tattoo needles, and contact with poorly laundered clothing and shared items such as towels, bed linens, athletic equipment, and razors. Another worrisome superbug found in hospitals is Clostridium difficile, or C. diff, which causes severe diarrhea and can live on surfaces such as bed rails and medical equipment. It causes about 250,000 infections and 14,000 deaths per year in the United States, according to the CDC.

Health officials warn that we could be moving into a post-antibiotic era of higher death rates. No new class of antibiotics has been developed since 1984, mostly because drug companies lose millions of dollars developing new antibiotics that are used for only a short time to treat infections. As a result, in 2017, only 15 of the world’s 50 largest drug companies were developing new antibiotics.

However, in 2015, researchers led by Kim Lewis discovered a new antibiotic called teixobactin, extracted from bacteria that live in dirt. In laboratory mice, it proved to be a powerful drug against tuberculosis, MRSA, and other infections. It works by breaking down a microbe’s outer cell walls—an approach that makes it difficult for bacteria to develop resistance to it. It will take years of testing to learn whether teixobactin offers a possible solution to the serious problem of antibiotic resistance.

In 2019, researchers and the CDC warned about Candida auris (or C. auris), a fungus that preys upon people with weakened immune systems. It is spreading around the world and is especially dangerous because it is genetically resistant to most anti-fungal medications. It is difficult to identify and kills nearly half of the patients who become infected within 90 days.

Critical Thinking

1. What are three steps that you think we could take to slow the rate at which disease-causing bacteria are developing resistance to antibiotics and fungi are developing genetic resistance to antifungals?

Individuals Matter 17.1

Hayat Sindi: Health Science Entrepreneur

Hayat Sindi /National Geographic Image Collection

Growing up in a home of humble means in Saudi Arabia, Hayat Sindi was determined to get an education, become a scientist, and do something for humanity. She was the first Saudi woman to be accepted at Cambridge University. She also earned a PhD in biotechnology at Cambridge and she taught in Cambridge’s international medical program. She was named a National Geographic Explorer and a United Nations Educational, Scientific, and Cultural Organization (UNESCO) Goodwill Ambassador for science education.

As a visiting scholar, Sindi worked with a team of scientists at Harvard University and co-founded a nonprofit company called Diagnostics for All to bring low-cost health monitoring to remote, poor areas of the world. The Harvard team sought to develop simple and inexpensive diagnostic tools that could be used to detect certain illnesses and medical problems in remote areas.

One such tool is a piece of paper the size of a postage stamp, with tiny channels and wells etched into it. A technician loads the channels with diagnostic chemicals and puts a drop of a patient’s blood, urine, or saliva on the paper. The fluid travels through the channels where the chemicals react with the fluid to change its color. Results show up in a minute. They can easily be read to diagnose different medical infections and conditions such as declining liver function, which can result from taking drugs to combat TB, hepatitis, and HIV/AIDS. The test can be conducted by a technician with minimal training and requires no electricity, clean water, or special equipment. After the paper is used, it can be burned on the spot to prevent the spread of any infectious agents.

Dr. Sindi has a passion for inspiring women and girls, particularly those in the Middle East, to purse science. As she explains, “I want all women to believe in themselves and know they can transform society.”

Learning from Nature

A shark’s skin is covered with tiny bumps that somehow help it to avoid bacterial infections. Scientists are using this information to create antibacterial films with a bumpy structure that could reduce human skin infections.

One reason why infectious disease is still a serious threat is that many disease-carrying bacteria have developed genetic immunity to widely used antibiotics (Science Focus 17.1). In addition, many disease-transmitting species of insects such as mosquitoes have become resistant to widely used pesticides such as DDT that once helped to control their populations.

Another factor that will likely keep infectious diseases high on the list of environmental health threats is climate change. Many scientists warn that warmer temperatures will likely allow some infectious diseases—especially those spread by mosquitoes and ticks that breed more rapidly in warmer climates—to spread to and thrive in formerly cooler parts of the world. An example is dengue fever. It is the world’s most widespread mosquito-borne viral disease, with nearly 400,000 new infections and thousands of deaths a year. West Nile virus, Zika virus, and yellow fever are also spread by mosquitoes. Other examples are Lyme disease and Rocky Mountain spotted fever, which are spread by ticks.

In 2016, melting permafrost in Siberia exposed the frozen carcass of a reindeer infected with deadly anthrax. When the reindeer carcass thawed out it released anthrax bacteria, which killed a boy, infected 20 other people, and killed more than 2,000 present-day reindeer.

