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Chapter 10 - Toxicology and environmental health
Although our modern lifestyle is made possible, in part, by artificially, produced chemicals, these chemicals also have
negative side effects. Environmental toxicology deals with toxicants that come from or are discharged into the
environment, and includes the study of health effects on humans, other animals, and ecosystems. In the 1960s, people
became concerned about the risks of synthetic pesticides and helped spur the environmental movement in the United
States. Toxicants can be classified into many categories, including carcinogens, mutagens, and teratogens. The endocrine
system may be especially vulnerable to effects from dispersed, dilute contaminants. Several methods of inquiry are
necessary for understanding the health effects of toxicants. Epidemiological studies measure the association between a
toxicant and an effect, but they can- . not prove that the toxicant is responsible for that effect. People in the environmental
health field assess environmental factors that influence human health and quality of life and they try 10 prevent adverse
effects. Humans can also suffer adverse health effects from indoor pollutants such as asbestos. Risk assessment and
management are involved in the steps between the collection of scientific data and the formulation of policy. There are
two basic philosophical approaches to categorizing a substance as dangerous, the "innocent-until-proven-guilty" and the
"precautionary principle" approaches. In developed countries, federal agencies such as the EPA regulate and control
diverse chemicals under various federal laws. Despite these laws, however, only 2% of synthetic chemicals have been
tested for carcinogenic, mutagenic, or teratogenic, less than 1 % are government regulated and 43% lack all basic data.
Alligators and Endocrine Disruptors at Lake Apopka, Florida .
In 1985, scientists found that alligators in Lake Apopka, Florida, had severe reproductive problems, including male
hatchlings with severely depressed levels of the male sex hormone testosterone, and female hatchlings with high levels of
the female sex hormone estrogen.
Lake Apopka had suffered a major spill of the pesticides dicofol and DDT and received high levels of chemical runoff
from agriculture and fertilizers.
Research showed that some environmental contaminants mimic hormones and can interfere with natural hormone
systems.
Endocrine disruptors mimic estrogen, and feminize males.
The hypothesis about events at Lake Apopka was that chemical contaminants were disrupting the endocrine systems of
alligators during egg development.
Similar problems occurred in other lakes receiving chemical runoff, especially with the pesticide atrazine, which can
convert testosterone into estrogen.
Because alligators and humans share some of the same hormones, scientists are worried that chemical contaminants can
also harm humans.
The Growth of Environmental Toxicology
Toxicology is the science that examines the effects of poisonous chemicals and other agents on humans and wildlife.
A chemical's toxicity is the degree of harm a substance can inflict.
Our modern lifestyle is made possible, in part, by artificially produced chemicals that provide food, medicine, and
materials.
Synthetic chemicals are ubiquitous in our environment
Many of the thousands of synthetic chemicals we produce are toxic to the health of wildlife and humans, but only a few
chemicals have been thoroughly tested for harmful effects.
These toxicants, especially pesticides, are found allover the world.
Pesticides in the United States come from agricultural fields, homeowners' lawns, and golf courses.
Natural chemical substances, such as oil oozing from the ground and radioactive radon gas, can also pose health risks.
The science of toxicology has grown with concern for health and the environment
Environmental toxicology deals with those toxicants that come from or
are discharged into the environment, and includes the study of health effects on humans, other animals, and ecosystems.
Animals are studied because of concern for their welfare and because they
can serve as indicators of health threats.
Studies have shown that pesticides such as atrazine can cause gonadal abnormalities in frogs all across the United States
at lower levels than commonly exist in drinking water.
After World War II, huge amounts of synthetic chemicals were developed, and their use greatly increased.
In the 1960s, people became concerned about the risks of exposure to
synthetic pesticides and helped spur the environmental movement in the United States.
Silent Spring began the public debate over pesticides
Rachel Carson's book, Silent Spring, brought together information from
many sources that showed that pesticides, especially DOT, were hazardous to people, wildlife, and ecosystems.
Carson's book, although attacked by the chemical industry, was a best-seller and generated significant social change in
views and actions toward the environment, and resulted in the banning of
DOT in the United States in 1973.
Toxic Agents in the Environment
Scientists must study the types and levels of risk a potential toxicant might pose.
Toxicants come in many different types
Carcinogens are chemicals that cause cancer, where certain cells grow un- controllably, creating tumors and often leading
to death.
Carcinogens can be hard to identify, because there may be years between exposure to a carcinogen and the onset of
cancer.
