Introduction
Every day in so many ways we are subjected to a variety of types and degrees of risk. Some of this risk is overt and obvious, much of it, covert and hidden. How do we determine what is an acceptable level of risk to which we are willing to be exposed? Are the levels different for home, work, school, recreation sites? What role do we as citizens play in determining the standards by which risk will be judged? What can we do when we feel the level of risk is beyond what we feel is safe? The following articles describe different types of risks in different situations. Use this information as you answer some of the questions at the end of the lab. The reading on toxicity is from Proctor and Gamble and will probably surprise you. There is little risk from reading it.
Read the following article from The New Scientist entitled "Assessing Risk."
It provides a balanced look at the benefits and the down side of biotechnology's recent workings.
Visit this site to look at another type of risk.
This site looks at species extinction from a global perspective.
Are some risks better to take than others?
Do your life style and your life choices play some role in the risks to which you may be exposed?
This study talks about the relationship between stress at work and the risks attendant to that stress.
TOXICITY
"Toxicology is the study of toxic or poisonous properties of substances. Toxicologists concern themselves with harmful effects of chemicals in our lives. All chemicals can have toxic effects. Toxic effects depend on a number of factors. One of these is dose. A two-year old child can swallow one teaspoon of salt safely, but 10 teaspoons might kill the child. Another of these factors is the route of exposure-the way the chemical enters the human body. This is generally through the mouth, through the skin, or through the lungs. Furniture polish accidently sprayed on the skin may have no toxic effects. The same polish may be very toxic if taken in the mouth or lungs. Other factors which relate to toxicity include frequency and duration of exposure--how often one is exposed to a chemical and the length of time one is exposed--and the toxicity, or harmful effects of the chemical itself.
Some of the most toxic substances in the world are natural. For example, botulism is produced by bacteria which cause a form of food poisoning. One gram (1/28 of an ounce) of botulism can kill 20 x 106 mice; the lethal human dose of botulism toxin is estimated to be 1/14 millionth of an ounce.
Many people feel that a chemical--whether natural or synthetic is either poisonous or it is not. This is not the case. In low doses DDD, a potent insecticide, is effective for treating certain forms of adrenal cancer. Coumarin compounds are excellent rodenticides (agents that kill or repel rodents), but they are also valuable drugs used to prevent the blood from clotting. The toxicity of these compounds is the same whether used as a drug or as a pesticide. Legally a poison is defined as a substance which has a lethal dose of 50 mg or less per kilogram of body weight. This is about 3/4 of a teaspoon for adults and about 1/8 teaspoon for a two-year old child. There are very few chemicals that are lethal in such small quantities. Even the majority of synthetic pesticides are not this toxic.
But how do toxicologists and other scientists determine the toxicity of a chemical? What methods are involved? How can these methods be improved to provide more accurate and reliable information? Toxicologists evaluating the safety of a new chemical or formulation first refer to computer data bases, scientific journals and books to determine whether anyone else has examined the effects of similar chemicals. If no information is available, the toxicolo-gist places the chemicals into appropriate testing, including tests using animals and tests using cell and tissue cultures. Early stages of the testing procedure may involve the use of bacterial cultures to detect any tendency of the chemical to cause mutations which might lead to cancer (the Ames test).
Often the effects of a particular chemical on humans cannot be reliably predicted by such methods and tests using animals are required. Over 90% of all animals used in testing are rodents, especially bred for this purpose. Scientists in most responsible major consumer product companies are committed to searching for alternatives to animals testing. The problem with many of these alternative tests is that they do not reliably predict the effects of chemicals on humans. Thus they are not usable for safety evaluation and are not accepted by the government agencies such as the Food and Drug Administration.
Toxicologists need to determine the level at which a particular chemical has an effect on a living system. This is called the threshold limit value and is one of the basic concepts of toxicity.
Individuals vary widely in their responses to chemicals. Some people require only one tablet of aspirin to cure a headache. Some need two or three. Different people have different tolerances for alcohol. Ten mg/kg of parathion, an extremely toxic insecticide, will kill about 1/2 of all rats receiving the dose. But 1/2 of the rats will live. These differences may it difficult to determine safe levels of chemicals in our lives. For example, there is a threshold at which a chemical might be effective as a food preservative because it kills bacteria that spoil food. There is also a threshold at which the same chemical might be harmful to humans, causing illness. A toxicologist compares the two values to determine whether the chemical has any real value as a food preservative. If the chemical is effective as a food preservative only at concentrations harmful to humans, it will not be used.
In order to make a complete safety evaluation, toxicologists must investigate both the short-term and long-term effects of exposure. Short term exposures are called acute effects. Examples of short-term exposures are accidental chemical spills at a factory or the accidental ingestion of a consumer product by a small child. These effects differ from the long-term effects of exposure, called chronic effects. An example of chronic toxicity is the danger of working for many years in an office where other people are smoking.