17.2bViral Diseases and Parasites

2 Million

The annual number of U.S. citizens who get infections that cannot be treated with any known antibiotics

Antibiotics do not affect viruses and some viruses are fatal. The biggest viral killer is the influenza or flu virus because it often leads to fatal pneumonia. The flu virus can be transmitted to others by body fluids or airborne droplets released when an infected person coughs or sneezes. Influenza often leads to fatal pneumonia. Flu viruses are transmitted so easily that an especially potent flu virus could spread around the world in only a few months. This could cause a pandemic and kill millions of people.

The second biggest viral killer is the human immunodeficiency virus, or HIV (see  Case Study  that follows). According to the Joint United Nations Programme on HIV, in 2017, HIV infected about 1.8 million people and 940,000 people died from AIDS-related diseases (down from 2 million in 2005). HIV is transmitted by unsafe sex, the sharing of needles by drug users, infected mothers who pass the virus to their babies before or during birth, and exposure to infected blood.

Case Study

The Global HIV/AIDS Epidemic

The spread of acquired immunodeficiency syndrome (AIDS), caused by HIV infection, is a major global health threat. This virus cripples the immune system and leaves the body vulnerable to infections such as TB and rare forms of cancer such as Kaposi’s sarcoma. A person infected with HIV can live a normal life, especially with proper but costly treatment. In time, however, HIV can develop into AIDS, which can be fatal. An estimated 20% of all people infected with HIV are not aware of the infection and can spread the virus for years before being diagnosed.

Since HIV was identified in 1981, this viral infection has spread around the globe. According to UNAIDS, in 2017, about 36.9 million people worldwide (about 1.1 million in the United States, according to the CDC) were living with HIV. In 2017, there were about 1.8 million new cases of AIDS (about 39,500 in the United States)—half of them in people ages 15 to 24.

Between 1981 and 2016, about 386 million people died of AIDS-related diseases, according to UNAIDS. According to the CDC, the U.S. death toll for the same period was more than 693,000. In 2016, AIDS killed about 940,000 million people (about 6,000 in the United States)—down from a peak of 2.3 million in 2005. AIDS has reduced the life expectancy of the 1 million people living in sub-Saharan Africa, the area south of the Sahara Desert, from 62 to 47 years on average, and to 40 years in the seven countries most severely affected by AIDS.

Deaths of people ages 15 to 49 affect the population age structures in several African countries, including Botswana ( Figure 17.6 ), where 23% of all people between ages 15 and 49 were infected with HIV in 2017. The premature deaths from AIDS of many young, productive teachers, health-care workers, farmers, and other adults in these countries has contributed to declines in education, health care, food production, economic development, and political stability. They have also led to large numbers of orphaned children.

Figure 17.6

In Botswana 23% of all people ages 15–49 were infected with HIV in 2017. This figure shows two projected age structures for Botswana’s population in 2020—one including the possible effects of the AIDS epidemic (red bars), and the other not including those effects (yellow bars).

Critical Thinking:

1. How might this affect Botswana’s economic development?

(Compiled by the authors using data from the U.S. Census Bureau, UN Population Division, and World Health Organization)

The treatment for HIV infection includes a combination of antiviral drugs that can slow the progress of the virus. However, such drugs cost too much to be used widely in the less-developed countries where HIV infections are widespread.

The third largest viral killer is the hepatitis B virus (HBV), which damages the liver. According to the WHO, it kills more than 780,000 people each year. It is spread in the same ways that HIV is spread.

Widespread screening of people for the Ebola virus can help reduce its spread ( Figure 17.5 ). However, those who care for patients are at a much higher-than-average risk of getting the disease, no matter where they are.

Figure 17.5

These health-care workers are screening a woman in China for the Ebola virus. They must wear special suits to avoid all direct contact between their own skin and anyone who might be infected with the virus.

plavevski/ Shutterstock.com

Another harmful virus is the Zika virus, which since 2010, has spread in 42 countries, most in Latin America. It is spread by the bite of a mosquito species that also spreads yellow fever and dengue fever. It can be transmitted through sex, and a pregnant woman can pass the Zika virus to her fetus. The mosquito species that spreads Zika is widespread in Latin America and, by 2016, had been found in 30 U.S. states, most of them warmer southern states. The disease can spread rapidly in less-developed countries with warm climates, where many houses have no window or door screens. The mosquitoes breed in standing water found near such homes.

