Summary

In this lab, students will learn about how the sun’s ultraviolet (UV) radiation affects their skin and how sunscreen protects it. They will compare how effective sunscreens of various SPF levels are at blocking UV radiation from the sun using a color-changing bead UV detector.

Grade Level

Middle School, High School

NGSS Alignment

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

• MS-PS3-5: Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object.
• 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.
• Science and Engineering Practices:
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

By the end of this lab, students should be able to

• Explain the health impacts of UV exposure and how sunscreen works to limit those effects.
• Compare how effective sunscreens of various SPF levels are at blocking UV radiation from the sun using a color-changing bead UV detector.

Chemistry Topics

• Radiation
• Electromagnetic Spectrum
• Energy

Time

Teacher Preparation: 20 minutes

Lesson: 60 minutes

Materials

• ChemMatters article “What’s in Sunscreens?” – print or digital access (see teacher notes)
• UV light detector beads (ex: Educational Innovations, other online purchase options)
• Multiple types of sunscreen, with various SPF ratings
• 1 box of clear, quart-sized sealable bags

Safety

• Do not look directly at the sun while using the UV detector as it may cause permanent eye damage, nor stay exposed to the sun's ray for a prolonged time period.
• Remind the students not to consume any of the material.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the experiment, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• Have students read the ChemMatters article “What’s in Sunscreens?” and complete the prelab questions prior to starting the investigation. Questions could be completed in class on the same day or the day before, or assigned as homework, depending on available time. The article is in the AACT ChemMatters archive and can be printed for students, or they could be given a student pass to access it digitally.
• If you would prefer not to assign the ChemMatters article referenced in the student handout, you could instead have students research the prelab questions on their own. You may want to direct them toward reliable resources or discuss how to determine if a source is reliable. Some resources with good background information on sunscreen, skin safety, and UV protection include:
• Sunscreen & Your Skin, Skin Cancer Foundation
• Sun Protection, American Academy of Dermatology
• Sunscreen: How to Help Protect Your Skin from the Sun, U.S. Food and Drug Administration
• Sun Safety Facts, Center for Disease Control
• For best results, plan to conduct this experiment on a bright sunny day.
• For consistency and ease of comparison, it is best to have all students use the same color beads for their UV detectors. Single color packs (as well as multicolor packs) are available from various suppliers, such as Educational Innovations.
• The instructions in the student activity have students describe the color of the beads qualitatively (ex: light, medium, or dark blue) and create a bar graph. If desired, students could instead compare their beads to a reference that assigns numerical values to various shades of color as the bead develops brighter color with more UV exposure. They could then create a scatterplot to look for trends. Sample reference for blue beads:

• During preparation, consider putting the sunscreen into smaller, labeled containers for each group, rather than having to distribute all of it at the time of experiment. For best comparison, use the same brand/type of sunscreen with different SPFs, if possible. This could also lead to a good discussion about experimental design and making sure that the independent variable (in this case, SPF) is the only thing that changes to affect the dependent variable.
• Students might be surprised by their results in this experiment, as there may not be much of a difference between the various sunscreens they test. SPF numbers communicate a ratio of how much of the sun’s UV rays reach your skin compared to when no sunscreen is applied. See the table below for percentages of UV radiation blocked by different SPFs (and notice that no sunscreen blocks 100% of UV rays!):

• Even though 30 is twice as much as 15, SPF 30 products do not block twice as much UV radiation as SPF 15 products. A more accurate way to think about it is that SPF 30 products allow half as much radiation through compared to SPF 15, but since SPF 15 already blocks a lot of UV radiation the difference is less than one might expect at first glance.

• This is why organizations such as the American Academy of Dermatologists recommend using at least SPF 30 for casual sun exposure (though recommendations may differ for individuals with certain skin conditions or other risk factors, such as regularly spending long periods of time outside or being outside in high UV index conditions). Sunscreens above SPF 30 offer minimally more protection than SPF 30, which can be useful in certain situations but is not always necessary because the difference is so slight.
• Another complicating factor is that SPF is a measure of protection from UVB rays and does not necessarily account for UVA protection.
• Human error/real-life conditions also can make sunscreen less effective, such as improper application (most people don’t use enough) and failure to reapply, especially after swimming or sweating.
• Encourage your students to reach out to a medical professional if they want guidance on their own sunscreen use!
• If there is not time for each group to conduct 3 trials for each SPF, all groups could compile their data into a class data set. Alternatively, if more beads are given to each group, they could run multiple trials simultaneously.
• There are several optional extension research questions at the end of the student handout. Remove, edit, or add to them as desired to suit the needs and interest of your classes.
• Differentiation and extension ideas:
• For more advanced classes, rather than telling students they will be comparing sunscreens with different SPF levels as the resource is written, they could be allowed to choose an independent variable, such as:
• Brand (same SPF, different brands)
• Physical (aka mineral) vs. chemical sunscreens
• Quantity of sunscreen applied
• Length of time in the sun
• Exposure to water (simulating swimming or sweat)
• Spray can vs. lotion formulations
• Sunscreen vs. sun-protective clothing
• Expired vs. un-expired, etc.

Students would need to then identify what factors they would need to hold constant for reliable results and create an appropriate data table. They would probably need to bring in their own sunscreen samples (or the teacher would need to provide a larger variety to choose from).

• Compare sun exposure to UV lightbulb exposure to see if there is a difference.
• Repeat the experiment on a cloudy day and compare results to sunny day and/or UV lightbulb (or other types of light sources).
• Have students research one or more of the extension questions at the end of the student handout.
• Another AACT resource that has student evaluate sunscreen and is a good option for older/more advanced students is Screen Your Sunscreen!