Summary

In this lesson, students will learn more about UV radiation, its effect on skin, and the different ways in which sunscreen protects skin. After exploring this, students will develop an investigation to assess the claims made by different sun protection products. Students will compare the effectiveness of a variety of sunscreens and/or clothing that claim to offer sun protection. The lab will be wrapped up with a CER-style report.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS4-4: Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter.
• Scientific and Engineering Practices:
  • Asking Questions and Defining Problems
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations
  • Engaging in Argument from Evidence
  • Obtaining, Evaluating, and Communicating Information
• Cross-Curricular Concepts:
  • Cause and effect

Objectives

By the end of this lesson, students should be able to:

• Describe the meaning of SPF ratings as indicated on various sun protection products.
• Develop a testable investigative question related to the protection factor of sunscreens and/or clothing.
• Design and implement procedures to gather and analyze data from multiple trials to answer the investigative question.

Chemistry Topics

This lesson supports students’ understanding of:

• UV-light and energy
• Electromagnetic radiation
• Energy absorption
• Experimental design
• Accuracy and precision

Time

Teacher Preparation: 30 minutes
Lesson:

• “Sun Protection Chemistry” activity and “Screen your Sunscreen” prelab section: 45-60 minutes
• Practice applying reproducible amounts of sunscreen to a surface: 15 minutes
• Complete at least 2 trials of their experiment: 45 minutes
• Photograph results, begin data analysis: 10-15 minutes (at least) after the paper has fully dried
• Write the full report and answer analysis questions (could be homework): 30-40 minutes

Materials

• A variety of sun protection products (ex: sunscreen cream with different SPF ratings and active ingredients, sun protective clothing, regular clothing etc.)
  • If possible, include expired sunscreen creams to test the effectiveness of the products that are past the expiration date
• SunPrint sun-sensitive paper (available through Amazon) or similar sun-sensitive paper
• Microscope slides, one for each test (these can be washed and reused)
• Disposable transfer pipets or beral pipets
• Permanent marker
• Popsicle sticks
• Bucket of water for processing sun-sensitive paper
• Paper towels
• Cardboard, cafeteria trays, or similar for transporting sun-sensitive paper and microscope slides
• Optional: 0.01 g balance (if measuring sunscreen by mass rather than volume)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• The National Science Board estimates that there are over 16 million jobs in the Skilled Technical Workforce (STW) that don’t require a 4-year college degree, but require STEM knowledge such as the use of chemicals, application of arithmetic and algebra, and the knowledge of quality control and other techniques for manufacturing goods. This lesson was developed as part of a content writing team to support the ACS Strategic Initiative on Fostering a Skilled Technical Workforce, with the goal of increasing awareness of and appreciation for STW opportunities in the chemistry enterprise at the high school level.
• This lesson plan introduces students to the product testing work chemical technicians might conduct to evaluate consumer products such as sunscreen. The first part of the lesson, “Sun Protection Chemistry,” has students think more deeply about what SPF really means in sun protection products and why it matters. Then in the lab section, “Screen your Sunscreen,” students practice using some of the skills needed by chemical technicians, including developing a testable investigative question, designing experimental procedures to answer that question, analyzing experimental data, and preparing a report to summarize results.
• The lab portion of this lesson was developed using sun-sensitive paper (specifically, the brand SunPrint) to evaluate the effectiveness of different types of sun protection. Sun-sensitive paper utilizes the cyanotype process: it has been pre-treated with chemicals that are sensitive to ultraviolet (UV) light (ferric ammonium citrate and potassium ferricyanide). When these chemicals are exposed to UV light, a reaction occurs between them to form an insoluble, vibrant blue compound known as Prussian blue. The reaction stops and the changes become permanent when the paper is washed in water to remove the remaining light-sensitive chemicals. The final image continues to darken until the paper is fully dry. (For more details on the chemical reactions that are occurring, see “UV Catalysis, Cyanotype Photography, and Sunscreens” in the Journal of Chemical Education.)
• One sheet of SunPrint paper is enough for 3 experiments and multiple trials.
• Have a large bowl of water set up in a shaded place, close to the location of the experiments so students can promptly stop the chemical reaction after their designated length of time in the sun. Each brand of paper likely comes with its own specific instructions.
• The final color does not fully develop until the paper is completely dry, so there will need to be time available during the next day for students to get their final results. A suggested 3-day schedule is as follows:
  • Day 1: complete the “Sun Protection Chemistry” activity and the pre-lab questions on the “Screen your Sunscreen!” lab.
  • Day 2: finalize the experimental design, complete the experiment, and obtain data.
  • Day 3: evaluate and analyze data, complete the written report.
  • If you have block periods, Days 1-2 or Days 2-3 can be combined. If time is short, some of the work designated for Day 1 or Day 3 can be done outside of class time, as needed.
• For question 3 in the “Sun Protection Chemistry” activity, it might be helpful to offer students examples of testable questions they can investigate. Alternatively, you can brainstorm an example of a question (and the procedure that will enable students to test the question) with the whole class. This will help them prepare for the lab activity as well. Example questions include:
  • How does the protection from SPF30 rated clothing compare to that of SPF30 rated sunscreens?
  • How does the protection from regular (non-SPF rated) clothing compare to that of SPF30 sunscreen?
  • Does expired SPF50 sunscreen offer the same protection as not-expired SPF50 sunscreen (with the same active ingredient and from the same brand)?
  • Does chemical SPF30 sunscreen offer the same level of protection as physical SPF30 sunscreen?
• The lab itself can be completed in a 45-minute class period if students have a clear investigative question and a procedure. Applying 1 mL of sunscreen to microscope slides is somewhat messy. It might be a good idea to have students practice this for 15 minutes during the lesson before the actual lab. Have students measure 1 mL of sunscreen and squeeze it out on a piece of paper until they develop the ability do this effectively. Alternatively, students can weigh 1.0 g of sunscreen on slides. Either method will take some practice and allows for a meaningful conversation about experimental errors. Use popsicle sticks to spread the sunscreen evenly on slides.
• Note that, while glass (such as the microscope slides used in this lab) filters out most UVB rays, UVA rays still penetrate the glass, which is what is primarily responsible for the discoloration of the UV-sensitive paper in this lab. (Exposure to both UVA and UVB rays cause skin damage and increase skin cancer risk.)
• Since the intensity of sunlight can vary depending on a number of conditions beyond students’ control (cloud cover, angle, time of day, etc.), it makes sense to have students run at least two trials of each experiment simultaneously so experimental conditions are as similar as possible.

