SUMMARY:
In this activity, students test the effectiveness of various sunscreen products using UV-sensitive bacteria.

Estimated class time:
One 90-minute block for background and set-up, 5-10 minutes of each class period for a week (maybe longer), one class period for summaries.

OBJECTIVES:

Students will:

• Define ozone and UV radiation
• Recognize the dangers associated with sun tanning
• Compare the effectiveness of several different sunscreen products

MATERIALS NEEDED:

• Bacteria culture (such as Bacillus subtilis or Serratia marcescens available from Carolina Biological www2.carolina.com, Science Kit www.sciencekit.com, or others)
• Nutrient agar
• Petri dishes
• Inoculating loop or sterile swabs
• Saran wrap
• Tape
• Marker to label dishes
• UV light source
• Various sunscreen lotions (brought in by students, perhaps)
• Internet access

PROCEDURES:

1. Direct students to the Hole in the 'Zone section of this website. Students should be able to answer the following questions after exploring the site:
• What is the ozone layer?
• What is UV radiation?
• How does the ozone layer help protect us?
• What causes sunburns and suntans?
• What are the dangers of sun tanning?
  
  
2. Tell the students that you are going to compare how well their favorite sunscreen products work. Have students bring in their own different brands and SPF levels or provide a variety of your own. Explain that many bacteria strains are easily killed by exposure to UV light and that this test will help to determine how effectively the sunscreens block UV radiation.
   
   
3. It's probably best for the teacher to prepare sterile agar culture plates ahead of time, following directions on agar mix. Make enough for each group of students to have at least 3 dishes to test 2 or 3 sunscreens, or assign each group to a different sunscreen and compare results as a class. Teacher may decide to inoculate with bacteria or allow students to do so, depending on skill level of students and time available. Keep dishes sterile and sealed.
   
   
4. Have students propose a hypothesis about what they expect to happen.
   
   
5. Working in small groups (no more than 3-4 students), label the agar/bacteria dishes according to the type of sunscreen being used. Be sure not to open the dishes and expose the agar to the air or anything else.
   
   
6. Provide each group of students with Saran wrap cut into sheets large enough to cover the culture dishes. (The plastic or glass lids have a tendency to reflect some of the UV light; Saran wrap has been used successfully for this purpose.) Students should apply a small amount of sunscreen samples to the pieces of Saran wrap and then quickly and carefully replace the plastic (or glass) dish covers with the appropriate wrap. Be careful not to expose the agar to the air too long and do not touch it. Be sure to seal the wrap around the dish with tape.
   
   
7. Make sure that some control dishes are being set up as well. One control dish should be set up directly under the UV source and should have very little bacterial growth. Another control should be placed away from the source of UV light as a comparison of normal growth. Each group can create their own controls (culture dishes with Saran wrap but with no sunscreen applied) or the teacher can set up controls that the whole class can use.
   
   
8. Set the dishes directly under a UV light source.
   
   
9. Check the dishes daily for a week or two, or until you have clear results. Those sunscreens that block the most UV radiation should have more bacteria growth than less effective sunscreens. Students can record results by counting colonies of bacteria or by qualitative comparisons and drawing the results.
   
   
10. Each student or group should then write a summary of the experiment, including his or her recommendation for the best sunscreen and whether or not his or her hypothesis was supported.

ASSESSMENT:

• Participation in discussion and lab groups
• Written summary or lab report

EXTENSION ACTIVITIES:

• Create a pamphlet on the dangers of tanning or on sun-safety
• Research different types of skin cancers and treatments, and write a report or make a presentation

RELEVANT CURRICULUM STANDARDS:
This lesson correlates to the following National Science Education Standards, located online at bob.nap.edu/html/nses/html/6e.html#csc912, and National Health Education Standards located online at www.aahperd.org/aahe/pdf_files/standards.pdf.

National Science Education Standards

Content Standard B:
As a result of their activities in grades 5-8, all students should develop an understanding of transfer of energy:

• The sun is a major source of energy for changes on the earth's surface. The sun loses energy by emitting light. The sun's energy arrives as light with a range of wavelengths, consisting of visible light, infrared, and ultraviolet radiation.

Content Standard F:
As a result of activities in grades 5-8, all students should develop understanding of personal health:

• Natural environments may contain substances (for example, radon and lead) that are harmful to human beings. Maintaining environmental health involves establishing or monitoring quality standards related to use of soil, water, and air.
• Important personal and social decisions are made based on perceptions of benefits and risks.

National Health Standards

Health Education Standard 2:
Students will demonstrate the ability to access valid health information and health-promoting products and services by:

• analyzing the validity of health information, products, and services.
• comparing the costs and validity of health products

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