Some chemicals are very toxic acutely but have no chronic toxic effects in small amounts. They may even be essential. Vitamin D is such a chemical. It has an acute oral toxicity of 10 mg/kg, yet all of us need a dose about one thousand times smaller daily to maintain good health. Vitamin D deficiency results in rickets, and severe deficiency can cause death. If Vitamin D were not exempted from the Hazardous Substances Labeling Act since it is a food (in milk) or a drug (as a vitamin), it would be required to carry a poison label. Metallic mercury has no acute oral toxicity; however, chronic exposure to mercury vapors is extremely hazardous to human health. If a child breaks an thermometer, the concern should not be whether any mercury is swallowed, but whether any spilled on the carpet or was ground into the fibers from which it could slowly vaporize.
To predict the long-term or chronic effects of low levels of chemicals in humans, we must test at higher levels of the chemical for shorter periods of time. Tests which last for thirty years would be too expensive or impractical. The chronic oral effects of the artificial sweetener saccharin were estimated by giving massive doses to laboratory rodents over a two-year period. Public outcry resulted over the disclosure that humans would have to drink 50,000 cans of diet soda in one day to obtain a similar dosage!
In the United States today, we interact with a staggering variety and number of chemicals as a normal part of our daily routine. We take over-the-counter and prescription medication and use a wide variety of consumer products without giving safety a second thought. The knowledge of toxicologists about the line between beneficial and toxic effects of chemicals makes this possible."
The worksheet following this information will give you an opportunity to see just how much of certain chemicals you take in during your lifetime. You may be surprised!
LIFETIME INGESTION OF TOOTHPASTE
Each time you brush your teeth, you swallow a tiny amount--even if you are very careful. How much toothpaste will you swallow in your lifetime? Certain assumptions are made to make the calculations easier.
- How many times do you brush your teeth daily?
(If your brushing habits vary, report the average value.)
I brush________________ times a day.
- Multiply the previous number by 365.25 (the number of days
in a year).
I will brush__________________ times in a year.
- Multiply your previous answer in item 2 by 66 if you are
male (_____________), by 72 if you are female (______________). These are the average life expectancies for your age group, according to gender, adjusted for the lack of regular brushing habits for the first
two years of your life. (Source: U.S. Bureau of Census, 1988).
I will brush____________________in my life.
- Multiply your answer from step 3 by .05. This is the number of
grams of toothpaste you swallow each time you brush. This is an
average amount, taking into account the fact that young children swallow more toothpaste than adults.
I will swallow___________________ grams of toothpaste in my life.
- The family-sized tube of Tartar Control Crest is 6.4 ounces or 182.4 grams. Divide the number of grams, found in step 4, by
182.4 to find out how many tubes of toothpaste you will swallow in your life.
I will swallow____________________ tubes of toothpaste in my life.
TARTAR CONTROL CREST
A Case Study in Consumer Product Safety
"The oldest recorded belief about toothaches is that they were caused by a "toothworm." Ancient Babylonian texts described toothaches caused by demons that took the shape of worms and invaded teeth. In ancient Greece, bleeding gums were cauterized with a red-hot iron. In France during the Middle Ages, open markets served as dental offices for the poor. Teeth were extracted by self taught gypsies. A red-hot knife was plunged into the gum to stop a tooth-ache. Dentists came to the homes of the wealthy, bringing their instruments with them. Their dental drills were thin metal rods turned by rolling them between the palms. Decayed teeth were sometimes drilled and then filled with soft materials that stopped the pain by keeping air from the open cavity. Usually the offending tooth was simply removed.
Regular dental hygiene is an important part of our daily lives. Americans today enjoy an unprecedented level of dental health in spite of a diet high in sugar and other foods that promote tooth decay. The above description of early dentistry bothers most readers. Today's visit to the dentist is very different from visits even our grandparents might have taken. We find it hard to believe that one of the most basic notions of dental science--the role of bacterial infection in tooth decay--was not discovered until the end of the 19th century, more than 4,000 years after the first descriptions of the Babylonian "toothworm."
Most adults brush their teeth at least twice daily. Many young people, especially those who wear braces, brush even more often. Can you think of another consumer product besides toothpaste we put into our mouths more often, and on a regular basis for our entire lives? Probably not. We expect toothpaste to cause problems for cavities, not for us. Few people realize the extensive behind-the-scenes scientific testing which goes on to make sure our dental care products continue to be safe and effective.
In the late 1930s, Proctor & Gamble invested an enormous amount of time, money and other resources to produce a toothpaste that would fight cavities. Many different formulas were tried before a compound called stannous fluoride was proven to be effective in preventing tooth decay. Numerous long-term clinical trials with children and adults were then conducted, supervised by doctors and scientists, to ensure the product was safe and effective. At the end of the tests, the number, location and severity of cavities were compared. The results were clear. People who brushed with a toothpaste containing stannous fluoride had significantly fewer cavities than people using the same toothpaste without the fluoride formula.
