CHAPTER 14 - Environmental Health and Toxicology
I. Central Case: Alligators and Endocrine Disruptors at Lake Apopka, Florida
A. In 1985, Louis Guillette discovered bizarre reproductive problems in American alligators in Lake Apopka.
B. He developed the hypothesis that certain chemical contaminants in Lake Apopka were disrupting the endocrine system of alligators during development in the egg.
C. Guillette and his co-workers found that lake Apopka alligators had abnormally low hatchling rates starting in the years after a pesticide spill. The lake also received high levels of chemical runoff from agriculture.
D. Guillette= s results showed that atrazine and nitrates act as endocrine disruptors, causing smaller penises and lower testosterone levels in juvenile male alligators.
E. Many scientists suspect that chemical contaminants could be affecting people just as they have alligators.
II. Environmental Health
1. The study and practice of environmental health assess environmental factors that influence human health and quality of life.
A. Environmental hazards can be physical, chemical, biological, or cultural.
1. Physical hazards are processes that occur naturally in our environment and can pose health hazards. These include discrete events such as earthquakes, fires, floods, and droughts, and also include ongoing natural phenomena such as ultraviolet radiation from the sun, which can cause sunburn and skin cancer.
2. Chemical hazards include many of the synthetic chemicals that our society produces, such as disinfectants and pesticides, and also include chemicals produced naturally by organisms.
3. Biological hazards result from ecological interactions among organisms, such as becoming sick from a virus, bacterial infection, or other pathogen. If this illness can spread to other humans, then it is an infectious disease, also called communicable or transmissible disease (e.g., malaria, cholera, flu).
4. Cultural hazards result from where we live, our socioeconomic status, our occupation, or our behavioral choices. Choosing to smoke, poor diet, and living in proximity to toxic waste are all cultural hazards.
B. Disease is a major focus of environmental health.
1. Many major killers such as cancer, hearth disease, and respiratory disorders have genetic bases but are also influenced by environmental factors.
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2. Malnutrition can foster a wide variety of illnesses, as can poverty, poor hygiene, lifestyle choices, and lack of exercise.
3. Infectious disease involves a pathogen that attacks us directly, or an infection may occur through a vector.
4. In order to predict and prevent infectious disease, experts deal with the often-complicated interrelationships among technology, land use, and ecology.
C. Environmental health hazards exist indoors as well as outdoors.
1. Indoor environmental health threats include radon, lead poisoning, and asbestos.
2. A recently recognized hazard is a group of chemicals known as polybrominated diphenyl ethers (PBDEs). These chemicals appear to be endocrine disruptors, affecting thyroid hormones in animals, and have been banned in Europe.
D. Toxicology is the study of poisonous substances.
1. Toxicology studies the effects of poisonous substances on humans and other organisms.
2. A toxicant, or toxic agent, must be compared to other substances to determine the toxicity, or the degree of harm that it can inflict.
3. With toxins, A the dose makes the poison@ means that the quantity received is an important factor in the damage done.
4. Environmental toxicology deals specifically with toxic substances that come from or are discharged into the environment, and includes the study of health effects on humans, other animals, and ecosystems.
III. Toxic Agents in the Environment
A. Synthetic chemicals are ubiquitous in our environment.
1. Thousands of synthetic chemicals have been manufactured and many have found their way into soil, air, and water.
2. Every one of us carries traces of numerous industrial chemicals in our bodies.
3. Very few of these chemicals have been tested for harmful effects.
B. Silent Spring began the public debate over pesticides.
1. Rachel Carson was a naturalist, author, and government scientist.
2. Using scientific studies, medical case histories, and other data, she showed that DDT and artificial pesticides in general were hazardous to people, wildlife, and the environment.
3. Carson= s book was a bestseller and generated significant social changes in views and actions toward the environment.
4. The United States manufactures and exports DDT to countries that still use it, especially for mosquito control. Certain species of mosquitoes are vectors for malaria, which is considered to be a greater risk than the toxic effects of the pesticide.
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C. Toxicants come in several different types.
1. Carcinogens are chemicals or types or radiation that cause cancer.
2. Mutagens are chemicals that cause mutations in the DNA of organisms.
3. Chemicals that cause harm to the unborn are called teratogens.
4. Allergens overactivate the immune system, causing an immune response when one is not necessary.
5. Neurotoxins assault the nervous system.
6. Endocrine disruptors are toxicants that interfere with the endocrine system.
D. Endocrine disruption may be widespread.
1. The idea that synthetic chemicals might be altering the hormones of animals was presented in the 1996 book Our Stolen Future.
2. One common type of endocrine disruptor involves the feminization of male animals.
3. To date, endocrine effects have been most widely found in nonhuman animals, but scientists attribute the striking drop in sperm counts among men worldwide to endocrine disruptors.
4. Because the endocrine system is geared to respond to minute concentration of hormones in the bloodstream, it may be especially vulnerable to effects from environmental contaminants.
E. Endocrine disruption research has generated debate.
1. Manufacturers of herbicides would lose many millions of dollars if their products were banned or restricted.
2. Many everyday household plastic products contain bisphenol-A, and estrogen mimic, but the plastic industry protests that the chemical is safe despite research showing birth defects in lab mice.
F. Toxicants may concentrate in surface water or groundwater.
1. Water runoff often carries low amounts of toxicants from large areas of land and concentrates them in small volumes of surface water.
2. Many chemicals are soluble in water, and thus are very accessible to organisms. This is why aquatic animals such as fish, frogs, and stream invertebrates are especially good indicators of pollution.
G. Airborne toxicants can travel widely.
H. Some toxicants persist for a long time.
1. DDT and PCBs have long persistence times, while Bt toxin has a very short persistence.
2. Some toxicants have breakdown products that are just as toxic as the original chemical, or more so (e.g., DDT breaks down into DDE, a highly persistent and toxic compound).
