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Define lethal dose 50% (LD₅₀) and evaluate dose response curves.

Lethal Dose 50% (LD₅₀)

A “Poison Scale” that ranks various substances based on their toxicity.
Each level of toxicity is represented by different substances, accompanied by icons. A red arrow on the left side indicates the increase in toxicity as it points downwards. — **This graphic compares the LD₅₀ and toxicity rating of various substances. You can see that water is the least toxic and botulinum toxin, the same bacteria used in Botox, is the most toxic.**

Toxicology is the study of the harmful effects of chemicals on human and environmental health. A dose is how much of the chemical has been applied or released and a response is an acute or chronic effect of a chemical in an organism. The threshold level of chemical exposure is the level at or below which no ill effects are observed.

The LD₅₀ of a chemical is the dose at which 50% of the exposed population is killed by the dose. We usually express this by writing the amount (usually in mg) of the chemical per body weight (usually in kg). A poison is any substance with an LD₅₀ < 50 mg/kg.

There is an inverse relationship between a chemical’s toxicity and the LD₅₀ of the chemical in that the lower the LD₅₀ the more toxic the chemical. The more toxic the substance, the lower the LD₅₀.

If you look at the graphic to the right, sugar is essentially nontoxic. It would take over a quart of sugar to reach LD₅₀. It would take about 1 teaspoon of caffeine to reach LD₅₀ and only about 7 drops of sodium cyanide to reach LD₅₀.

Today, in part due to Rachel Carson and her book Silent Spring, chemicals, including pesticides, must list warnings, usage guidelines, and disposal guidelines as well as give guidance on the toxicity of the pesticide. The following “signal words” are used to indicate the level of toxicity:

CAUTION — Pesticides that are the least harmful to you.
WARNING — More toxic than those with a Caution label.
DANGER — Very poisonous or irritating and should be used with extreme care.

Essential Knowledge

Lethal dose of 50% (LD₅₀) is the dose of a chemical that is lethal to 50% of the population of a particular species.

Dose Response Curves

The survival rates of fish populations exposed to increasing doses of a substance. The LD50 (dose at which 50% of the population survives) is highlighted at 300 mg/kg exposure. — **A simplified dose-response assessment.**

Dose-response assessments are processes used to determine an acceptable level of exposure. For example, in the image to the right, the starting population has 8 individuals. The researcher continually increases the dose of the toxin. Looking at the results, the researcher can determine the LD₅₀, because that is where half of the individuals were killed. So, the LD₅₀ for this population and this toxin is 300 mg/kg.

In these types of experiments, the dose is the concentration of the chemical and the response is how many individuals died as a result of exposure.

Once we have this data from a dose-response assessment, we can create a dose-response curve, which provides us with a visual for how organisms react to different dosages of a chemical. In a dose-response curve, the threshold dose and LD₅₀ are shown. These graphs have a very typical shape as seen in the graph below:

The relationship between dose (mg/kg) and response (percent).
The X-axis represents dose and
The Y-axis represents response percentage. An orange dashed line intersects the curve at approximately 40 mg/kg, indicating the LD50. — ***This dose-response curve shows that for this particular chemical, a dose of 40 mg/kg will result in the death of 50% of those exposed.***

Let's do a couple of examples! Watch the video below to learn more about dose-response curves and LD_{50 ^.}

Essential Knowledge

A dose response curve describes the effect on an organism or mortality rate in a population based on the dose of a particular toxin or drug.

AP Exam Tip

You should be able to view a dose-response curve and pick out the LD₅₀.

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