Scientists estimate that throughout history, more than half of all infectious diseases were originally transmitted to humans from wild or domesticated animals. The development of such diseases has spurred the growth of the new field of ecological medicine ( Science Focus 17.2 ). GREEN CAREER: Ecological medicine

Science Focus 17.2

Ecological Medicine: Tracking Infectious Diseases from Animals to Humans

Scientists estimate that throughout history, more than half of all infectious diseases were originally transmitted to humans from wild or domesticated animals. Examples of such diseases and their origins include the following:

· HIV—moves from primates (apes and monkeys) to humans

· Lyme disease—moves from wild deer and mice through ticks to humans

· Ebola—thought to have come from bats

· West Nile virus—transmitted from birds via mosquito bites

· Avian flu—a severe flu strain from birds

· Plague—moved from rats to rat fleas to humans

· Dengue fever—spread through mosquitoes and thought to have come from apes

· African sleeping sicknesses—moves from wild and domestic grazing animals through tsetse flies to humans

In order, the three largest sources of diseases likely to infect people are bats, primates, and rodents (rats and mice).

The development of such infectious diseases has spurred the growth of the relatively new field of ecological medicine. It is devoted to tracking down infectious disease connections between animals and humans and investigating other factors, such as climate change, that can affect populations of the wild species involved.

Scientists in this field have identified several human practices that encourage the spread of diseases among animals and people:

· The clearing or fragmenting of forests to make way for settlements, farms, and expanding cities.

· The hunting of wild game for food. In parts of Africa and Asia, local people who kill monkeys and other animals for bushmeat regularly come in contact with primate blood and can be exposed to a simian (ape or monkey) strain of HIV, which causes AIDS.

· The illegal international trade in wild species.

· Industrialized meat production. For example, a deadly form of E. coli bacteria sometimes spreads from livestock to humans when people eat meat contaminated by animal manure. Salmonella bacteria found on animal hides and in poorly processed, contaminated meat can cause food-borne disease. Each year, 48 million Americans get sick, 128,000 are hospitalized, and 3,000 die from preventable food-borne diseases.

In the United States, the push of suburban development into forests has increased the chances of many suburbanites becoming infected with Lyme disease. The bacterium that causes this disease lives in the bodies of deer and white-footed mice and is passed between these two animals and to humans, mostly by certain types of ticks ( Figure 17.A , left). It is the most common tick-borne disease in the United States. Left untreated, Lyme disease can cause debilitating arthritis, heart disease, and nervous disorders.  Figure 17.A , right, shows the rash spot that appears.

Figure 17.A

A deer tick (left) can carry the Lyme disease bacterium from a deer or mouse to a human. The right figure shows the rash that can appear due to a Lyme disease infection.

Dariusz Majgier/ Shutterstock.com; AnastasiaKopa/ Shutterstock.com

Another growing health hazard is infectious diseases caused by parasites, especially malaria (see the second  Case Study  that follows).

Case Study

Malaria is a life-threatening blood disease that is transmitted to humans by a mosquito bite. About 3.2 billion people—42% of the world’s population—are at risk of getting malaria ( Figure 17.7 ). Most of them live in poor African countries. People traveling to malaria-prone areas are also at risk because there is no vaccine that can prevent this disease.

Figure 17.7

About 42% of the world’s population lives in areas in which malaria is prevalent. As the earth warms, malaria may spread to some temperate areas such as the southern half of the United States.

(Compiled by the authors using data from the World Health Organization and U.S. Centers for Disease Control and Prevention.)

Malaria is caused by a Plasmodium parasite transmitted to humans through the bite of a female Anopheles mosquito ( Figure 17.8 ) infected with the parasite. The mosquito bites an infected person, picks up the parasite, and passes it to the next person it bites. The parasites multiply and destroy many of the victim’s red blood cells. This causes intense fever, chills, drenching sweats, severe abdominal pain, vomiting, and headaches. Without treatment, severe cases of malaria can be fatal.

Figure 17.8

The bite of a female Anopheles mosquito infected with the Plasmodium parasite can lead to malaria in its victim.

Sanimfocus/ Shutterstock.com

Over the course of human history, malarial protozoa probably have killed more people than all the wars ever fought. The spread of malaria slowed during the 1950s and 1960s, a time when widespread draining of swamps and marshes, mostly to grow crops, sharply reduced mosquito-breeding areas. These areas were also sprayed with insecticides, and drugs were used to kill the parasites in victims’ bloodstreams.

Scientists have made progress in developing a malaria vaccine, but currently no effective vaccine is available. Another approach is to provide poor people in malarial regions with free or inexpensive insecticide-treated bed nets ( Figure 17.9 ) and window screens. Between 2000 and 2014, the percentage of Africa’s population sleeping under mosquito nets increased from 2% to more than 50% saving 6.2 million lives, according to the WHO. Children can also be given zinc and vitamin A supplements to boost their resistance to malaria.

Figure 17.9

This baby in Senegal, Africa, is sleeping under an insecticide-treated mosquito net to reduce the risk of being bitten by malaria-carrying mosquitoes.

