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
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:
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
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
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.
For a graphic representation of this, go to the following
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?
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
1ml of water weighs 1 gram)
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,
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
how many cans would a 77kg (155 pound) person have
to drink to reach the LD_50?