env100 note 59.docx

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28 Mar 2012
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ENV100 Lecture Notes Mar 26, 2012
Environmental Health continued
Reading: Chapter 19
Toxins can act in several different ways
Carcinogens cause cancer
Mutagens cause DNA mutations
o Can lead to severe problems, including cancer
Teratogens cause birth defects
Allergens overactivate the immune system
Neurotoxins damage the nervous system
Endocrine disruptors interfere with the endocrine (hormone) system
Airborne toxins can travel widely
Pesticide drift = airborne transport of pesticides
Synthetic chemical contaminants are found globally
o They appear in arctic polar bears, Antarctic penguins, and people living in
Greenland
Some toxins persist for a long time
Persistent toxins can either degrade quickly or remain unaltered and persist for decades
o Rates of degradation depends on temperature, moisture, and sun exposure
Breakdown products = toxicants degrade into simpler products
o May be more or less harmful than the original substance
o DDT degrades into DDE, which is also highly persistent
Toxins can accumulate over time and up the food chain
Bioaccumulation = toxicants build up in animal tissues over time
Biomagnification = toxicants concentrate in top predators (= “food
chain” concentration)
Studies rely on human case histories, epidemiology, and animal testing
Case histories = observation and analysis of individual patients
Epidemiology = large-scale comparisons among groups of people over a long period of
time
o Comparing exposed and unexposed people
Manipulative experiments = lab experiments that expose subjects to toxicants to
establish causation
o Animals used as models (e.g. rats, mice)
Observations of animals in the wild
o Conspicuous mortality events
Dose-response analysis is a mainstay of toxicology
Dose = the amount of toxicant the test animal receives
Response = the type or magnitude of negative effects of the animal
Dose-response curve = the plot of dose given against response
LD50/ED50= the amount of toxicant required to kill (affect) 50% of the subjects
Threshold = the dose level where certain responses occur
Individuals vary in their responses to hazards
Different people respond differently to hazards
o Affected by genetics, surroundings, etc.
o People in poor health are more sensitive
o Sensitivity also varies with sex, age, and weight
o Fetuses, infants, and young children are more sensitive
Standards are set by Health Canada
o Do not account for risks to fetuses, infants, children, older adults, and the
immunocompromised
The type of exposure can affect the response
Acute exposure = high exposure for short periods of time to a hazard
o Easy to recognize
o Stem from discrete events: ingestion, oil spills, nuclear accident
Chronic exposure = low exposure for long periods of time to a hazard
o Hard to detect and diagnose
o Affects organs gradually: lung cancer, liver damage
o Cause and effect may not be easily apparent
Mixes may be more than the sum of their parts
Synergistic effects = interactive impacts that are more than or different from the simple
sum of their constituent effects
o Mixed toxicants can sum, cancel out, or multiply each other’s effects
o New impacts may arise from mixing toxicants
We don’t know everything there is to know about all of the chemicals that are in the
environment
o Mainly, we don’t know how they interact with each other and with
environmental media
Two approaches exist for determining safety
“Innocent until proven guilty” approach: product manufacturers must prove a product is
safe
o Slows down technological innovation and economic advancement
o Puts into wide use some substances that may later on turn out to be dangerous
“Precautionary principle” approach: substances are harmful until they are shown to be
harmless
o Identifies troublesome toxicants before they are released
o May impede the pace of technology and economic advance
Conclusions
We (and other organisms) are exposed to a wide variety of hazards in our environment
every day
Synthetic chemicals have brought hazards but also convenience, a larger food supply,
and medical advances
Solutions to protect the world’s people, wildlife, and ecosystems from toxic chemicals
and environmental hazards come from:
o Governments
o Consumer choice
o Scientific research