Olivier Asselin/Alamy Stock Photo

17.2cReducing the Incidence of Infectious Diseases

According to the WHO, the percentage of all deaths worldwide resulting from infectious diseases dropped by at least a third between 1970 and 2016, primarily because a growing number of children were immunized against major infectious diseases. Between 1990 and 2016, the estimated annual number of children younger than age 5 who died from infectious diseases dropped from nearly 12 million to 5.4 million, according to the WHO. This is important progress but it still amounts to an average of 15,000 under-five deaths per day in 2016.

Learning from Nature

The African resurrection plant completely dries out during annual droughts and revives itself during the rainy season. Scientists hope to learn how they do this and use this information to store and transport vaccines throughout the world without the need for refrigeration.

Figure 17.10  lists measures that could help prevent or reduce the incidence of infectious diseases—especially in less-developed countries. The WHO has estimated that implementing the solutions listed in  Figure 17.10  could save the lives of as many as 4 million children younger than age 5 each year. Improving sanitation and access to clean drinking water can also reduce infectious diseases. According to the WHO, poor sanitation and unsafe drinking water kill about 1.4 million children under age 5 per year—an average of more than 3,800 deaths per day. GREEN CAREER: Infectious disease prevention

Figure 17.10

Ways to prevent or reduce the incidence of infectious diseases, especially in less-developed countries.

Critical Thinking:

1. Which three of these approaches do you think are the most important? Why?

Top: Omer N Raja/ Shutterstock.com. Bottom: Rob Byron/ Shutterstock.com.

There is growing concern about the effects of toxic chemicals on human health. A  toxic chemical  is an element or compound that can cause temporary or permanent harm or death to humans. The U.S. Environmental Protection Agency (EPA) has listed arsenic, lead, mercury (Core Case Study), vinyl chloride (used to make PVC plastics), and polychlorinated biphenyls (PCBs; see the Case Study that follows) as the top five toxic substances in terms of human health.

Case Study

PCBs—A Toxic Legacy from the Past

Polychlorinated biphenyls (PCBs) are a class of more than 200 chlorine-containing organic compounds that are very stable and nonflammable. They exist as oily liquids or solids but, under certain conditions, they can enter the air as a vapor. Between 1929 and 1977, PCBs were widely used as lubricants, hydraulic fluids, and insulators in electrical transformers and capacitors. They also were ingredients in a variety of products including paints, fire retardants in fabrics, preservatives, adhesives, and pesticides.

The U.S. Congress banned the domestic production of PCBs in 1977 after research showed that they could cause liver cancer and other cancers in test animals. Studies also showed that pregnant women exposed to PCBs gave birth to underweight babies who eventually suffered permanent neurological damage, sharply lower-than-average IQs, and long-term growth problems.

Production of PCBs has also been banned in most other countries, but the potential health threats from these chemicals will be with us for a long time. For decades, PCBs entered the air, water, and soil as they were manufactured, used, and disposed of, as well as through accidental spills and leaks. Because PCBs break down very slowly in the environment, they can travel long distances in the air before landing far from where they were released. Because they are fat-soluble, PCBs can also be biologically magnified in food chains and food webs (Figure 17.11).

Figure 17.11

Biological magnification of polychlorinated biphenyls (PCBs) in an aquatic food chain in the Great Lakes.

As a result, PCBs are now found almost everywhere—in the air, soil, lakes, rivers, fish, birds, most human bodies, and even the bodies of polar bears in the Arctic. According to the EPA, about 70% of all the PCBs made in the United States are still in the environment.

There are three major types of potentially toxic agents.  Carcinogens  are chemicals, some types of radiation, and certain viruses that can cause or promote cancer. Cancer is a disease in which malignant cells multiply uncontrollably and create tumors, or masses of abnormal cells. Tumors can damage the body and often lead to premature death. Examples of carcinogens are arsenic, benzene, formaldehyde, gamma radiation, PCBs, radon, ultraviolet (UV) radiation, vinyl chloride, and certain chemicals in tobacco smoke.

Typically, 10 to 40 years can pass between the initial exposure to a carcinogen and the appearance of detectable cancer symptoms. This time lag helps explain why many healthy teenagers and young adults have trouble believing that their habits such as smoking and poor diet could lead to some form of cancer before they reach age 50.

Mutagens are the second major type of toxic substance.  Mutagens  include chemicals or forms of radiation that cause or increase the frequency of mutations, or changes, in the DNA molecules found in cells. Most mutations cause no harm, but some can lead to cancers and other disorders. For example, nitrous acid , formed by the digestion of nitrite  preservatives in foods, can cause mutations linked to increases in stomach cancer in people who consume large amounts of processed foods and wine containing such preservatives. Harmful mutations occurring in reproductive cells can be passed on to offspring and to future generations.

