•According to the Environmental Protection Agency (EPA), the maximum level of: •Lead contamination in water cannot exceed 0 ppb (parts per billion);•Mercury contamination in water cannot exceed 2 ppb;•Chromium contamination in water cannot exceed 100 ppb.

Think-Pair-Share

•Why is the level of contamination set by the EPA different from pollutant to pollutant?•How does the EPA (or other agencies) come up with these values?

Essential Questions

•How do scientists determine and evaluate the toxicity (dangers) of a toxin/chemical?

•How do scientists use research and data to inform and set safety guidelines for wildlife and humans?

Interpreting Toxicity Values: LD50 & LC50

Objectives•Students will be able:• Explain the concept of LD50 & LC50 and describe

how it is used in risk assessment.•Describe threshold toxicity and identify threshold

toxicity on a dose-response curve.• Interpret dose-response curves to determine the

threshold and LD50 & LC50.

•Dose: The amount of a substance an organism has inhaled (breathed), ingested (eaten), or absorbed through the skin.

LD50 & LC50

•LD50: Lethal dose 50•Amount of a substance required to kill 50% of the test population.

•LC50: Lethal concentration 50•Amount of a substance in air (gas, vapor, mist, fume) or dissolved in water required to kill 50% of the test population.

Rules of Thumb about LD/LC501. Results are applicable to only the test population

a. Example: The data indicate a dosage of xx.xxx kills 50% of this test population of rats.

2. The lower the amount (dose), the more lethal the substance

a. Example: an LD50 of 10 mg/kg is more toxic than an LD50 of 100 mg/kga. An LD50 of 10 mg/kg is 10x more toxic than an LD50 of 100

mg/kg

Checking for Understanding

1. The LD50 of a substance indicatesa. The dosage that kills 50% of the population.b. The dosage that kills 50% of the test

population.c. The dosage that kills 50% of a species.d. The dosage that kills 50% of humans.e. The dosage that kills 50% of anyone.

Checking for Understanding

•True or false: The higher the dose (i.e., LD50), the more toxic the substance.

Dose-Response Curve

•Illustrates the response of a population to a dose of a chemical/toxin•Shows a relationship (correlation) between the toxin and its effects on the test population

Threshold of Toxicity

•The dose (level) below which no toxic (lethal) effects are observed and/or above which the toxic (lethal) effects are apparent.•At what dose is NO test subject dead?

• Slope = 0 (on the dose-response curve)

Using a Dose-Response Curve to determine LD/LC50 and threshold of toxicity

Threshold of toxicity

LD50

0.01 0.1 1 10 100