The new formula was given the name "Crest." In 1960, the American Dental Association (ADA) publicly recognized Crest's effectiveness in reducing cavities "...when used in a conscientiously applied program of oral hygiene and regular professional care." All toothpastes now on the market have ADA endorsement with the same disclaimer statement.
But how does Crest reduce the chance of tooth decay? The important role of bacteria in tooth decay was mentioned earlier. In fact, just minutes after you brush, a thin coating of bacteria called placque may form on tooth surfaces. These bacteria break down the sugar and similar molecules in food into organic acids which then attack tooth enamel. Over time, this erosion can lead to cavities. The stannous fluoride in Crest attaches to the calcium compounds in tooth enamel, making the enamel stronger and more resistant to decay.
Recently, dental health specialists have become concerned with tartar, or dental calculus, as it is sometimes called. Tartar is caused by a gradual buildup of placque over time until it becomes thick and hard. Uncontrolled build-up of placque can produce a condition called gingivitis which is an inflammation of the gum tissue that surrounds the tooth. Other problems can also occur and may result in premature tooth loss. The scientists at Proctor & Gamble decided that a toothpaste that would inhibit tartar formation would be very important.
Chemically, tartar is a calcium precipitate. A precipitate is a solid mass or substance that forms (or settles out of) from liquid solutions. Certain conditions are necessary for precipitates to form. If these conditions are not present or are altered, the precipitate will not form properly. Proctor & Gamble scientists had to find a safe way to change the conditions in the mouth that allow tartar to form. Scientists must know how a chemical molecule is constructed in order to find a way to attack it. A thorough search of various chemicals that would inhibit tartar formation was conducted. On the basis of safety factors, effectiveness and manufacturing costs, sodium pyrophosphate was chosen for more testing.
Sodium pyrophosphate fights tartar by a process known as chelation. A chelate is a compound in which the central atom (in this case, calcium) is attracted by two or more bonds, forming a surrounding structure. Normally the tartar that forms on your teeth is a hard, calcium-containing deposit. When a toothpaste containing sodium pyrophosphate is used, the tartar that forms is weaker. It crumbles easily under the action of your toothbrush. How does sodium pyrophosphate make the tartar weaker? It forms chemical bonds with the calcium network in crystals of tartar, reducing the cross-linking bonds that would normally form. The tartar that forms is not nearly as strong. But there were many parts to the problem. In order to allow the active ingredient to spread evenly over the tooth surface, the right surfactant or spreading agent had to be formulated.
But was pyrophosphate safe to use? It was known that ingestion of certain chelating agents can lead to health problems in humans. Since millions of people would be using the new toothpaste, it was necessary to test the product to make sure it was absolutely safe for humans. After an extensive battery of safety tests, including animal tests, it was concluded that the new formulation was safe for human use.
The product was then tested in clinical trials supervised by scientists and dental care professionals, often in double blind studies. A "double blind" study is one in which the exact identity of the substance being tested is kept secret from both the parti-cipants and the researchers. In carefully controlled studies, various formulations containing sodium pyrophosphate proved effective in controlling tartar, reducing deposits by more than 44% in some cases. Participants in the study represented a cross-section of the population with regard to age, brushing habits and other factors.
Decisions regarding the flavor formulation were needed. Sodium pyrophosphate has a very salty taste. Several flavors were tried before one was found that masked the salty taste but which consumers did not find too strong. In 1985, Tartar Control Crest was first marketed."
Based on the articles you have read, answer the following questions.
1. Write a job description for a toxicologist. Tell about the various duties that such a person would be involved in and have responsibility for.
2. Name the five factors that influence toxicity. Give examples of each factor taken from the articles you have read.
3. What is the chemical definition of a poison? Can chemicals which are poisons have beneficial uses? Give at least two examples.
4. Describe the steps in the safety evaluation of a new chemical. Compare these procedures to the sequence of steps associated with scientific methodology.
5. What is the threshold limit value? Why is it important?
6. Distinguish between acute and chronic toxicity. Give at least two examples, taken from the readings, that represent each type and why it is important to know either the acute or chronic toxicity of a chemical.
7. Describe some of the problems in determining the chronic toxicity of a chemical.
8. What is a chelate or chelating agent? How does it work?
9. What is the difference between placque and tartar? What kinds of problems do each cause?
10. How does sodium pyrophosphate inhibit tartar formation? Why were clinical trials necessary to prove the effectiveness of both sodium fluoride and sodium pyrophosphate?
11. Is it possible to create an environment which has zero risk? Would it be desirable to do so? Why or why not? How would you or how do you determine what an acceptable level of risk is in the workplace environment?
Source: Proctor & Gamble Educational Services, Decisions About Product Safety: A Teaching Unit for Science Educators, Cincinnati, Ohio, 1990