Abstract
What should you NOT forget when going outside on a sunny day? To put sunscreen on! Sunscreen is important because it protects your skin from sunburn. But there are so many sunscreens to choose from: different methods of application (sprays and lotions), different Sun Protection Factors (SPFs), and different ingredients. Which one is best? In this science project, you will test the effectiveness of different sunscreens and find out how water-soluble they are.Summary
Objective
To measure the effectiveness of different sunscreens for blocking ultraviolet (UV) light and to assess their water solubilities.
Introduction
Do you love being in the sun? Studies have shown that sunlight can have positive effects on our health. For example, our body needs the sun to produce vitamin D. Too much sunlight, however, can be harmful—especially to our skin. In the short term, it can cause bad sunburn, and in the long term, it can lead to skin cancer.
Not all wavelengths of sunlight are equally harmful, though. When it comes to sunburn and skin damage, ultraviolet (UV) light is the most dangerous component. But how do we know how much sunlight or UV radiation is too much for our skin?
Scientists have developed a tool called the UV Index to help us understand the potential danger of sunlight. The UV index is a number ranging from 0 to 12. The UV Index Scale, shown in Figure 1, tells us what effects to expect from being in the sun at different intensities. A lower UV Index means less UV light exposure.
Figure 1. The UV Index Scale helps to determine expected sun exposure risks.
Sun protection is recommended even at a low UV Index. You can protect your skin using sunscreen. Sunscreen prevents your skin from getting damaged by UV radiation. There are two different types of sunscreen, each with a different way of protecting your skin. The two different types are physical sunscreen and chemical sunscreen.
If you look at the ingredients of physical sunscreen, you will find that they include minerals like zinc oxide or titanium dioxide. This is why physical sunscreens are also called "mineral sunscreens." When you apply a physical sunscreen, the titanium dioxide or zinc oxide particles form a layer on top of the skin. Just like a mirror, this layer reflects the UV radiation away before it reaches the skin, as shown on the left side of Figure 2.
On the left is a schematic drawing of a cross-section of skin with the epidermis on the top, the dermis in the middle, and the hypodermis at the bottom. A blue line on top of the skin represents the physical sunscreen. Red arrows representing sunrays hit the physical sunscreen layer, then reflect away from the skin. On the right is a schematic drawing of a cross-section of skin with the epidermis on the top, the dermis in the middle, and the hypodermis at the bottom. A green gradient along the skin depth represents chemical sunscreen that gets absorbed into the skin. Red lines representing sunlight fade as they penetrate the epidermis and dermis, because the sunlight is absorbed by the sunscreen chemicals inside the skin.
Figure 2. Physical sunscreen (left) reflects UV light, whereas chemical sunscreen (right) absorbs UV light and transforms it into harmless radiation.
Chemical sunscreen uses different ingredients. These include chemicals like avobenzone, homosalate, octocrylene, or octisalate. When chemical sunscreen is applied, these chemicals are absorbed into the skin. Once there, they transform the harmful UV radiation that penetrates the skin into non-damaging radiation or heat, as shown on the right side of Figure 2. It takes some time for the chemicals to get into the skin and become effective. For this reason, it is important to apply chemical sunscreens at least 15 minutes before going into the sun.
Sunscreens differ not only by their ingredients but also by their sun protection factor (SPF). SPF tells you how well sunscreen protects the skin against sunburn. However, the number does not tell you how long you can stay in the sun. Instead, it tells you how much it multiplies the time you can stay in the sun before you get a sunburn (when used as directed). For example, with an SPF 30 sunscreen, it would take 30 times as long to get a sunburn as if you were not wearing sunscreen. A higher SPF means more sun protection.
With so many different sunscreen choices, how do you know which sunscreen is the best? In this science project, you will put different sunscreens to the test.
The first test will look at the effectiveness of different sunscreens. In this test, you will investigate how well different sunscreens block harmful UV light. The results will tell you which sunscreen is best at protecting you from sunburn.
You might be curious about how you can test sunscreen effectiveness. You will use a special device called a UV meter. This meter uses a UV sensor to measure the UV Index. To find out how effective your sunscreen is, you will apply sunscreen to a clear plastic sheet and hold it in front of the UV sensor. If the sunscreen blocks the UV light, the UV meter readings should decrease. The more the UV readings decrease, the more effective the sunscreen is in blocking the UV light.
