• Prevalence of toxic agents in
the environment
• Persistent, organic pollutants
(POP’s)
• Bioaccumulation
• Biomagnification
• Epidemiology, animal
testing, and dose-response
analysis
• Factors affecting toxicity
Toxicology
• The study of poisonous substances and their effects on
humans and other organisms
• Toxicologists assess and compare toxic agents, or
toxicants, for their toxicity, the degree of harm a
substance can inflict.
• Environmental toxicology focuses on effects of
chemical poisons released into the environment.
Synthetic chemicals are everywhere !
• Many thousands have
been produced and
released.
• Some persist for long
time periods or travel
great distances.
• Of the 100,000
synthetic chemicals on
the market today, very
few have been
thoroughly tested for
harmful effects.
Environmental Toxicology
• Studies toxicants that come from or are discharged into
the environment, and:
Health effects on humans
Effects on animals
Effects on ecosystems
• Animals are studied:
For their own welfare
To warn of possible effects on humans
• Frogs show reproductive
abnormalities in response to
small doses of the herbicide
atrazine.
• Others suggest that atrazine
may have effects on humans
as well.
• The fierce criticism from
atrazine’s manufacturer
reflects the high stakes in
environmental toxicology.
Tyrone Hayes (UC Berkeley)
Synthetic Chemicals
People are largely unaware of the health risks of
many toxicants.
Types of Toxicants
•
•
•
•
•
•
Carcinogens: cause cancer
Mutagens: cause mutations in DNA
Teratogens: cause birth defects
Allergens: cause unnecessary immune response
Neurotoxins: damage nervous system
Endocrine disruptors: interfere with hormones
Types of Toxicants: Teratogens
• The drug thalidomide,
used to relieve nausea
during pregnancy, turned
out to be a potent
teratogen, and caused
thousands of birth
defects before being
banned in the 1960s.
Thalidomide survivor Butch Lumpkin
Endocrine Disruption
• Some chemicals, once
inside the bloodstream, can
“mimic” hormones.
• If molecules of the
chemical bind to the sites
intended for hormone
binding, they cause an
inappropriate response.
• Thus these chemicals
disrupt the endocrine
system.
• The hormone system is geared to working with
tiny concentrations of hormones, so it can respond
to tiny concentrations of environmental
contaminants.
A 1992 study summarized results of sperm count
studies worldwide since 1938. Data showed a
significant decrease in men’s sperm counts over 50
years.
Testicular Cancer
• Others hypothesize that endocrine disruptors are behind
the rise in testicular cancer in many nations.
Toxicants Concentrate in Water
• Surface water and groundwater can accumulate toxicants.
• Runoff from large areas of land drains into water bodies,
becoming concentrated.
• Toxicants in groundwater or surface water reservoirs used
for drinking water pose potential risks to human health.
Airborne Toxicants
• Volatile chemicals can travel long distances
on atmospheric currents.
• PCBs are carried thousands of miles from
developed nations of the temperate zone up
to the Arctic, where they are found in tissues
of polar bears and seals.
Persistence
• Some chemicals are more stable than others,
• persisting for longer in the environment.
DDT and PCBs are persistent.
Bt toxin in GM crops is not persistent.
• Temperature, moisture, sun exposure, etc., affect rate of
degradation.
• Most toxicants degrade into simpler breakdown
products. Some of these are also toxic.
•
(DDT breaks down to DDE, also toxic.)
Typical Arctic
Atmospheric
Circulation
Pattern in
Winter
Atmospheric
Circulation
Patterns from
DenselyPopulated Areas
of Europe into
the Canadian
Arctic
Source: Environmental
Protection Division,
Department of Renewable
Resources, Government
of the NWT
Arctic Haze
1950’s
Reddish –brown haze first observed in arctic
atmosphere
Pollution peaks from January to April, as strong
low pressure systems in the Atlantic and Pacific
bring pollution northward in lowest 1-2 km
of atmosphere: most visible when sun returns
Composed of solid and liquid natural
compounds and contaminants (sulphates,
soot from coal and petroleum production)
Commoner, B., P.W. Bartlett, H. Eisl and K. Couchot. 2000. Long range air transport of dioxin from North American
sources to ecologically-vulnerable receptors in Nunavut, Arctic Canada. Center for Biology of Natural Systems, Queen’s
College, CUNY. (http://www.cec.org/programs_projects/pollutants_health/develop_tools/dioxins/dioxrep.pdf)
Arctic haze is also of greater significance in winter since
the atmosphere becomes stratified, with a stable layer of
cold air near the surface (inhibits mixing)
Summer values of pollutants are much lower (only 2-5%
of winter values)
Why ? Transport mechanisms are weaker, there is more
photochemical breakdown, there is more vertical mixing
and rain helps to flush the atmosphere
Source is mainly from Europe due to atmospheric
circulation; in North America, much of the pollution
produced is pulled eastward, south of the Icelandic low
Production
Of Arctic
Haze
“Cleansing” of Arctic Haze
Snow, rain, and dry deposition cleanse the air and
contaminate the land
Rivers carry and process contaminants by transportation,
sedimentation and re-suspension of particles.
Lakes, estuaries, and deltas serve as sediment traps and
sinks for contaminants.