Ideas for differentiation

• If you’d like to focus more on the quantitative aspects of energy and wavelengths, you can add a question that has students calculate and compare the energy of UVA, UVB, and visible light. They can practice unit conversions (nm to m) and use to calculate energy. This will support students’ understanding of the fact that shorter wavelengths carry more energy (and can therefore do more damage).

• Additional depth can be added by having students research the amount of energy that is needed to break or damage bonds in DNA molecules. This energy is often listed in kJ/mol or eV. Students can convert this to Joule (1 eV = 1.6·10^-19 J) and then calculate the wavelength of light that can cause damage to DNA bonds. They’ll realize these wavelengths are in the UV region. This source can be used by more advanced students to learn more about the specific DNA damage (pyrimidine dimers) caused by UV light.
• Energies known to break or damage DNA bonds are 0-3eV (see this article for more details). Photons with a wavelength of 242 nm carry 3eV of energy. This corresponds to UVC light, most of which is absorbed by our atmosphere. This can be used to discuss broad-spectrum sunscreen.
• Typical C-C bond energies are ~350 kJ/mol, corresponding to wavelengths of 285 nm in the UVB region. (See this LibreText page for more on bond energies.)
• Sunscreens often state the active ingredients accompanied with a w/w percentage. You can use this to have your students calculate how much of the active ingredient is present in a container of their sunscreen. Another quantitative exploration is to compare sunscreens of the same brand and type (ex: physical, Alba Botanica, SPF15, 30, and 50) to see how the % of active ingredient correlates with the stated protection factor.

Ideas for extension

• There are health concerns with some of the ingredients in sunscreens. This activity can be extended to explore some of these as follows:
  • Have students practice reading Safety Data Sheets. Caution should be exercised – due to a lack of understanding, students can easily become overly concerned after reading SDS information. I have had my students use the oral rat LD₅₀ to calculate realistic dangerous exposures – this makes for meaningful and interesting learning about toxicity, dosage, etc. The Flinn SDS database lists several active sunscreen ingredients; another, more comprehensive one can be accessed through ChemicalSafety.com. The data table for pre-lab question 3 could then look like this:

• Besides health concerns for people, active ingredients in sunscreen can also damage biologically diverse and significant areas such as coral reefs. This source introduces students to coral reefs, their importance, and the threats they face. The website dedicates a section to reef-safe sunscreens and one to non-reef-safe sunscreens and how to tell them apart. Instead of a report, students can create a public service announcement/ awareness poster about their findings. NOAA is another valuable source of information on this topic, as is Chemical and Engineering News.