Lesson 74 Lethal Doses: Toxicity

Lesson 74: Lethal Doses

Toxicity

How do chemists determine the toxic dose of a

substance?

Today, we are learning
about how scientists

determine lethal doses so

that we are can explain
the role that both dose
and body weight play in
determining the toxicity

of a substance.

You will know you have
been successful when you
are able to:

✓ calculate the toxic dose

for a variety of
substances, given the
LD50

✓ explain the role of

dosage in toxicity

✓ Explain why the LD50

are different for
childrens and adults

A measure of toxicity is used by chemists is

the lethal dose, LD50

The LD50 is defined as the

amount of an ingested
substance that kills 50%

of a test sample of

animals. It is expressed in
mg/kg, or milligrams of
substance per kilogram

of body weight.

The video to the right explains
how scientists calculate the LD50

for a substance.

It is easy to say that it is not

ethical to test toxic substances on
humans; however, there is debate
over whether it is ethical to test
on animals. Here are resources

from Stanford Medical School and
Lone Star College exploring both

sides of the issue.

We have referred to the

phrase “The dose makes the

poison” in this class many

times. It is important to
remember that harmful

substances in low doses may
not be harmful. Conversely,
“safe” substances, like sugar
and salt, can also be toxic in

large enough doses.

The lower the LD50, the more

toxic the substance. A low
LD50 means that only a small
amount needs to be ingested

to observe toxic effects.

There are a wide range of doses that can be

considered lethal--some grams, some milligrams,

some micrograms.

There are many toxic substances that can be
therapeutic (helpful) at doses below the LD50.

How can we use the LD50 to determine

safe dosages?

Toxicity depends on both the amount of substance

ingested AND the mass (body weight) of the

person ingesting substance.

First

Then

Finally

In the previous example, we used conversion factors

to solve the problem.

Conversion factors are ratios
(fractions) that relate two units
and are equal to 1; this means

that the numerator and

denominator have the same

value, just different units.

It is necessary to use conversion
factors to convert units. Simply

multiplying or dividing by a

number changes the value, but

NOT the units.

To review conversion factors, you
can read this article or watch this

video.

When deciding what form of a
conversion factor to use, you

should always pick the one where
the units you want to convert TO

are on TOP.

It is important to note that the toxic dose also

depends upon a person’s body mass

In the caffeine example, we saw that
the lethal dose of caffeine for a 120-lb
person was 8160 mg of caffeine. The

toxic dose for a 50-lb child is much less:

Differences in body mass are not the

only reason children (including

teenagers!) are more susceptible to a

substance’s toxic effects.

Younger bodies are also still growing and
developing, This means that their bodies

metabolize, or process, substances

differently than an adult. Additionally,

substances might have an effect on

organs and body systems that are still
developing, which can cause lifelong
problems. This is one of the reasons

that “legal” substances are only legal

for people aged 18 or over.

For more on the importance of age in

toxicity considerations, you can read this

article.

50

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3405

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22.7

In summary:

● Every substance is a potential toxin.
● Toxicity is relative to dosage: The toxicity (or

therapeutic effect) of a substance depends on the
dose in which it is received.

● The lethal dose (or therapeutic dose) of a substance

is often expressed as a ratio between a certain
mass of the substance and 1 kilogram of the body
mass of an organism exposed to the substance.

Ready to test your knowledge? Use this link to access

the Quizziz lesson and self-test!

Multiple Choice

The LD₅₀ of saccharin is 17.0 g/kg. What is the smallest mass of saccharin that would be lethal to a person who weighs

165 pounds? (2.2 lb = 1 kg)

• 1.28 kg

• 107 g

• 10.7 g

• 107 mg

Multiple Choice

How many grams of sodium chloride (LD₅₀ = 3.00 g/kg) would be lethal to a 154-pound person? (2.2 lb = 1 kg) (1 point)

• 5.80 g

• 12.8 g

• 210 g

• 1850 g

Multiple Choice

The LD₅₀ for aspirin is 200 mg/kg. How many 250-mg aspirin tablets would a 175-pound person need to consume to get a lethal dose? (1 kg = 2.2 lb)

• 50 tablets

• 64 tablets

• 241 tablets

• 280 tablets

Reorder

Reorder the following substances from MOST to LEAST toxic:

• arsenic oxide

  (LD₅₀ =15 mg/kg)

• soft drink

  (LD₅₀ = 127 mg/kg)

• lorchel mushroom (LD₅₀ = 320 mg/kg)

• vitamin A

  (LD₅₀ = 1510 mg/kg)

1

2

3

4

• 1
• 2
• 3
• 4

Reorder

Reorder the following substances from LEAST to MOST toxic: Which substance is the least toxic? (1 point) ✓ a. b. c. d.

• table salt

  (LD₅₀ = 3000 mg/kg)

• vitamin A

  (LD₅₀ = 1510 mg/kg)

• cola

  (LD₅₀ = 127 mg/kg)

• aspirin

  (LD₅₀ = 200 mg/kg)

1

2

3

4

• 1
• 2
• 3
• 4

Open Ended

Why is the amount of a substance that would be lethal different for adults and children?