Ice Shelves may also accumulate, transport and release
pollutants in the biologically productive shelf seas, where
they can be taken up into the food chain.
Snowmelt: A springtime surge of contamination
Melting snow facilitates the transport of contaminants
Snow accumulates contaminants from the air throughout the
long winter.
Deep snow packs retain volatile contaminants, which would
otherwise be released back into the air.
Water-soluble chemicals concentrate in the melt-water during
melt
Meltwater generally flows over frozen ground directly into
streams and lakes
Sediments and contaminants in the ice are released during the
spring melt
Biological productivity within the water peaks early in the
summer (coincides with high pollution)
Contaminants can, therefore, build up in the winter and be
efficiently incorporated into animal and plant life in the spring
Bioaccumulation of toxic contaminants
Biomagnification is the increase in contaminant load as
predators obtain the chemicals eaten by their prey, thus
further concentrating the toxic material at each trophic level
Primary concern: levels of persistent organic pollutants
(POP’s) are nine times higher in breast milk of northern
women than in southern Canada
Why is this ?
Contaminants are preserved in cold climates due to the lack
of microbial activity to degrade them
People eat more country foods
Why are POP’s so persistent ?
POP’s are synthetic organic chemicals that
(1)
(2)
break down very slowly
are fat-soluble
They are found in high concentrations in arctic animals because:
(1)
(2)
storing energy as fat is required for survival in
cold climates
many arctic animals are carnivorous (high on the
food chain
Poisons move up the food chain
• At each trophic level,
chemical concentration
increases:
biomagnification.
• DDT concentrations
increase from plankton
to fish to fish-eating
birds.
Figure 10.9
Poisons Accumulate in Tissues
• The body may excrete, degrade, or store toxicants.
• Fat-soluble ones are stored.
•
DDT is persistent and fat soluble,
•
… so builds up in tissues: bioaccumulation.
• Bioaccumulated chemicals may be passed on to animals
that eat the organism—up the food chain…
Effect of POP’s on living organisms
In birds:
Impacts upon ability to conceive and raise young.
Affects egg development and mating behaviour in bird species
In mammals:
1.
2.
3.
4.
Leads to malformations in reproductive organs, fewer
young or infertility
Hormone disrupters mimic hormones because they are similar
enough in structure to fit into the body’s biochemical receptors
Can affect the immune system, especially in the young
Carcinogenic
Specifics on other contaminants:
DDT
Pesticide: toxic to more organisms than intended to kill
Birds of prey began to die in large numbers in affected areas
Evidence of long-range transport:
Detected in the blubber of ringed seals in 1970
Banned for decades in circumpolar countries
PCB’s
Used in transformer fluids
Carcinogenic, mutagenic
Dioxins and furans
Used in high temperature processes (stable) and herbicides
Disturbed reproduction, suppressed immune function and
highly carcinogenic
Hexachlorobenzene (HCB)
Used in pesticides and produced in waste incineration and
metallurgical processes
Affects reproductive and immune function
Annual Contaminant Deposition in
Precipitation (Gregor, 1990)
Compound
HCH
DDT
PCB’s
HCB
C. Ont.
14
0.02
1.6
0.08
N. Sask.
2.1-6.5
0.002-0.1
1.5-1.7
0.003
Arctic
0.01-2.3
0.01-0.2
0.01-0.3
<0.01
Organic compound concentrations in
Animals of the Northwest Territories
(Environment Canada, 1992)
Caribou
Arctic Hare
Ptarmigan
Marine Fish
Seabirds
Polar Bear
Seal
Whale
DDT
HCH
chlordane
1-4
1-42
0.2-8
<0.2-0.9
1-50
0.6-4
0.3-110.4-53
0.2-7.4
3-21
2-110
1-44
3-13000
5-10
7-230
10-1200
300-870
1800-7000
2-4900
1-270
2-4600
320-800
150-240
30-2300
All toxicants are not synthetic
• Although toxicology tends to focus on manmade chemicals, it’s important to keep in mind
that there are plenty of natural toxicants.
• Many are toxins produced by animals or plants
for protection against predators and pathogens.
Dose-Response Analysis
• Method of determining toxicity of a substance by
measuring response to different doses
• Lab animals are used.
• Mice and rats breed quickly, and give data relevant to
humans because they share mammal physiology with us.
• Responses to doses are plotted on a dose-response curve.
Dose-response curve
Threshold = dose at
which response begins
LD50 = dose
lethal to 50%
of test animals
Figure 10.10
Dose-Response Curve
• Dose-response curves allow us to predict effects of higher
doses.
• By extrapolating the curve out to higher values, we can
predict how toxic a substance may be to humans at
various concentrations.
• In most curves, response increases with dose.
But this is not always the case; the increase may not be
linear.
Factors Affecting Toxicity
• Not all people are equal. Sensitivity to toxicant can vary
with sex, age, weight, etc.
• Babies, older people, or those in poor health are more
sensitive.
• Type of exposure:
•
acute = high exposure in short period of time
•
chronic = lower amounts over long period of time
Mixtures of Toxicants
• Substances may interact when combined together.
• Mixes of toxicants may cause effects greater than the sum
of their individual effects.
These are called synergistic effects.
• A challenging problem for toxicology:
There is no way to test all possible combinations!
(And the environment contains complex mixtures of many
toxicants.)