The Dose-Response Curve: Determining Health Effects of Environmental Pollutants

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  • 0:07 Lethal Effects of Toxicants
  • 1:43 Non-Lethal Effects of…
  • 2:43 Threshold Doses
  • 3:43 Understanding the Data
  • 5:09 Exposure to Toxicants
  • 5:56 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this video lesson, you will learn about dose-response curves and how toxicologists measure the toxicity and effects of toxicants. You will also learn about how this data is applied to human health.

Lethal Effects of Toxicants

I'm guessing that as a kid you sometimes pushed your parents' buttons. You tried different things just to see what their response would be. You didn't know it, but you were actually carrying out a scientific experiment!

Believe it or not, toxicologists use this same method of testing to determine the toxicity of a substance; though, they tend to run experiments on lab animals, not your parents. Toxicologists will administer a dose, or amount of toxicant the animal receives, and then measure the response, which is the type and magnitude of negative effects the animal shows from the dose.

This process is repeated over and over, and then the percentage of animals exhibiting negative effects from the dose is plotted on a graph.

Since the dose is being specifically manipulated in the experiment, it is on the X-axis; therefore, response is on the Y-axis. The curve that results from these experiments is called a dose-response curve.

The dose-response curve is important because it provides scientists with a quick way to describe the toxicity of a substance. They do this by identifying the amount of toxicant it takes to kill half of the animals used in the study. Because this is a lethal dose for 50% of the population, it is called the LD50.

Low toxicity means a high LD50 because it takes a large dose to kill 50% of the population. Likewise, high toxicity means a low LD50 because it takes a much smaller amount of toxicant to kill 50% of the study population.

Non-Lethal Effects of Toxicants

In addition to the lethal effects of a toxicant, scientists may also want to identify other non-lethal effects of the substance. When something other than death is studied in a toxicant, it is the effective dose for 50% of the population, or the ED50. Effective dose responses may be things like: what dose of the toxicant causes 50% of the study animals to lose their hair, or what dose of the toxicant causes nervous system malfunction in 50% of the study population?

Though testing for different responses, the same is true for both lethal-dose tests and effective-dose tests: results from low doses (low LD50 s or ED50 s) indicate a high toxicity because a lower dose is needed to elicit a response. When higher doses are needed, this indicates a low toxicity of the substance because it takes more to get the desired result.

Threshold Doses

A threshold is like a dividing line. Think of the threshold of your house. When you are on one side of the threshold, you are outside. Once you step over the threshold into your house, you are now inside. It separates one situation from the other, and it's pretty black and white.

Toxicologists sometimes find that a toxicant has a threshold dose, the minimum amount needed for a response to occur. This happens in your body. Your organs can excrete some toxicants at low doses but after a certain level become overwhelmed and can no longer do it. Likewise, if your cells can only repair DNA to a certain point and a toxicant damages DNA beyond this point, this would be the threshold dose.

With a threshold dose, there is no response until a certain threshold has been reached. At this point, there is a sharp change in the effect. You can see this on the graph below because the response is '0' and runs along the X-axis as such until it hits the threshold dose and then skyrockets.

After the threshold dose, there is a sharp increase.
graph showing threshold dose

Understanding the Data

Toxicologists give lab animals much higher doses (relative to their body size) than we would receive in nature. This allows for two important things: 1) the response is great enough to be measured; and 2) the differences between low and high doses can be clearly seen.

For any given toxicant, multiple dose-response experiments are performed in order to generate an accurate dose-response curve. Scientists then extrapolate the data to estimate what effects these toxicants have on humans. However, because these are not actual tested results, they are not entirely precise estimates. Therefore, maximum allowable levels of toxicants are set far below the minimum toxicity levels generated from these studies to account for any error.

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