Mutagens are chemicals that cause mutations in the DNA of organisms, and can lead to cancer and many other disorders.
Teratogens cause harm to unborn young, and can cause birth defects or death.
Allergens over-activate the immune system, causing an immune response or weakening the immune system, making the
body less able to defend itself against disease or allergy-causing agents.
Neurotoxins affect the nervous system, and include heavy metals (i.e., lead and mercury), pesticides, and chemical
weapons.
Endocrine disruptors interfere with the endocrine system and can disrupt growth, development, sexual maturity, brain
function, appetite, and sexual drive.
The potential for endocrine disruption appears to be widespread
Our Stolen Future (1996), by Theo Colburn, Dianne Dumanoski, and J.
P. Myers, gathered a large amount of scientific data
from toxicologists, medical doctors, wildlife biologists, and others about the hormonal impacts of synthetic chemicals on
humans and wildlife.
Endocrine disruption can involve the feminization of male animal, such as alligators, frogs, and fish.
Strong evidence suggests that dropping sperm counts in human males may also be caused by endocrine disruptors.
Other reproduction system effects may be testicular cancer and undescended testicles in males, and breast cancer in
women.
Brain and nervous system impairment may also result from endocrine disruptors.
For example, Michigan mothers who ate Great Lakes fish contaminated with PCBs had babies with birth defects who
grew into weak children with jerky reflexes and lower intelligence.
Endocrine disruption research is ongoing and contentious
Bisphenol-A, an estrogen mimic, is used in plastic products such as baby bottles and cavity-preventing coating for
children's teeth.
\ Evidence shows that the chemical leaches out of these products into water and food, and has been linked to birth defects
in lab mice.
The plastics industry protests that the chemical is safe and points to research showing its safety.
Endocrine disruptors may affect organisms in very small doses because the endocrine system is geared to respond to
minute concentrations of substances.
Toxicants may become concentrated in surface water or groundwater
Water can carry toxicants from large areas of land and concentrate them in small volumes of water.
Soil contaminants can leach into groundwater.
Aquatic organisms such as fish and frogs are good indicators of contamination, because they are the first to be exposed to
water-soluble chemicals.
Airborne toxicants can move widely through the environment
Airborne transport of pesticides, called "pesticide drift," carries pesticides long distances.
Despite being manufactured and applied in temperate and tropical zones, contaminants show up allover the globe, even in
the most remote and seemingly pristine places.
Some toxicants persist for a long time
The rate at which chemicals degrade depends on the temperature, moisture, sun exposure, and the chemistry of the
toxicant.
Those toxicants that persist in the environment for a long time have the most potential to cause harm.
Highly toxic chemicals such as PCBs are persistent, while atrazine's persistence is variable, depending on environmental
conditions.
The products from degradation of the toxicant can be less, or more, harmful than the original substance.
For example, DDT breaks down into DOE, a highly persistent and toxic compound.
Toxicants may accumulate and move up the food chain
Once a toxicant enters an organism, it can be excreted, broken down, or stored in the body.
Toxicants like DDT are stored in the fatty tissues, and can build up in an animal, in a process called bioaccumulation.
When one organism eats another, it takes in any stored fat-soluble toxicants and stores them itself.
Biomagnification is the process of increasing the concentration of
toxicants with each step of the food chain, so that top predators end up with huge concentrations of the toxicant.
DDT is the most famous example of biomagnification, and led to the near extinction of birds such as ospreys, eagles,
peregrine falcons, and brown pelicans.
Not all toxicants are synthetic Chemical toxicants exist naturally in the environment and in our food. Some scientists argue
that natural foods contain much higher levels of toxicants than synthetic chemicals from pesticides.
Others say natural toxicants are more readily metabolized and excreted, while synthetic ones persist and accumulate.
Studying Effects of Toxicants
Several methods of inquiry are necessary for understanding the health effects of toxicants.
Wildlife toxicology utilizes careful observations in the field and the lab
Scientists can piece together evidence of toxicant effects by taking measurements from animals in the wild, then
experimenting in the lab to test hypotheses.
Wildlife deaths can provide evidence of some toxic event; for example, scientists hypothesize that the death of otters off
the California coast in 1998-2001 was caused by a parasite that came from cat litter box waste that washed into the ocean.
Human toxicology relies on case histories and on epidemiology
Scientists often study sick and dead humans through the study of individual patients in what. is labeled a "case history"
approach.
Case histories are not useful for new or rare chemicals, or those existing in extremely small levels or that have minor
long-term effects.
