Chapter 7
Chapter 7
Human Health and Environmental Toxicology
Lecture Outline:
I. Human Health
A. Two indicators of human health in a given country are life expectancy (how long people are expected to live) and infant mortality (how many infants die before the age of one)
B. Health issues in highly developed countries
i. Average life expectancy for American women is 80 and 75 for American men
ii. A significant fraction of premature deaths in the United States is caused in part by individual lifestyle habits (poor diet, lack of exercise, smoking)
iii. Healthcare professionals use the body mass index (BMI) to determine whether a person is overweight and/or obese
C. Health issues in developing nations
i. Malnutrition, unsafe water, poor sanitation, and air pollution still prevail in many less developed countries despite gradual improvements in sanitation and drinking water
ii. Average overall life expectancy for developing countries is 65
iii. Average overall life expectancy for the very poorest developing countries is <45
iv. 18% of the 57 million deaths that occur worldwide each year are children less than 5 years of age
D. Emerging and reemerging diseases
i. Emerging diseases are infectious diseases that were not previously found in humans; they typically jump from an animal host to the human species (i.e., HIV/AIDS, Lyme disease, West Nile virus, influenza (new strains), SARS, Ebola)
ii. Reemerging diseases are infectious diseases that existed in the past but for a variety of reasons are increasing in incidence or in geographic range (i.e., tuberculosis, yellow fever, malaria, dengue fever)
iii. The main factors involved in the emergence or reemergence of infectious diseases include
1. Evolution of the infectious organisms
2. Evolution of antibiotic resistance
3. Urbanization (overcrowding, poor sanitation)
4. An increase in elderly who are more susceptible to infection
5. Pollution, environmental degradation, changing weather patterns
6. Growth in international travel and commerce
7. Poverty and social inequality
II. Environmental Pollution and Disease
A. Persistence, bioaccumulation, and biological magnification of environmental contaminants
i. Persistence is a characteristic of certain chemicals that are extremely stable and may take years to be broken down in to simpler forms by natural processes
ii. Bioaccumulation is the buildup of a persistent toxic substance
iii. Biological magnification is the increased concentration of toxic chemicals in the tissues of organisms that are at higher levels in food webs
1. Toxic substances that exhibit all three of these characteristics include certain pesticides (DDT), radioactive isotopes, heavy metals (lead/mercury), flame retardants (PBDEs), and industrial chemicals (PCBs)
2. Natural decomposers (bacteria) have not yet evolved ways to degrade many synthetic pesticides, therefore leading to the accumulation of pesticides in the environment and food web
B. Endocrine disrupters are chemicals that mimic or interfere with the actions of the endocrine systems in humans and wildlife
1. Examples include chlorine-containing industrial compounds (PCBs and dioxins), heavy metals (lead and mercury), pesticides (DDT, kepone, dieldrin, chlordane, endosulfan), flame retardants (PBDEs), and certain plastics and plastic additives such as phthalates
a. Phthalates are ingredients in cosmetics, fragrances, nail polish, medications, and common plastics used in food packaging, toys and household products
b. Phthalates have been implicated in birth defects and reproductive abnormalities
c. Like hormones, they are active at very low concentrations and appear to alter reproductive development in males and females of various animal species
2. Hormones are chemical messengers produced by organism in minute quantities to regulate growth, reproduction and other important biological functions
III. Determining Health Effects of Environmental Pollution
A. All chemicals, even “safe” chemicals such as sodium chloride (table salt), are toxic if exposure is high enough
B. The study of toxicants, or toxic chemicals, is called toxicology
i. Toxicity is measured by the extent to which adverse effects are produced by various doses of a toxicant
1. Acute toxicity, which ranges from dizziness and nausea to death, occurs immediately to within several days following a single exposure
2. Chronic toxicity generally produces damage to vital organs, following a long-term, low-level exposure to chemicals
ii. One complication of toxicology is that each individual’s genes largely determine that person’s response to a specific toxicant
1. Environmental susceptibility genes affect how the body metabolizes toxicants
2. Other gene variations allow certain toxicants to bind strongly – or less so – to DNA
iii. Children and chemical exposure
1. Children are more susceptible to most chemicals than are adults because their bodies are still developing and are not as effective in dealing with toxicants
