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Science Olympiad» Disease Detectives Event » National Event Exercises
Skin Cancer Module: Practice Exercises

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Skin Cancer Home
Module 6: Ultraviolet Radiation
Module 8: A Diversity of Skin Types
Skin Cancer Glossary

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What Is Ozone?
What Is Ozone Depletion?

Module 7: The Earth's Ozone Layer and the UV Index

Objective: Learn about the ozone layer and ozone depletion. Practice using and interpreting the UV Index.

What Is Ozone?

Ozone is a gas. Its molecules are made up of three atoms of oxygen. The term ozone comes from the Greek word meaning "smell." That's because ozone has a strong odor. Most of the ozone in Earth's atmosphere is in a layer about 6 to 30 miles above the Earth. This layer is called the stratosphere.

Ozone in the stratosphere absorbs much of the sun's harmful ultraviolet (UV) rays, especially UVB. The ozone keeps the rays from reaching the Earth. This is how the ozone protects us.

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What Is Ozone Depletion?

Measurements suggest that more UVB radiation is reaching the Earth in recent years. It is worse in some parts of the world than others. More UVB is reaching Earth because the ozone layer is being used up, or depleted. This is caused by man-made chemicals, such as chlorofluorocarbons. Ozone depletion is very dangerous for all of us.


A. Interpreting the UV Index for a Given City

Step 1: Go to the EPA Sunwise Web site at
Step 2: Find the "UV Index" link in the far left column.
Step 3: Under "UV Index," click on "Graphs of yearly values for each city."
Step 4: Find the major city closest to your home.
Step 5: Click on the most recent year.

Note that two indices are graphed (for most cities):

  • "Clear Sky UV Index" and
  • "UV Index Forecast"

Remember: Clouds keep some UV rays from reaching the Earth's surface. This explains differences between the two graphs.


Questions to consider after reviewing the Web site resource.

  1. For how many days did your city have a UV Index forecast in each of the following ranges:
    • Very High
    • High
    • Moderate
    • Low
  2. In what months did the highest and lowest readings occur?
  3. Is there is a big difference between the "Clear Sky UV Index" and the "UV Index Forecast?" If so, try to explain this difference.

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B. Using the UV Index and Skin Type to Decide How Much Sun Protection You Need

People with different skin types respond differently to a given level of UV radiation. (See module 8 for more on skin type.)

Step 1: Go back to the UV Index homepage at
Step 2: Click on "UV Index" again. Find "Information about UV radiation and how it varies".
Step 3: From here, click on "How to make use of the UV Index information."
Step 4: Click on "Figure 1" in the paragraph on this page.

Figure 1 has sun protection guidelines based on skin type and UV index. Use your skin type and your city to answer the following questions:


Questions to consider after reviewing the Web site resource.

  1. How many "minutes to skin damage" do you have during the month when your city has its highest UV Index?
  2. What sort of protection could you use to prevent damage to your skin during this month?
  3. What are some possible problems with using this public health message?
  4. What would be a more sun-safe, healthier message?

Do you want sun protection guidelines tailored to your location, skin type, and planned activities? If so, check out* Insert your information. The site will tell you how to protect yourself during your favorite outdoor activities. Read the descriptions to identify your skin type.

To find out more about the UV Index, go to:

To learn more about the ozone layer and ozone depletion, go to:

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Back to Skin Cancer Home

* Links to non-Federal organizations are provided solely as a service to our users. Links do not constitute an endorsement of any organization by CDC or the Federal Government, and none should be inferred. The CDC is not responsible for the content of the individual organization Web pages found at these links.

This page last reviewed April 24, 2007

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