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TOXICOLOGY
AND HUMAN
HEALTH
© 2011 Pearson Education, Inc.
The four types of environmental hazards
• Physical hazards = occur naturally in our environment
- Earthquakes, volcanoes, fires, floods, droughts
- We can’t prevent them, but we can prepare for them
- We increase our vulnerability by deforesting slopes
(landslides), channelizing rivers (flooding), etc.
- We can reduce risk with better environmental choices
• Chemical hazards = synthetic chemicals such as
pharmaceuticals, disinfectants, pesticides
- Harmful natural chemicals (e.g., venom) also exist
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• Biological hazards = result from ecological interactions
- Viruses, bacteria, and other pathogens
- Infectious disease = species parasitize humans, fulfilling
their ecological roles
- Vector = an organism that transfers a pathogen to a host
- We can’t avoid risk, but we can reduce infection
• Cultural = result from where we live, our socioeconomic
status, our occupation, our behavioral choices
- We can minimize some, but not all, of these hazards
- Smoking, drug use, diet and nutrition, crime, mode of
transportation
- Health factors (e.g., living near toxic waste) are often
correlated with poverty
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Four types of environmental hazards
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Disease is a major focus of environmental
health
• Despite our technology,
disease kills most of us
• Disease has a genetic and
environmental basis
- Cancer, heart disease,
respiratory disorders
- Poverty and poor
hygiene foster illnesses
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Infectious diseases kill millions
• Infectious diseases kill 15
million people/year
- Half of all deaths in
developing countries
• Money lets developed
countries have access to
hygiene and medicine
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Health workers fight disease
• The best way to reduce disease? Improve the basic living
conditions of the poor
- Food security, sanitation, clean drinking water
• Expanded access to health care
- Health clinics, immunizations, pre- and postnatal care
• Education campaigns work in rich and poor nations
- Public service and governments give advice
- Packaging and ads advise us on smoking, etc.
- Sex and reproductive health education slows
population growth and spread of HIV/AIDS
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Toxicology studies poisonous substances
• Toxicology = the study of the effects of poisonous
substances on humans and other organisms
- Toxicity = the degree of harm a toxicant can inflict
- Toxicant = any toxic substance (poison)
- “The dose makes the poison” = toxicity depends on
the combined effect of the chemical and its quantity
• Environmental toxicology = deals with toxic substances
that come from or are discharged into the environment
- Studies health effects on humans, other animals, and
ecosystems
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Toxic substances in the environment
• The environment contains
natural chemicals that may pose
health risks
• Toxins = toxic chemicals made
in tissues of living organisms or
synthetically by humans
- Every human carries traces of
industrial chemicals
The U.S. makes or imports 250 lb of
chemicals for every person in the country
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Examples: Indoor environmental toxins
• Radon = a highly toxic,
radioactive gas that is
colorless and undetectable
- It can build up in
basements
• Asbestos = a mineral that
insulates, muffles sounds,
and resists fire
- Asbestosis = scarred
lungs that cease to
function
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Chemicals are in the air, water, and soil
• 80% of U.S. streams contain 82 contaminants
- Antibiotics, detergents, drugs, steroids, solvents, etc.
• 92% of all aquifers contain 42 volatile organic
compounds (from gasoline, paints, plastics, etc.)
- Less than 2% violate federal health standards for
drinking water
Pesticides are present in
streams and groundwater
in levels high enough to
affect aquatic life
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Lead poisoning
• Lead poisoning = caused by lead, a heavy metal
- Damages the brain, liver, kidney, and stomach
- Causes learning problems, behavior abnormalities, and
death
• Exposure is from drinking water that flows through lead
pipes or from lead paint
Education led to declines
in poisoning, but China
still used it in toy paint
until recently
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Synthetic chemicals are in all of us
• Every one of us carries traces of hundreds of industrial
chemicals in our bodies
- Including toxic persistent organic pollutants restricted
by international treaties
• Babies are born “pre-polluted” – 232 chemicals were in
umbilical cords of babies tested
• Not all synthetic chemicals pose health risks
- But very few of the 100,000 chemicals on the market
have been tested
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A recently recognized hazard
• Polybrominated diphenyl ethers (PBDEs) = has fireretardant properties
- Used in computers, televisions, plastics, and furniture
- Persist and accumulate in living tissue
- Mimic hormones and affect thyroid hormones
- Also affect brain and nervous system development and
may cause cancer
• Concentrations are rising in breast milk
- Now banned in Europe, concentrations have decreased
- The U.S. has not addressed the issue
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Case: Poison in the bottle: is bisphenol A
(BPA) safe?