Teratogens , a third type of toxic agent, are chemicals that harm a fetus or embryo or cause birth defects. Ethyl alcohol, an ingredient in alcoholic beverages is a teratogen. Women who drink alcoholic beverages during pregnancy increase their risk of having babies with low birth weight and a number of physical, developmental, behavioral, and mental problems. Other teratogens are mercury (Core Case Study), lead, PCBs, formaldehyde, benzene, phthalates, and PCP (angel dust).

17.3bSome Chemicals Can Affect Our Immune and Nervous Systems

Since the 1970s, research on wildlife and laboratory animals along with some studies of humans suggest that long-term exposure to some chemicals in the environment can disrupt important body systems, including immune and nervous systems.

The immune system consists of specialized cells and tissues that protect the body against disease and harmful substances. For example, it forms antibodies, or specialized proteins, that detect and destroy invading agents. Some chemicals such as arsenic and methylmercury (Core Case Study), can weaken the human immune system. This leaves the body vulnerable to attacks by allergens and infectious bacteria, viruses, and protozoa.

Neurotoxins are natural and synthetic chemicals that can harm the human nervous system, which includes the brain, spinal cord, and peripheral nerves. Neurotoxins can cause behavioral changes, learning disabilities, attention-deficit disorder, paralysis, and death. Examples of neurotoxins are PCBs, arsenic, lead, and certain pesticides.

Methylmercury (Core Case Study) is an especially dangerous neurotoxin because it persists in the environment and, like DDT and PCBs, can be biologically magnified in food chains and food webs (Figure 17.12). According to the Natural Resources Defense Council, predatory fish such as tuna, orange roughy, swordfish, mackerel, grouper, and sharks can have mercury concentrations in their bodies that are 10,000 times higher than the levels in the water around them.

Figure 17.12

Movement of different forms of toxic mercury from the atmosphere into an aquatic ecosystem where it is biologically magnified in a food chain.

Critical Thinking:

1. What is your most likely exposure to mercury?

In one study, the EPA found that almost half of the fish tested in 500 lakes and reservoirs across the United States had levels of mercury that exceeded safe levels (Figure 17.1). Similarly, a study by the U.S. Geological Survey of nearly 300 streams across the United States found mercury-contaminated fish in all of the streams surveyed, with one-fourth of the fish exceeding the safe levels determined by the EPA.

The symptoms of mercury poisoning in adults include poor balance and coordination, muscle weakness, tremors, memory loss, insomnia, hearing loss, loss of hair, and loss of peripheral vision. The EPA estimates that about 1 of every 12 women of childbearing age in the United States has enough mercury in her blood to harm a developing fetus. Figure 17.13 lists ways to prevent or reduce human inputs of mercury (Core Case Study) into the environment.

Figure 17.13

Ways to prevent or control inputs of mercury (Core Case Study) into the environment from human sources—mostly coal-burning power plants and incinerators.

Critical Thinking:

1. Which two of these solutions do you think are the most important? Why?

Top: Mark Smith/ Shutterstock.com. Bottom: tuulijumala/ Shutterstock.com

The endocrine system is a complex network of glands that release tiny amounts of hormones into the bloodstreams of humans and other vertebrate animals. Very low levels of these chemical messengers (often measured in parts per billion or parts per trillion) regulate bodily systems that control sexual reproduction, growth, development, learning ability, and behavior. Each hormone has a unique molecular shape that allows it to attach to certain parts of cells called receptors, and to transmit a chemical message (Figure 17.14).

Figure 17.14

Each type of hormone has a unique molecular shape that allows it to attach to specially shaped receptors on the surface of, or the inside of, a cell and to transmit its chemical message (left). Molecules of hormonally active agents (center and right), have shapes similar to those of natural hormones, allowing them to attach to the hormone molecules and disrupt endocrine systems.

Molecules of certain pesticides and other synthetic chemicals, called hormonally active agents (HAAs) or endocrine disrupters, have shapes similar to those of natural hormones (Figure 17.14). This allows them to attach to the receptors for natural hormones and disrupt endocrine systems of humans and some other animals.

Examples of HAAs include some herbicides, organophosphate pesticides, dioxins, lead, phthalates, various fire retardants, and mercury (Core Case Study). Some HAAs, including bisphenol A, or BPA (Science Focus 17.3) act as hormone imposters, or hormone mimics. They are chemically similar to estrogens (female sex hormones) and can disrupt the endocrine system by attaching to estrogen receptor sites. Other HAAs, called hormone blockers, disrupt the endocrine system by preventing natural hormones such as androgens (male sex hormones) from attaching to their receptors.