I. Toxicants may accumulate and move up the food chain.
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1. Fat-soluble toxicants such as DDT and DDE are absorbed and stored in fatty tissues and may build up in animals in a process called bioaccumulation.
2. Toxicants that bioaccumulate in the tissues of one organism may then be transferred to other organism in the food chain, in a process called biomagnification.
3. An example is polar bears in arctic Norway that are suffering from PCB contamination because of biomagnification, resulting in high cub mortality.
J. Not all toxicants are synthetic.
1. Chemical toxicants also exist naturally in the environment around us and in the foods we eat.
2. Scientists have actively debated just how much risk is posed by natural toxicants.
IV. Studying Effects of Hazards
A. Wildlife studies use careful observations in the field and the lab.
B. Human studies rely on case histories, epidemiology, and animal testing.
1. Much knowledge has been gained by studying sickened individuals directly, a case history approach.
2. Epidemiological studies involve large-scale comparisons among groups of people, usually contrasting a group known to have been exposed to some toxicant with a group that has not.
3. The advantages of epidemiological studies are their realism and ability to enable relatively accurate predictions about risk; the drawbacks include the length of time it takes to obtain results and the inability to address possible effects of new products.
4. Manipulative experiments are needed to truly nail down causation. However, this is not possible with human subjects, so animals are substituted.
C. Dose-response analysis is a mainstay of toxicology.
1. The standard method of testing lab animals in toxicology is called dose-response analysis.
2. The dose is the amount of toxicant the test animal receives, and the response is the type or magnitude of negative effects the animal exhibits as a result. The response is generally quantified by measuring the proportion of animals exhibiting negative effects.
3. Once a dose-response curve is plotted, scientists can calculate a convenient shorthand gauge of a substance= s toxicity - the amount of toxicant it takes to kill half the population of study animals used (LD50).
4. Nonlethal health effects are determined by the level of toxicant at which 50% of the population is affected (ED50).
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5. Common sense suggests that the greater the dose, the stronger the response will be. However, sometimes responses occur only above a certain dose, called the threshold dose.
6. Sometimes responses decrease with dose. Some dose-response curves are U-shaped, J-shaped, or shaped like an inverted U; these curves appear to apply to endocrine disruptors.
D. Individuals vary considerably in their response to hazards.
E. The type of exposure can affect the response.
1. The toxicity of many substances varies according to whether the exposure is in high amounts for short periods of time - acute exposure - or in lower amounts over long periods of time - chronic exposure.
2. Acute exposure is easier to recognize but chronic exposure is more common, and is more difficult to detect and diagnose.
F. Mixes may be more than the sum of their parts.
1. Interactive impacts may arise when toxicants are mixed together, and when these impacts are more than or different from the simple sum of their constituent effects; these are called synergistic effects.
2. Traditionally, environmental health has tackled the effects of single hazards one at a time, and single-substance tests have received priority. This is changing, but scientists will never be able to test all possible combinations.
V. Risk Assessment and Risk Management
A. Risk is expressed in terms of probability.
1. Exposure to a toxin causes some probability of harm, a statistical chance that damage will result. The probability depends on the toxin, its strength, the frequency and duration of the encounter, the sensitivity of the organism, and other factors.
B. Our perception of risk may not match reality.
C. Risk assessment analyzes risk quantitatively.
1. Risk assessment involves the scientific study of toxicity and the likely extent of the exposure.
2. Risk assessment studies are often performed by scientists associated with the industries that manufacture toxicants, which in many people= s minds may undermine the objectivity of the process.
D. Risk management combines science and other social factors.
1. In most developed nations, risk management is handled largely by federal agencies.
2. Scientific assessment of risk are considered in light of economic, social, and political needs and values.
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VI. Philosophical and Policy Approaches
A. Two approaches exist for determining safety.
1. One approach is to assume that substances are harmless until shown to be harmful - the innocent-until-proven-guilty approach. This approach encourages technological innovation but may put some dangerous substances into wide use.
2. The other approach is to assume that substances are harmful until shown to be harmless. This enables us to identify toxicants before they are released into the environment, but may also impede technological and economic advances.
B. Philosophical approaches are reflected in policy.
1. Most nations follow a blend of the two approaches.
2. At present, European nations follow the precautionary principle.
3. The United States largely follows the innocent-until-proven-guilty approach.
4. In the United States, the tracking and regulation of synthetic chemicals is shared among several federal agencies.
5. The EPA also regulates diverse chemicals under the Toxic Substances Control Act (TSCA).
a. TSCA was the first law to require screening of substances before they entered the marketplace.
b. Many public health and environmental advocates view TSCA as being too weak.
c. Industry= s critics say chemical manufacturers should be made to bear the burden of proof for the safety of their products before they market them.
C. The EPA regulates pesticides and other substances.
1. The registration process involves risk assessment and risk management.
2. Critics say the process allows hazardous chemicals to be approved if they offer enough economic benefits.
D. Toxicants are regulated internationally.
1. In 2003 the European Union commissioners proposed legislation that would require chemical manufacturers to test and register 30,000 chemicals already in use and would impose restrictions on 1,500 chemicals already considered hazardous. In late 2005 the proposal was slowly making its way toward becoming law.
2. An international treaty, the Stockholm Convention, aims first to end the use and release of 12 persistent organic pollutants (POPs) called the A dirty dozen.@ It appears to be on its way to ratification.
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A. International agreements such as the Stockholm Convention represent a hopeful sign that governments will act to protect the world= s people, wildlife, and ecosystems from harm by toxic chemicals and other environmental hazards.
B. A society= s philosophical approach to risk management will determine what policy decisions are made.
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