• 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 humansEffects on animalsEffects on ecosystems

• Animals are studied:

For their own welfareTo 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 ArcticAtmosphericCirculationPattern in Winter

AtmosphericCirculation Patterns fromDensely-Populated Areasof Europe into the CanadianArcticSource: Environmental Protection Division, Department of Renewable Resources, Government of the NWT

Arctic Haze

1950’s Reddish –brown haze first observed in arcticatmosphere

Pollution peaks from January to April, as stronglow pressure systems in the Atlantic and Pacificbring pollution northward in lowest 1-2 kmof 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 sincethe atmosphere becomes stratified, with a stable layer ofcold 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 morephotochemical breakdown, there is more vertical mixingand 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

ProductionOf ArcticHaze

“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 contaminantsBiomagnification 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 northernwomen than in southern Canada

Why is this ?Contaminants are preserved in cold climates due to the lackof microbial activity to degrade them

People eat more country foods

Why are POP’s so persistent ?

POP’s are synthetic organic chemicals that

(1) break down very slowly(2) are fat-soluble

They are found in high concentrations in arctic animals because:

(1) storing energy as fat is required for survival in cold climates

(2) 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…

In birds:

Impacts upon ability to conceive and raise young. Affects egg development and mating behaviour in bird species

In mammals:

1. Leads to malformations in reproductive organs, fewer young or infertility

2. Hormone disrupters mimic hormones because they are similar enough in structure to fit into the body’s biochemical

receptors3. Can affect the immune system, especially in the young 4. Carcinogenic

Effect of POP’s on living organisms

Specifics on other contaminants:

DDT

Pesticide: toxic to more organisms than intended to killBirds 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 andhighly 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 C. Ont. N. Sask. ArcticHCH 14 2.1-6.5 0.01-2.3DDT 0.02 0.002-0.1 0.01-0.2PCB’s 1.6 1.5-1.7 0.01-0.3HCB 0.08 0.003 <0.01

Organic compound concentrations inAnimals of the Northwest Territories(Environment Canada, 1992)

DDT HCH chlordaneCaribou 1-4 1-42 0.2-8

Arctic Hare <0.2-0.9 1-50 0.6-4

Ptarmigan 0.3-11 0.4-53 0.2-7.4

Marine Fish 3-21 2-110 1-44

Seabirds 3-13000 5-10 7-230

Polar Bear 10-1200 300-870 1800-7000

Seal 2-4900 1-270 2-4600

Whale 320-800 150-240 30-2300

All toxicants are not synthetic

• Although toxicology tends to focus on man-made 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

LD50 = dose lethal to 50% of test animals

Figure 10.10

Threshold = dose at which response begins

Dose-Response CurveDose-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.)