The second test will look at how the sunscreens interact with water. This is important for two reasons. The first reason is because to be effective, the sunscreen needs to stay on our bodies rather than be washed away. The more water-soluble a sunscreen is, the more likely it is washed off of you. The second reason has to do with how eco-friendly the sunscreen is. Some ingredients used to make some sunscreens can be harmful to animals and the environment. You can research more about these ingredients using this video as a starting place.
Designing products, like sunscreen, in a way that reduces their harm while maintaining their effectiveness is done through green chemistry. There are 12 principles that guide green chemistry. These include reducing waste, using and creating less harmful ingredients, and being more efficient with energy and natural resources. You can hear Paul Anastas, one of the founders of the green chemistry movement, explain green chemistry in his video.
In this science project, you will look at how well different types of sunscreen fit the green chemistry principles by evaluating their effectiveness at blocking harmful UV rays and how reef-safe they are. (Note: Here, we define reef safety as not dissolving easily in water, but you may find additional definitions if you read up on this topic.) Do you think physical and chemical sunscreens will perform the same or differently? Grab a UV meter, a few sunscreens, and some water, and find out!
Terms and Concepts
- Ultraviolet (UV) light
- UV radiation
- UV Index
- Physical sunscreen
- Chemical sunscreen
- Mineral
- Zinc oxide
- Titanium dioxide
- Reflect
- Avobenzone
- Homosalate
- Octocrylene
- Octisalate
- Sun Protection Factor (SPF)
- UV meter
- UV sensor
- Green chemistry
- Sustainable
Questions
- What is the UV Index, and what does it mean?
- How do sunscreens protect us from getting a sunburn?
- What is the Sun Protection Factor (SPF), and what does it mean?
- What are the 12 principles of green chemistry?
Bibliography
- Ducksters. (n.d.). The Science of Light Spectrum. Retrieved February 16, 2023.
- Environmental Protection Agency (EPA). (2004, May). A Guide to the UV Index. Retrieved February 16, 2023.
- Australian Academy of Science (n.d.). What does the SPF rating really mean?. Retrieved February 20, 2023.
- Richey, J. (2018, March). How does sunscreen work to protect your skin?. Colorescience. Retrieved February 16, 2023.
- Environmental Working Group (n.d.). The Trouble with Ingredients in Sunscreens>. Retrieved February 16, 2023.
For help creating graphs, try this website:
- National Center for Education Statistics, (n.d.). Create a Graph. Retrieved June 25, 2020.
Materials and Equipment
- UV meter
- Clear, resealable plastic bags, 1 gallon size, one for each sunscreen. Try to use bags that have nothing printed on them.
- Scissors
- Permanent marker
- A variety of sunscreen lotions
- To compare physical versus chemical sunscreens, you need to have one physical sunscreen (with titanium dioxide or zinc oxide) and one chemical sunscreen (with chemicals like avobenzone, homosalate, octocrylene, or octisalate). They should both have the same SPF, and ideally, they should be from the same brand. If you can't find both from the same brand, it is ok if they are from different brands.
- To compare sunscreens with different SPFs, you need the same type of sunscreen (physical or chemical) with different SPFs. To make a fair comparison, the sunscreens should all be from the same brand.
- Digital scale, 0.1 gram accuracy
- Mini cups with lids, 2 oz each, two for each sunscreen
- Disposable gloves, one for each sunscreen
- Paper towels
- Rubbing alcohol
- Water
- 1 Teaspoon measuring spoon
- Spoons for stirring, one for each sunscreen
- Pen or pencil
- Lab notebook
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Experimental Procedure
Testing Sunscreen Effectiveness
- Prepare your different sunscreen samples:
- Cut the top part (with the zipper) off the resealable plastic bag. Then cut along the folds on the side and bottom of the bag, so you have two individual plastic sheets, as shown in Figure 3. Only use sheets that do not have anything printed on them. You want the sheet to be clear.
Figure 3. Cut plastic bag with the top zipper part, the front and the back of the bag. The top sheet shown in this picture should not be used, as it has printed text on it. - Repeat step 1.a until you have a plastic sheet for each sunscreen you want to test.