2. Pesticides and children
a. The EPA estimates that 84% of U.S homes use pesticide products
b. More than 65,000 reports of exposure and possible poisoning from household pesticides, involving children, occur each year
3. Identifying cancer-causing substances
a. Toxicology and epidemiology are the two most common methods for determining whether a chemical causes cancer
b. Although epidemiological studies have the advantage that they look at people who were actually exposed to the chemical, several limitations still exist
i. It is difficult to reconstruct, or estimate, historical doses
ii. Various confounding factors exist (additional exposures)
iii. Individuals respond differently
iv. Chemical mixtures interact by additivity, synergy, or antagonism
1. Additivity – the effect is exactly what one would expect, given the individual effects of each component of the mixture (1+1=2)
2. A synergistic chemical mixture has a greater combined effect than expected (1+1=3)
3. An antagonistic interaction in a chemical mixture results in a smaller combined effect than expected (1+1=1.3)
IV. Ecotoxicology: Toxicant Effects on Communities and Ecosystems
A. There has been a paradigm shift in the way people think about toxicant effects on communities and ecosystems
i. People used to think - and some still do – that “the solution to pollution is dilution” (the dilution paradigm)
ii. Today virtually all environmental scientists have rejected the dilution paradigm in favor of the boomerang paradigm – “what you throw away can come back and hurt you”
B. Ecotoxicology (aka environmental toxicology) studies contaminants in the biosphere, including their harmful effects on ecosystems
i. Its scope is broad – from molecular interactions to global climate change
ii. It helps policymakers determine the costs and benefits of the many industrial and technological “advances” that affect us and the ecosystems
1. Obtaining higher-level information is complicated because natural systems are exposed to many environmental stressors
2. Also, natural systems must be evaluated for extended periods to establish trends, and results must be clear enough for evaluation by policymakers and the public
V. Decision Making and Uncertainty: Assessment of Risks
A. Risk management is the process of identifying, assessing, and reducing risks
i. Risk is the probability that a particular adverse effect will result from some exposure or condition
ii. A hazard is a condition that has the potential to cause harm
iii. Effective risk management cannot be based on calculated risks alone, but must also account for intuition, trust, and social conditions
B. Cost-benefit analysis of risks analyzes the estimated cost of some regulation to reduce risk compared with potential benefits associated with that risk reduction
C. The precautionary principle is the idea that no action should be taken or product introduced when the science is inconclusive and unknown risks may exist; it puts the burden of proof on the developers
D. Ecological risk assessment involves hazard identification, dose-response assessment, exposure assessment, and risk characterization
In-Class Activities:
Instructor Notes for In-Class Activity 1
Title: / Contrasting Human Health Issues in the Developed and Developing WorldTime: / 5 – 10 minutes prep; 15 – 20 minutes in class
Materials: / Tally master sheet or template (see below)
Handouts: / One
Procedures: / Distribute handouts to all students. Students will spend one or two minutes filling out a worksheet that assesses the types of diseases and other causes of death they have encountered at a personal level. They will then tally their results in small groups, and discuss any uncertainties they may have encountered. One student will then report back to the larger group, and the instructor or a student will tally the list for the full class. This list will represent diseases familiar to the US. The instructor can then challenge the students to think about rates of the same diseases and causes of death in less developed countries.
Student
Instructions: / Fill out the worksheet. Then, working in groups of 3 - 4, tally your group’s results, and talk about any uncertainties you may have. Choose one member of your group to report back to the class.
After full class has been tallied, ask students to return to their group of 3 – 4, and discuss this question: how do you think these numbers would differ in a less developed country (perhaps Bangladesh or Zaire). Why? Would the differences be more pronounced among the young, middle aged, or old?
Have groups take turn reporting their ideas back to the full class (number of ideas per group will depend on total size of class).
Specific
Suggestions: / 1. Prepare a master tally sheet for the classroom—whether an overhead, PowerPoint slide or template to copy onto the board.
2. Feel free to add other “conditions” to the table below, or allow students to add other conditions by adding blank lines.