Science Focus 17.3

In 2013, the FDA indicated that the chemicals triclosan and triclocarban, widely used in antibacterial soaps and some deodorants, are likely hormone disrupters and could be contributing to bacterial resistance to antibiotics. The FDA also said that there is no evidence that using these chemicals is any more effective in preventing bacterial infections than is thoroughly washing your hands with plain soap and water. Since 2000, several European countries have restricted the use of triclosan in consumer products.

Concerns about BPA, phthalates, and other HAAs show how difficult it can be to assess the potential harmful health effects from exposure to very low levels of various chemicals. Resolving these uncertainties will take decades of research. Some scientists argue that as a precaution during this period of research, people should sharply reduce their exposure to products that contain potentially harmful hormone disrupters, especially in products frequently used by pregnant women, infants, young children, and teenagers (Figure 17.15).

Figure 17.15

Individuals matter: Ways to reduce your exposure to hormone disrupters.

Critical Thinking:

1. Which three of these steps do you think are the most important ones to take? Why?

17.4aMany Factors Determine the Toxicity of Chemicals

Toxicology  is the study of the harmful effects of chemicals on humans and other organisms.  Toxicity  is a measure of the ability of a substance to cause injury, illness, or death to a living organism. A basic principle of toxicology is that any synthetic or natural chemical can be harmful if ingested or inhaled in a large enough quantity .  However, the critical question is: “What level of exposure to a particular toxic chemical will cause harm?”

This is a difficult question to answer because of the many variables involved in estimating the effects of human exposure to chemicals. A key factor is the  dose , the amount of a harmful chemical that a person has ingested, inhaled, or absorbed through the skin at any one time.

Age is another variable that impacts how a person is affected by exposure to a particular chemical. Toxic chemicals usually have a greater effect on elderly adults. Fetuses, infants, and children are also more vulnerable to exposure to toxic chemicals than adults. Current research suggests that exposure to chemical pollutants in the womb may be related to increasing rates of autism, childhood asthma, and learning disorders.

Toxicity also depends on genetic makeup, which determines an individual’s sensitivity to a particular toxin. People vary widely in their degrees of sensitivity to chemicals (Figure 17.16), and some are sensitive to a number of toxins—a condition known as multiple chemical sensitivity (MCS). Another factor is how well the body’s detoxification systems, including the liver, lungs, and kidneys, are working.

Figure 17.16

Individuals in a human population can vary in how sensitive they are to a particular dose of a toxic chemical.

Several other variables can affect the level of harm caused by a chemical. One is its solubility—whether a chemical can be dissolved in water or in oily substances. Water-soluble toxins can move throughout the environment and get into water supplies, as well as the aqueous solutions that surround our bodies’ cells. Oil- or fat-soluble toxins can penetrate the membranes that surround our cells, because these membranes allow similar but non-toxic oil-soluble chemicals to pass through them as part of their normal functioning. Thus, oil- or fat-soluble toxins can accumulate in body tissues and cells.

Bioaccumulation and biological magnification (see Figure 9.14) can also play a role in toxicity. Animals that eat higher on the food chain are more susceptible to the effects of fat-soluble toxic chemicals because of the magnified concentrations of the toxins in their bodies. Examples of chemicals that can be biomagnified include DDT, PCBs (Figure 17.11), and methylmercury (Core Case Study).

The health damage resulting from exposure to a chemical is called the  response . An acute effect is an immediate or rapid harmful reaction ranging from dizziness to death. A chronic effect is a permanent or long-lasting consequence of exposure to a single dose or to repeated lower doses of a harmful substance. Kidney and liver damage are examples of chronic effects.

Natural and synthetic chemicals can be safe or toxic. In fact, many synthetic chemicals, including many of the medicines we take, are quite safe if used as intended, while many natural chemicals such as lead and mercury (Core Case Study) are deadly.

Case Study

Protecting Children from Toxic Chemicals

In one study, the Environmental Working Group analyzed umbilical cord blood from 10 randomly selected newborns in U.S. hospitals. Of the 287 chemicals detected in that study, 180 have been shown to cause cancers in humans or animals, 217 have damaged the nervous systems of test animals, and 208 have caused birth defects or abnormal development in test animals. Scientists do not know what harm, if any, might be caused by the very low concentrations of these chemicals found in the infants’ blood.

However, more recent science has caused some experts to suggest that exposure to chemical pollutants in the womb may be related to increasing rates of autism, childhood asthma, and learning disorders. In 2009, researchers for the first time found a connection between the exposure of pregnant women to air pollutants and lower IQ scores in their children as they grew. A team of researchers led by Frederica Perera of Columbia University reported that children exposed to high levels of air pollution before birth scored 4–5 points lower, on average, in IQ tests than did children with less exposure.