Epidemiological studies compare a group of people, known to have been exposed to a chemical, to a group that has not.
Advantages of epidemiological studies are: (1) they are realistic, and (2) they enable relatively accurate predictions about
risk.
Drawbacks of epidemiological studies are: (1) they take years, (2) the future of new chemicals can't be tested, and (3)
participants encounter many other factors that influence their health.
Epidemiological studies measure the association between a toxicant and an effect, but they cannot prove that the toxicant
is responsible for that effect.
Although manipulative studies are needed to determine what causes the
effects, scientists cannot experiment on humans, so they use other animals to test toxicity.
Animals may serve as models for humans
Although some people object to animal testing on ethical grounds, animal testing has been used because mammals,
especially, have similar systems to humans.
Other techniques, using cells, bacteria, or tissue, can help decrease the number of live animals used.
Dose-response analysis is a mainstay of toxicology
The dose-response analysis is the standard method of testing with lab animals in toxicology.
The dose is the amount of toxicant the animal receives, and the response is the degree or type of damage the animal
suffers.
The data are plotted on a graph, and the resulting curve is called the dose-response curve, which scientists can use to
estimate the dose that causes harm.
Scientists can calculate LD5o, the amount of toxicant it takes to kill half the population of study animals used; a high
LD50 indicates a low toxicity.
The EDso, called the effective dose, is the level of toxicant at which 50% of the population of animals is affected (but not
killed) in whatever way is of interest in the study.
Chemicals with low LDso and EDso are assumed to be poisonous to humans.
Because these are simply estimations, regulatory agencies set standards for maximum allowable levels of toxicants below
the minimum toxicity levels estimated in the lab,
Individuals vary considerably in their responses to toxicants
Sensitivity to a certain amount of toxicant may be influenced by genetics, health, ability to detoxify the substance, age,
gender, and weight.
The very young are much more sensitive to toxicants than adults, and regulatory agencies are criticized for not being strict
enough in protecting children.
The type of exposure can affect toxicity
Acute exposure occurs when an organism is exposed to high amounts of toxicants for a short time.
Chronic exposure occurs when an organism is exposed to low amounts of toxicants for a long time, and is more common,
but more difficult to detect and diagnose.
Mixes may be more than the sum of their parts
When mixed, substances act in ways that cannot be predicted from their effects when alone.
When substances are mixed, synergistic effects may result, in which the interactive impacts are more or different than the
simple sum of their constituent effects.
For example, DDE can either help cause or inhibit sex reversal, depending on what other substances are present.
Hazards to Environmental Health
Environmental health assesses environmental factors that influence human health and quality of life and tries to prevent
adverse effects.
Environmental hazards can be chemical, physical, biological, or socioeconomic Physical or climatic events include natural
catastrophes such as floods, blizzards, and droughts, that humans can do little to prevent.
Biological hazards include bacterial infection, viruses, or other pathogens. Lifestyle hazards result from the place we live,
our socioeconomic status, jobs, and behavioral choices, such as smoking, drug use, nutrition, and Crime.
Risk is expressed in terms of probability
To understand the potential for producing some form of harm, a scientist must know (1) the strength, or toxicity, of the
substance; (2) the chances that an organism will encounter it; (3) the frequency of encounters;
(4) the amount of substance the organism is exposed to; and (5) the organism's sensitivity.
The risk posed by a substance is the probability that some harmful outcome (i.e., injury or death, or environmental or
economic loss) will result from a given action, event, or substance.
Our perception of risk may not match reality
Although some actions and decisions are more risky than others, most of us try our best to minimize the risks we take.
People often believe an activity is more risky when they are not in control of the situation; thus, they feel that nuclear
power, toxic wastes, and pesticides are more dangerous than overeating or smoking cigarettes.
Risk assessment analyzes risk subjectively and quantitatively
Risk assessment is the quantitative measurement of risk and the comparison of risks of different activities or substances.
Risk assessment is the crucial first step in determining whether a substance or activity poses a substantial health threat to
people or wildlife.
The multi-step process of risk assessment is very hard to do for environmental health because of the complexity of the
natural environment.
Risk management combines science and other social factors
Risk management consists of decisions and strategies to minimize risk.
Developed countries have federal agencies, such as the EPA and Food and Drug Administration (FDA), to handle risk
management.
In risk management, science is coupled with economic, social, and political needs and values and the costs and benefits of
addressing risks in various ways.