3. Be sure to ask whether any students are from or have personal contacts in developing countries.
Objectives: / Contrast health issues in highly developed and developing countries
In-Class Activity 1: Handout
Malaria
Obesity
Heart Disease
Diarrhea (infant)
Tuberculosis
Car accident
Polio
HIV / AIDS
Starvation
Instructor Notes for In-Class Activity 2
Time: / 5 - 10 minutes prep; 15 - 25 minutes in class (depends on students’ math skills)
Materials: / Calculators or computers with spreadsheets
Handouts: / One (see below)
Procedures: / Students will calculate a hypothetical bioaccumulation of the common lipophilic pesticide DDT as it progresses through the food chain from algae to fish to humans. First, students will work through an example calculation: a starting concentration of DDT found in algae is used that to calculate the total amount of DDT consumed by a shrimp. The amount consumed by the shrimp divided by the weight of the shrimp will yield an approximate concentration of DDT in the shrimp. A similar calculation will yield the concentration of DDT in a fish, and another will yield the concentration in an adult human.
Student
Instructions: / Complete the calculations on the worksheet below.
Specific
Suggestions: / 1. Assess the math skills of your students before doing this exercise. If they have solid basic math skills, they should be able to do this alone. Alternatively, have them work in small groups.
2. If you have access to computers, these equations can easily be set up in a spreadsheet program.
3. For advanced students, consider varying the various consumption rates and body weights, consider the case of children, and / or plot the bioaccumulation in humans as they age.
Objectives: / Demonstrate how bioaccumulation can lead from small exposures to large exposures through the food chain.
In-Class Activity 2: Handout
Estimating Bioaccumulation: DDT in a Lake
(Note: this is a hypothetical example.)
A medium sized lake can serve as a major food source for a subsistence community. Suppose that a community of about 100 people gets much of its protein from fishing on such a lake. Every day, each person eats one or two fish, which weigh on average about 150 grams. The fish feed primarily on shrimp, as well as insects and other small animals. You have learned that an alga that is one of the primary food sources for shrimp in the lake is contaminated with DDT, a persistent organic pesticide that farmers upstream have begun using to control ants in food crops. You want to know how much of this DDT might eventually end up in members of the community.
The following shows how to calculate the concentration of DDT in the shrimp, given the concentration in algae and the eating habits of the shrimp.
Knowns:
§ Algae contain about 0.0002 mg of DDT per gram of algae. That is about 0.2 parts per million (ppm).
§ Over its (short) life, a shrimp will grow to weigh about 1 gram
§ The average shrimp eats about 10 grams of contaminated algae during its life.
The amount of DDT consumed by the shrimp can then be calculated:
DDT consumed by shrimp = amount algae consumed × concentration of DDT in algae
So
§ DDT consumed by shrimp = 10 galgae × 0.0002 mgDDT per galgae = 0.002 mgDDT
§ Since a shrimp weighs 1 gram, the concentration of DDT in the shrimp is
§ 0.002 mgDDT / gshrimp, or about 2 ppm
§ This assumes that ALL of the DDT consumed by the shrimp stays in its body (a “worst case” assumption that is probably not true).
Now, use this same method to estimate the eventual concentration of DDT in fish and in humans, given the following information:
§ The average fish eats about 1.7 kg of shrimp as it grows to weigh 150 g.
§ The average person eats 1 – 2 fish per day over a 60 year life.
§ The average adult weighs about 70 kg.
§ Note that you will have to compute the total amount of fish eaten by multiplying the amount eaten per day by the number of days in the individual’s life.
Instructor Notes for In-Class Activity 3
Time: / 10 minutes prep; 15 - 25 minutes in class
Materials: / None
Handouts: / Instruction sheet, graphs
Procedures: / Students will chart
Student
Instructions: / Working in groups of 3-4, create a Dose-Response curve given the data and instructions on the worksheet. Use this dose-response curve to answer the questions on the worksheet. Discuss as a class.
Specific
Suggestions: / Provide students with multiple copies of the graph sheet.
Consider using additional real or hypothetical dose-response data so that students will see a range of possible curves, including thresholds and even hormetic (beneficial) effects as shown in the Environmental news clip.
Objectives: / Describe how a dose-response curve helps determine the health effects of environmental pollutants.
In-Class Activity 3: Handout 1