Infants and young children are more susceptible to the effects of toxic substances than are adults, for three major reasons. First, they generally breathe more air, drink more water, and eat more food per unit of body weight than do adults. Second, they are exposed to toxins in dust and soil when they put their fingers, toys, and other objects in their mouths. Third, children usually have less well-developed immune systems and body detoxification processes than adults have. Fetuses are also highly vulnerable to trace amounts of toxic chemicals such as methylmercury (Core Case Study) that they can receive from their mothers.

The EPA has proposed that in determining any risk, regulators should assume that children have a 10-times higher risk factor than adults have. Some health scientists suggest that to be on the safe side, we should assume that this risk for children is 100 times the risk for adults.

Critical Thinking

1. Do you think environmental regulations should require that the allowed levels of exposure to toxic chemicals for children be 100 times lower than those for adults? Explain your reasoning.

17.4bMethods for Estimating Toxicity

Chemicals vary widely in their toxicity (Table 17.1). Some can cause serious harm or death after a single very low dose. For example, swallowing a few drops of pure nicotine (found in e-cigarettes) would make you very sick, while a teaspoon of it could kill you. Other chemicals such as water or table sugar cause such harm only at dosages so huge that it is nearly impossible to get enough into the body to cause injury or death. Most chemicals fall between these two extremes.

Table 17.1

Toxicity Ratings and Average Lethal Doses for Humans

The most widely used method for determining toxicity involves tests with live laboratory animals. Scientists expose a population of such animals to measured doses of a specific substance under controlled conditions. Mice and rats are widely used because, as mammals, their systems function somewhat similarly to human systems. They are also small and can reproduce rapidly under controlled laboratory conditions.

Scientists estimate the toxicity of a chemical by determining the effects of various doses of the chemical on test organisms and plotting the results in a  dose-response curve  (Figure 17.17). One approach is to determine the lethal dose—the dose that will kill an animal. A chemical’s median lethal dose (LD50) is the dose that can kill 50% of the animals (usually rats and mice) in a test population within a given time period, usually expressed in milligrams of the chemical per kilogram of body weight (mg/kg). Then scientists use mathematical models to extrapolate, or estimate, the effects of the chemical on humans, based on the lab testing results.

Figure 17.17

Dose-response curves. Scientists estimate the toxicity of various chemicals by determining how a chemical’s harmful effects change as the dose increases. Some chemicals behave according to the nonthreshold model (left curve) with harmful effects increasing with the dose. Others behave according to the threshold model (center curve), with harmful effects not occurring until a threshold dose is reached. Still others are unconventional in how they behave (right curve) with the harmful effects decreasing after a certain dose level.

There are three general types of dose-response curves. With the nonthreshold dose-response model (Figure 17.17, left), any dosage of a toxic chemical causes harm that increases with the dosage. With the threshold dose-response model (Figure 17.17, center), a certain level, or threshold, of exposure to the chemical must be reached before any detectable harmful effects occur, presumably because the body can repair the damage caused by low dosages of some substances. With the third type, called the unconventional model (Figure 17.17, right), the harmful effects increase with dosage to a certain point and then begin decreasing.

Establishing which of the three models in Figure 17.17 applies at low dosages is extremely difficult and controversial. To be on the safe side, scientists often choose the nonthreshold dose-response model. High dosages are used to reduce the number of test animals, usually mice or rats (Figure 17.18) needed, obtain results quickly, and lower costs. Using low dosages would require running tests on millions of laboratory animals for many years, in which case chemical companies and government agencies could not afford to test most chemicals. For the same reasons, scientists usually use mathematical models to extrapolate the effects of low-dose exposures based on the measured results of high-dose exposures. Then they extrapolate these results from test organisms to humans as a way of estimating LD50 values for acute toxicity.

Figure 17.18

Laboratory worker injecting a white rat to learn about the toxicity of a chemical.

Oleksandr Lysenko/ Shutterstock.com

Four factors can limit the usefulness of epidemiological studies. First, in many cases, too few people have been exposed to high enough levels of a toxic agent to detect statistically significant differences. Second, the studies usually take a long time. Third, closely linking an observed effect with exposure to a particular chemical is difficult because people are exposed to many different toxic agents throughout their lives and can vary in their sensitivity to such chemicals (Figure 17.16). Fourth, epidemiological studies cannot evaluate hazards from new technologies or chemicals to which people have not yet been exposed.

17.4cAre Trace Levels of Toxic Chemicals Harmful?

