The Portion is the Poison

Adapted from "The Dose Makes the Poison" Lesson #2 in Chemicals, the Environment, and You: NIH Curriculum Supplement Series (Grades 7-8)

Adapted by: Brink Harrison

Time: one class period
Preparation Time: 5 minutes
Materials: Holiday Dinner Menu Overhead


Students will calculate the amount of everyday food products or liquids that would need to be consumed in order to become toxic. The students should see that the amount of a substance determines it toxicity. By using unit analysis, students will calculate the amount of certain common food items that would need to be consumed in order to become toxic. They will also become aware of the amount of arsenic present in several brands of bottled water and the amount of caffeine in several everyday products.


Students will be able to:

i. Use unit analysis to check measurement computations

National Mathematics Education Standard

Measurement: Apply appropriate techniques, tools, and formulas to determine measurement

Teacher Background
See attached sheet above

Related and Resource Websites







1. Start class by sharing with the students, “Because we are exposed to many naturally occurring and synthetically produced chemicals everyday, it is important to know when to be worried about exposure and when not to be worried."

2. Ask the students to give you examples of chemicals they are exposed to by simply eating a Thanksgiving dinner. (See Teacher Background for a sample list compiled by the ACSH: http://www.acsh.org/publications/pubID.103/pub_detail.asp)

3. Once students have provided some ideas show the students the list and share that all of the chemicals listed there are known to cause adverse affects in rats. Ask, " We are being exposed to all of these chemicals and many more each day. Why are we not being poisoned by our own food supply?" Discuss this, and get the students to realize that 1) we are different from rats; and 2) that exposure to a chemical that can cause harm does not necessarily result in harm. Stress that it is the amount of the chemical you ingest that makes it toxic and with most chemicals, the amount that makes the chemical toxic is virtually impossible to consume. As an example, tell the students that the chemical malonaldehyde is a chemical found in poultry meats. Here are the facts we will use:

  • There are 6 milligrams of malonaldehyde in one gram of turkey
  • Let's assume that a slice of turkey weighs 5 grams
  • The rodent toxic dose of malonaldehyde is 632 milligrams per kilogram of body weight per day, fed everyday of its life. (Rats are fed this dose everyday of their lives)

A student who weighs 155 pounds is roughly 70 kilograms because there are approximately 2.2 pounds per kilogram

4. Now let's do the math to see how much turkey the student would need to consume to get a toxic dose of malonaldehyde.

To get an equivalent toxic dose, as that given to the rodents, the student would need to eat 1474 (5-gram) slices of turkey every day for several years.

5. Discuss with the students the example of arsenic. In very small doses, arsenic may be essential to humans, and because arsenic is so prevalent in nature, on an average, there is about 10-20 milligrams of arsenic in the body. Higher levels are toxic and do lead to problems.

Arsenic trioxide, an inorganic arsenic, is used industrially and is found as a food and water contaminant. It is the strongest poison of the arsenics. The toxic dose is probably 200 milligrams. Arsenic is often found in deep wells and the World Health Organization recommends a limit of the amount of arsenic in water to be 10ppb.There is a village in Bangladesh where 72 out of 73 tube wells are contaminated with arsenic and 21 of these wells contain arsenic at a concentration higher than 1000 ppb, the highest being 4000 ppb.

These are levels of exposure that are deadly, and yet the problem has not been resolved. In contrast, prior to 1999 the National Resources Defense Council tested more than 1000 bottles of 103 different brands of bottled water. Many bottles were found to exceed the WHO requirement and the California requirement of 5ppb, and the companies were taken to court and forced to meet the requirements.

6. For a graphic representation of this, go to the following web site:

Using the information and the example above students should start work on the following problem:
A slice of bread contains 167 micrograms of furfural, a rat carcinogenic substance. The rat carcinogenic dose is 197 milligrams/kilogram of body weight per day, which is the same as 197,000 micrograms/kg/day. How many slices per day would a 155 pound student need to eat to get the equivalent of the rodent carcinogenic dose?

I) The Crystal Geyser Bottling Company had a product called Napa Valley Sparkling Mineral Water, which had an arsenic level of 35.2 ppb. How many liters of this product would you need to drink to reach 200 milligrams of arsenic? (hint: 1ml of water weighs 1 gram)

II) The LD_50 (lethal dose reported to kill 50% of the people given the dose) for caffeine is 150 milligrams/kg of body weight. This means for people weighing 50 kg, the LD_50 is 7.5 grams and for people weighing 80 kg, he LD_50 is 12 grams. If there are 71milligrams in a can of Jolt, how many cans would a 77kg (155 pound) person have to drink to reach the LD_50?

Embedded Assessment

Informal discussion about the possible harm from being exposed to a chemical will help you assess their understanding of the concept of the amount of exposure being the key to the harmful effects. Informal observations as the students calculate the toxic doses in the homework will also help you assess how the students use unit analysis to reach their solutions.


PULSE is a project of the Community Outreach and Education Program of the Southwest Environmental Health Sciences Center and is funded by:

NIH/NCRR award #16260-01A1
The Community Outreach and Education Program is part of the Southwest Environmental Health Sciences Center: an NIEHS Award


Supported by NIEHS grant # ES06694

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Last update: March 7, 2007
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