| Photo by John Pickle
Perhaps you know someone who likes to lay for hours in the sunlight. Many light-skinned people want to get a “tan” during the summer. But exposure to the ultraviolet energy from the Sun can cause burning of the skin and DNA damage associated with skin cancer, in addition to the increased production of melanin in the skin known as tanning.
There are numerous products—creams, lotions, or gels—available which claim to offer protection from the Sun’s harmful ultraviolet radiation. They usually list a Sun protection factor, or SPF. How well do sunscreens really work? Are the SPF ratings accurate? Would a regular lotion that makes no SPF claim offer any protection?
Rather than using sunscreen, you may use clothing to protect yourself from the Sun. People who tan may often be seen with tan lines that outline where clothing was worn while in the Sun, so clothing must stop UV. But how much protection do they provide compared to sunscreens?
The purpose of this experiment is to determine how effective sunscreens and clothes are at filtering out ultraviolet energy. To do this you will expose sunprint paper to sunlight. The sunprint paper is sensitive to ultraviolet energy and changes color when exposed to sunlight.
You can test a sample of sunscreen by smearing it onto a strip of clear acetate and then taping the acetate over the sunprint paper. Clothing can be tested by placing a piece of the cloth on the sunprint paper. When the sun-sensitive paper is exposed to sunlight for a few minutes, you’ll find out how well you are being protected from the Sun’s UV.
- sunprint paper
- various sunscreens (different brands and SPFs)
- swatches of different cloth (jeans, cotton T-shirt, sweatshirts)
- acetate sheet for overhead projectors
- cellophane or plastic tape
- glass stirring rod
- manila folder
- water bowl
- paper towels
Strategies for Investigation
1. Plan your experiment. Look at the collection of sunscreens and cloth swatches, and decide on four or five for your experiment. If you select sunscreens to test, design your experiment to test a single variable—such as SPF, or brand of sunscreen, or thickness of the sunscreen layer. If you choose to test clothing, you may want to test the effect of color, thickness, fabric, weight, or wetness (some people use T-shirts while swimming). Write a hypothesis, stating how you think the variable you selected will affect UV energy from the Sun. Decide which variables should be controlled (kept constant) in your experiment.
2. Prepare your samples. Smear a layer of each kind of sunscreen onto a separate strip of acetate. Here is a suggestion for smearing uniform layers of sunscreen:
a. Cut a strip of acetate, about 2 cm by 5 cm.
b. Tape the acetate to a table on three sides.
c. Squeeze a bead of sunscreen on an end of the strip.
d. Use a glass rod to smear the sunscreen down the strip, using a single, smooth motion. The sunscreen thickness should be the same as the thickness of the tape.
If you are working with cloth swatches cut the pieces so they all fit onto one sheet of sunprint paper. If you are testing the effect of water on the clothing’s protection, place a piece of acetate between the wet cloth and the sunprint paper. Wet swatches will lie on the acetate while dry swatches must be taped to the acetate.
3. Place the acetate with the samples on the sunprint paper. When the samples are ready, get a piece of sunprint paper. Immediately put it into a folder, and work on it in subdued light, so it is not exposed to ultraviolet energy before the experiment is ready. Tape the sunscreen samples or dry cloth swatches to the sunprint paper so they do not overlap. Allow a part of the sunprint paper to be covered by clear acetate alone to act as a “control.” If you have too many samples, get an additional sheet of sunprint paper and put it into the same folder, so all samples are exposed the same length of time. Label the sunprint paper with the brand and SPF of each sunscreen sample or cloth type, color, and thickness.
4. Expose the sunprint paper. Take the folder outdoors, open it, and place the sunprint paper in the Sun. Expose it for 1-2 minutes if the sunlight is bright, and up to 5-6 minutes if the sky is a bit overcast. Weight the folder around the edges with rocks if it is windy.
5. Develop the sunprint paper. Back in the room, remove the acetate strips from the sunprint paper, and take the paper out of the folder. (Be sure that it is labeled correctly before removing the samples.) Rinse the exposed paper in a bowl of water for at least one minute to develop and fix the color change, then remove it and lay it on a table to dry.
Results and Conclusions
1. Tape the dry sunprint paper to a larger sheet of paper, and describe the results.
2. Was your hypothesis supported or contradicted by the results? Try to give an explanation for why your hypothesis was supported or contradicted.
3. Can you advise people about Sun protection based on this experiment? For example, if you tested different brands, can you advise people about what brand is best? If you tested SPF, how would you advise someone about what SPF provides significant protection from the Sun? If you tested the effect of different thicknesses of sunscreen, what advice would you give someone about how to put on sunscreen so it offers protection? If you tested clothing, would you recommend a fair-skin person to use that clothing if outdoors all day?
4. What additional experiments do you think should be done to prepare a full report about the effectiveness of different types of Sun protection?