Almost everyone who lives in a more-developed country is exposed to potentially harmful chemicals (Figure 17.19) in their environment. Many of these chemicals build up to trace levels in their blood and in other parts of their bodies. CDC studies have found that the blood of an average American contains traces of 212 different chemicals, including potentially harmful chemicals such as arsenic and BPA.

Figure 17.19

A number of potentially harmful chemicals are found in many homes.

Critical Thinking:

1. Does the fact that we do not know much about the long-term harmful effects of these chemicals make you more likely or less likely to minimize your exposure to them? Why or why not?

(Compiled by the authors using data from the U.S. Environmental Protection Agency, Centers for Disease Control and Prevention, and New York State Department of Health.)

Should we be concerned about trace amounts of various synthetic chemicals in our air, water, food, and bodies? In most cases, we simply do not know because there are too few data to determine the effects of exposures to low levels of these chemicals.

Some scientists view exposures to trace amounts of synthetic chemicals with alarm, especially because of their potential long-term effects on the human body. Other scientists view the threats from such exposures as minor. They point out that average life expectancy has been increasing in most countries, especially more-developed countries, for decades. Other scientists contend that the concentrations of such chemicals are so low that they are harmless. All agree that there is a need for much more research on the effects of trace levels of synthetic chemicals on human health.

Some scientists and health officials, especially those in European Union countries, push for much greater emphasis on pollution prevention. To them chemicals that are known or suspected to cause significant harm should not be released into the environment at pollutant levels. Preventing such pollution requires finding harmless or less harmful substitutes for toxic and hazardous chemicals. It also requires recycling toxic chemicals within production processes to keep them from reaching the environment, as companies such as DuPont and 3M have been doing (see the Case Study that follows).

Case Study

Pollution Prevention Pays

The U.S.-based 3M Company makes 60,000 different products in 100 manufacturing plants around the world. In 1975, 3M began a Pollution Prevention Pays (3P) program. Since then, it has reformulated some of its products, redesigned equipment and processes, and reduced its use of hazardous raw materials. It has also recycled and reused more waste materials and sold some of its potentially hazardous but still useful wastes as raw materials to other companies. As of 2019, this program had prevented more than 2.1 million metric tons (2.3 million tons) of pollutants from reaching the environment and saved the company $1.9 billion.

The 3M 3P program has been successful largely because employees are rewarded if the projects they come up with eliminate or reduce a pollutant; reduce the amount of energy, materials, or other resources required in production; or save money through reduced pollution control costs, lower operating costs; or increase sales of new or existing products. Employees at 3M have now completed more than 13,000 3P projects.

Since 1990, a growing number of companies have adopted similar pollution and waste prevention programs that have led to cleaner production. They are learning that, in addition to saving money by preventing pollution and reducing waste production, they have a much easier job of complying with pollution laws and regulations.

Pollution prevention is a strategy for implementing the precautionary principle. According to this principle, when there is substantial preliminary evidence that an activity, technology, or chemical substance can harm humans, other organisms, or the environment, decision makers should take measures to prevent or reduce such harm, rather than waiting for more conclusive scientific evidence.

There is controversy over how far we should go in using the precautionary principle. Those who favor a precautionary approach argue that a person or company, proposing to introduce a new chemical or technology should bear the burden of establishing its safety. This would require two major changes in the way we evaluate and manage risks. First, we would assume that new chemicals and technologies could be harmful until scientific studies show otherwise. Second, the existing chemicals and technologies that appear to have a strong chance of causing harm would be removed from the market until their safety is established. For example, after decades of research revealed the harmful effects of lead, especially on children, lead-based paints and leaded gasoline were phased out in most developed countries.

Many manufacturers and businesses contend that widespread application of the precautionary approach and requiring pollution prevention would make it too expensive and almost impossible to introduce any new chemical or technology. They note that there is always some uncertainty in any scientific assessment of risk.

However, applying the precautionary principle can be good for business. It reduces health risks for employees and society, frees businesses from having to deal with pollution regulations, and reduces the threat of lawsuits from injured parties. It also focuses companies on finding solutions to pollution problems that are based on prevention rather than cleanup. Businesses could also improve their images by operating in this manner.

Finally, proponents argue that society has an ethical responsibility to reduce known or potentially serious risks to human health, to the environment, and to future generations. This is in keeping with the ethical principle of sustainability.

17.5cMost People Do a Poor Job of Evaluating Risks

Most of us are not good at assessing the relative risks from the hazards that we encounter. Many people deny or shrug off the high-risk chances of death or injury from the voluntary activities they enjoy. These include risks of death by smoking (1 in 250 by age 70 for a pack-a-day smoker), motorcycling (1 in 1,000), hang gliding (1 in 1,250), and driving (1 in 3,300 without a seatbelt and 1 in 6,070 with a seatbelt).