- With a permanent marker, label each plastic sheet with the name of a sunscreen. Write it at the very top of the sheet.
- Prepare one more plastic sheet and label it "blank." This will be your control sheet without sunscreen.
- Pour a little bit of rubbing alcohol on a paper towel and clean each plastic sheet with rubbing alcohol on both sides. Let the rubbing alcohol dry.
- With a permanent marker, label one set of mini cups. Write a sunscreen name on each of the mini cups. You should have one mini cup for each sunscreen you want to test.
- Place one mini cup on the scale and zero the scale. Add 0.3 grams of the sunscreen you wrote on that cup.
- Repeat step 1.g for each sunscreen.
- Place each mini cup next to the plastic sheet labeled with the same sunscreen name.
- Put a disposable glove on one of your hands. Use this hand to apply ALL the sunscreen from one mini cup to the front of the corresponding plastic sheet. Make sure to spread the sunscreen evenly across the whole sheet.
- Remove your disposable glove and repeat step 1.j for each sunscreen you want to test.
- Let all the prepared plastic sheets dry for at least 15 minutes. Now your samples are ready to be measured.
- Cut the top part (with the zipper) off the resealable plastic bag. Then cut along the folds on the side and bottom of the bag, so you have two individual plastic sheets, as shown in Figure 3. Only use sheets that do not have anything printed on them. You want the sheet to be clear.
- Measure your prepared sunscreen samples.
- Make a data table like Table 1 to record your measurements.
Table 1. Data table to record your UV Index measurements.Sample Type of sunscreen (physical or chemical) Sunscreen protection factor (SPF) UV Index Measurement 1 UV Index Measurement 2 UV Index Measurement 3 UV Index Average Just the Sun Blank sheet Sunscreen 1 Sunscreen 2 ... ... - Familiarize yourself with the UV meter by reading the directions that came with it. Locate the UV sensor and do some test measurements. Do your measurements on a sunny day with a clear sky, preferably between 10 a.m. and 2 p.m. when the sun is still high in the sky. Note: All your measurements need to be taken at the same background UV Index. If your UV Index changes during measurements due to clouds covering the sun, you have to start all your measurements again. Pick a time for your measurements where you can be sure that your sunlight intensity will not change over the course of at least 30 minutes.
- Find a place to put your UV meter upright with the UV sensor facing the sun. For your measurements, ideally, the UV meter should read at least 6 when facing the sun. Record the UV Index measured with just the sun in your data table.
- Next, measure your blank sheet. Hold the plastic sheet right in front of the UV sensor, as shown in Figure 4. Try to minimize the air gap between the sensor and the plastic sheet and make sure that only the plastic sheet is between the sun and the detector. The UV meter reading should drop by about 1.0 with the blank plastic sheet. If it drops more than that, play with the positioning of the sheet in front of the UV sensor until you find the right location, and make sure you don't block the sun with your body or move the UV meter during the measurements. Also, do not press the Max or Hold buttons during the measurements. Once the UV Index readings are stable, record the UV Index for the blank sheet in your data table. This will be your control reading and will be a baseline to compare the other readings to and see if they increase or decrease.
Figure 4. Hold the plastic sheet with the sunscreen in front of the UV detector. - Continue measuring all your samples the same way. When you hold the sample sheets in front of the UV meter, the side with the sunscreen should face the sun. It is important not to get any sunscreen on the UV sensor itself. Record the measured UV Index for each sunscreen in your data table. Note: To make sure your measurement is representative of your sunscreen effectiveness, move the plastic sheet with the sunscreen to the left and right and up and down in front of the UV sensor. Try to keep the same distance to the sensor while moving the sheet. If you spread the sunscreen equally across the sheet, the measured UV Index should not change. However, if you missed a spot on the sheet with your sunscreen, the UV Index will be different at that location. If the measurements change more than 0.5 across the sheet, prepare a new plastic sheet for that sunscreen and try to spread the sunscreen more evenly.
- Between measurements, make sure the background reading with just the sun hasn't changed. This might be the case if the sunlight got more intense or a cloud got in front of the sun. All the measurements need to be done with the same background UV Index. If the background readings have changed, start measuring all your samples again. Record each of your measurements in your data table.