Risk management in environmental health and toxicology is difficult because benefits are often economic and provide
money to many, while costs involve health, where only a very few suffer greatly.
Philosophical Approaches to Environmental Health
There are two basic philosophical approaches to categorizing a substance as dangerous that center around who has the
burden of proving a substance is safe.
The "innocent-until-proven-guilty" approach assumes that a substance is harmless until it is shown to be dangerous.
This approach does not slow technology or economic advancement, but may allow a dangerous substance to be widely
circulated and cause great harm before its effects are discovered.
Some health threats are part of the natural environment
Some hazards, such as electromagnetic radiation or sunshine, are unavoidable because they are a natural part of the
environment,
Radiation and radon (a highly toxic radioactive gas) can seep out of rocks and pose health hazards.
Disease is a major focus of environmental health
Despite human technology, we still battle infectious (or communicable or transmissible) disease..
Humans can be directly attacked, or the disease organism can invade via a vector, which is an organism that transfers the
pathogen to the host.
Communicable diseases account for almost half of all deaths in developing countries, but very few deaths occur in
developed countries, because of access to medicine and hygiene.
Many disease-causing organisms have developed resistance to modem medicines such as antibiotics.
Environmental health workers also study non-transmissible diseases, such as those caused by malnutrition, poverty, and
poor hygiene.
Many non-communicable diseases have both genetic and environmental components.
Environmental health is an issue indoors as well as outdoors
Indoor health threats, including radon and asbestos, also threaten humans.
Asbestos, a mineral with a fibrous structure, has been used as insulation in buildings for many years.
When inhaled, the fibers lodge themselves in the lungs, which causes the body to release acid to try to dissolve the
asbestos.
The acid does not dissolve the asbestos, but it does scar the tissues to such a degree that they may no longer function, or
they may develop cancer.
People, especially children, can suffer lead poisoning from drinking water that passes through lead pipes or from ingesting
lead-based paint, leading to brain damage, behavioral abnormalities, anemia, hearing loss, and death.
Other indoor hazards, such as the endocrine disruptor polybrominated diphenyl ethers (PBDEs), which provides fireretardant properties, can persist and accumulate in tissues and affect brain function..
The European Union banned PBDEs in 2003, but the United States has not addressed the issue.
Risk Assessment and Risk Management
Policy decisions involving chemicals reach beyond the scientific results of testing and incorporate economic and ethical
considerations.
Risk assessment and risk management are involved in the steps between the collection of scientific data and the
formulation of policy.
The "precautionary principle" assumes that a substance is dangerous until proven safe; this enables harmful toxicants to be
identified before they are released, but may slow technology and economic gain.
Policy on Toxicants
The philosophical approach to toxicants directly affects which materials we release into the environment.
Europe generally follows the precautionary principle regarding regulating synthetic chemicals and genetically modified
foods, while the United States does not.
Federal agencies within the United States track and regulate synthetic chemicals: the Food and Drug Administration
(FDA) regulates foods, food additives, cosmetics, and drugs; the EPA regulates pesticides; and the Occupational Safety
and Health Administration (OSHA) regulates workplace hazards.
According to the 1976 Toxic Substances Control Act, the EPA regulates synthetic chemicals not covered by other laws.
Pesticides in the United States are registered through the EPA
The 1947 Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) was first intended to assure consumers that
products actually worked as their manufacturers claimed.
In later years, FIFRA shifted the focus toward protecting health and safety and charged the EPA with registering new
pesticides.
Critics say that hazardous products are almost always approved if their economic benefits are great enough.
The EPA also regulates diverse chemicals under the Toxic Substances Control Act The EPA also monitors and can ban any of
the more than 75,000 industrial chemicals manufactured in or imported into the United States through the direction of the
Toxic Substances Control Act of 1976.
The TSCA was the first law requiring screening of substances, particularly asbestos, radon, and lead, before they entered
the marketplace.
Critics say TSCA is too weak because it requires only minimal screening of industry and the EPA must show proof of the
chemical's toxicity be- fore it can order more tests, which it can't do because it doesn't have prior proof of toxicity.
Only 2% of synthetic chemicals have been tested for carcinogenicity, mutagenicity, or teratogenicity, while less than 1 %
are government regulated and 43% lack all basic data.
Industry advocates say the critics will never be satisfied and mandating
more research will slow the introduction of products that consumers want.
Toxicants are regulated internationally
In 2003, the European Union proposed legislation that would require testing and registering 30,000 chemicals and would
impose restrictions on 1,500 chemicals considered hazardous.