• BPA causes cancer, nerve
damage, and miscarriages
- In extremely low doses
• It is in hundreds of products
- Cans, utensils, baby
bottles, laptops, toys
• BPA leaches into food,
water, air, and bodies
- 93% of Americans have
it in their bodies
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Bisphenol A mimics estrogen
• BPA mimics estrogen, a female hormone
- Effects occur at lower levels than set by regulatory
agencies
• Researchers, doctors, and consumer advocates want
regulation
- The chemical industry insists it is safe
• Some countries and states have banned it
- Many companies are removing it voluntarily
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Toxicants come in different types
• Carcinogens = cause cancer
- Hard to identify because of the long time between
exposure and onset of cancer
• Mutagens = cause DNA mutations
- Can cause cancer
• Teratogens = cause birth defects in embryos
• Neurotoxins = assault the nervous system
• Allergens = overactivate the immune system
• Endocrine disruptors = affect the endocrine
(hormone) system
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Endocrine disruptors mimic hormones
• Hormones stimulate growth,
development, sexual maturity
• Synthetic chemicals
- Block hormones
- Mimic hormones
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Many products mimic female hormones
• Bisphenol A binds to
estrogen receptors
• Phthalates in plastics
disrupt hormones
- Toys, perfumes,
makeup
- Birth defects, cancer,
reproductive effects
- In the bodies of
everyone in the U.S.
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Toxins may concentrate in water
• Runoff carries toxins from land to surface water
• Chemicals in the soil can leach into groundwater
- Contaminating drinking water
• Chemicals enter organisms through drinking or
absorption
- Aquatic organisms (fish, frogs, etc.) are good pollution
indicators
• Contaminants in streams and rivers enter drinking water
and the air
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Routes of chemical transport
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Airborne substances can travel widely
• Chemicals can travel by air
- Their effects can occur far
from the site of use
• Pesticide drift = airborne
transport of pesticides
• Synthetic chemicals are
found globally
- In arctic polar bears,
Antarctic penguins, and
people in Greenland
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Some toxicants persist
• Toxins can degrade quickly and become harmless
- Or they may remain unaltered and persist for decades
- Rates of degradation depend on the substance,
temperature, moisture, and sun exposure
• Breakdown products = simpler products that toxicants
degrade into
- May be more or less harmful than the original
substance
- DDT degrades into DDE, which is also highly
persistent and toxic
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Toxicants can accumulate and biomagnify
• Toxicants in the body can be
excreted, degraded, or stored
- Fat-soluble toxicants are
stored in fatty tissues
• Bioaccumulation = toxicants
build up in animal tissues
• Biomagnification =
concentrations of toxicants
become magnified
- Near extinction of peregrine
falcons and bald eagles
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Not all toxicants are synthetic
• Toxic chemicals also exist naturally and in our food
- Don’t assume natural chemicals are all healthy and
synthetic ones are all harmful
• Some scientists feel that natural toxicants dwarf our
intake of synthetic chemicals
- Natural defenses are effective against synthetics
• Environmentalists say synthetic toxins:
- Are harder to metabolize and excrete
- Persist and accumulate
- Enter people in ways other than in food
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Wildlife studies integrate field and lab work
• Museum collections provide data from times before
synthetic chemicals were used
• Measurements from animals in the wild can be compared
to controlled experiments in the lab
Alligators and frogs show
reproductive abnormalities
due to endocrine
disruption from pesticides
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Human studies
• Case history approach = studies individual patients
- Autopsies tell us about lethal doses
- Don’t tell about rare, new, or low-concentration toxins
- Don’t tell about probability and risk
• Epidemiological studies = large-scale comparisons
between exposed and unexposed groups
- Studies can last for years
- Yield accurate predictions about risk
- Measure an association between a health hazard and an
effect – but not necessarily the cause of the effect
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Manipulative experiments show causation
• Animals are used as test subjects
• Mammals share evolutionary history
- Substances that harm rats and mice probably harm us
• Some people object to animal tests
- Medical advances would be
far more difficult without
them
• New techniques may replace
some live-animal testing
- Human cell cultures,
bacteria, etc.
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Dose-response analysis
• Dose-response analysis = measures the effect a
toxicant produces or the number of animals affected
- At different doses
• Dose = amount of substance
the test animal receives
• Response = the type or
magnitude of negative effects
• Dose-response curve = the
dose plotted against the
response
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Dose response curves
• LD50/ED50= the amount of toxicant required to kill
(affect) 50% of the subjects
- A high number indicates low toxicity
• Threshold dose = the level where certain responses
occur
- Organs can metabolize or
excrete low doses of a
toxicant
Scientists extrapolate
downward from animal
studies to estimate the
effect on humans
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Individuals vary in their responses to
hazards
• Different people respond differently to hazards
- Affected by genetics, surroundings, etc.
- People in poor health are more sensitive
- Sensitivity also varies with sex, age, and weight
- Fetuses, infants, and young children are more sensitive
• The Environmental Protection Agency (EPA) sets
standards for responses based on adult responses
- Often, standards are not low enough to protect babies
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The type of exposure affects the response
• Acute exposure = high exposure to a hazard for short
periods of time
- Easy to recognize
- Stem from discrete events: ingestion, oil spills,
nuclear accident, etc.
• Chronic exposure = low exposure for long periods of
time
- More common but harder to detect and diagnose
- Affects organs gradually: lung cancer, liver damage
- Cause and effect may not be easily apparent
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Perceptions of risk may not match reality
Everything we do involves some risk
• We try to minimize risk
- But perception may
not match reality
- Flying versus driving
• We feel more at risk
when we do not control a
situation
- We fear nuclear power
and toxic waste
- But not smoking or
overeating
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Analyzing risk quantitatively
• Risk assessment = the quantitative measurement of risk
- Compares risks involved in different activities or
substances
- It identifies and outlines problems
• Risk assessment has several steps:
- The scientific study of toxicity
- Assessing an individual or population’s exposure to
the substance (frequency, concentrations, length)
• Teams of scientific experts review hundreds of studies
- Regulators and the public benefit from informed
summaries
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Risk management
• Risk management = decisions and strategies to
minimize risk
• Federal agencies manage risk
- The U.S. has the Centers for Disease Control (CDC),
the EPA, the Food and Drug Administration (FDA)
• Scientific assessments are considered with economic,
social, and political needs and values
• Comparing costs and benefits is hard
- Benefits are economic and easy to calculate
- Health risks (costs) are hard-to-measure probabilities
of a few people suffering and lots of people not
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EPA regulation is only partly effective
• The Toxic Substances Control Act (1976) = the EPA
monitors chemicals made in or imported into the U.S.
- The EPA can ban substances that pose excessive risk
• Many health advocates think the TSCA is too weak
- Of 83,000 chemicals, only five have been restricted
- To push for more testing, toxicity must already be
proven, but testing is minimal
• Only 10% of chemicals have been tested for toxicity
- Fewer than 1% are regulated
- Almost none have been tested for endocrine,
nervous, or immune system damage
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FIFRA (1974)
• The Federal Insecticide, Fungicide, and Rodenticide Act
- Charges the EPA with “registering” new pesticides
manufacturers want to market
• The EPA asks the manufacturer to provide information
on safety assessments
- The EPA examines ingredients, use, etc. to determine
risks to people, other organisms, water, or air
- It approves, denies, or sets limits on the chemical’s
sale and use and approves language used on the label
• Hazardous chemicals are approved if economic benefits
outweigh hazards
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The Stockholm Convention on Persistent
Organic Pollutants (POPs)
• Enacted in 2004 and ratified by over 150 nations
• POPs = toxic, persistent chemicals
- Bioaccumulate and biomagnify
- Can travel long distances
• The “dirty dozen” = the 12 most dangerous POPs
• The Stockholm Convention sets guidelines for phasing
out these chemicals
- Encouraging transition to safer alternatives
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