- Do two more rounds of measurements with all samples and record your results in your data table. Repeating measurements is good practice in science and helps to ensure that you can trust your results. All the measurements need to be done with the same background UV Index as the first round. If the background UV Index changes, you need to start measuring all your samples again.
- Make a data table like Table 1 to record your measurements.
Testing Sunscreen Water Solubility (Eco-Friendliness)
- With a permanent marker, label another set of mini cups. Write a sunscreen name on each of the mini cups. You should have one mini cup for each sunscreen you want to test.
- Place a mini cup on the scale and zero the scale. Add 0.3 grams of the sunscreen you wrote on that cup.
- Repeat step 2 for each sunscreen.
- Add 2 teaspoons of room-temperature water to each labeled mini cup. Swirl the cup or use a new spoon for each cup to mix the solution. Observe what happens in each cup and record your observations in a data table like Table 2. The sunscreen is water soluble when the water becomes milky white when swirling or mixing the solution. Some clumps of sunscreen might still be visible, but most of it dissolves in the water and gives it the milky white color. The sunscreen is not water soluble if the water stays clear when swirling or mixing the solution and clumps of solid sunscreen are still visible in the clear water.
Table 2. Data table to record your water-solubility data.Sunscreen name Type of sunscreen
(physical or chemical)Is the sunscreen water soluble?
(yes or no)Comments and observations
Analyzing Your Data
- Start with your sunscreen effectiveness data.
- Use your data in Table 1 to calculate the average UV Index for each sunscreen. For each row, add the three measured values together and divide the result by three. Record the results in your data table.
- Graph your data. If you need help graphing your data, use the Create a Graph resource from the National Center for Education Statistics. Make a vertical bar graph. Put the sunscreen names on the horizontal x-axis and the average UV Index measured for each sample on the vertical y-axis.
- Compare the average UV Indexes for the different sunscreens. Remember, the lower the measured UV Index for a sunscreen is, the more it was able to block the UV light from the UV sensor and the more effective it is. Note: Depending on what sunscreens you chose for your test, you may be able to use your data to compare sunscreens with the same SPF and different types (physical versus chemical), or you may be able to compare sunscreens of the same type and brand with different SPFs.
- Which sunscreen is most effective at blocking UV light? Did you expect this result?
- How do sunscreens with different SPFs compare? Try to keep this comparison within the same brand.
- How do chemical and physical sunscreens with the same SPF compare?
- Review your water solubility data in Table 2. Use your data to answer the following questions.
- How do your water solubility results differ between sunscreens?
- Do you see any trends in your data? For example, do different sun protection factors make a difference? What about the water solubility of chemical sunscreens versus physical sunscreens?
- What do your water solubility results tell you about the eco-friendliness or reef safety of the different types of sunscreens?
Ask an Expert
Variations
- In this science project, you compared different sunscreen lotions, but there are also sunscreen sprays. Repeat the experiment with different sunscreen sprays. You can spray the sunscreen right into the mini cup. It will liquify there, and you can collect the liquid until you have the desired amount of sunscreen in the cup. How do sprays compare to lotions?
- This project focused on comparing chemical versus physical sunscreens and sunscreens with different SPFs. You can use the same experimental procedure to compare sunscreens from different brands. How does an SPF 30 sunscreen from one brand compare to an SPF 30 from another brand?
- The water solubility test you did in this science project told you whether the sunscreen was water-soluble or not. If you are curious about finding out how much of the sunscreen ends up in the water, try doing the Science Buddies project How Eco-Friendly is Your Sunscreen? This project will show you how to measure the amount of UV-absorbing sunscreen chemicals in a water sample.
- The UV Index tells you how risky it is for your skin to stay in the sun. Do you think your sunburn risk goes up or down on a cloudy day versus a sunny day? Find out by comparing the UV Index on cloudy and sunny days. Based on the UV Index scale, do you think you still need sunscreen on cloudy days?
- The position of the sun in the sky changes throughout the day and peaks at solar noon. Does this have an effect on the amount of sunlight or UV radiation reaching your skin? Use the UV meter to compare the UV Index at different times of the day (without cloud cover). Make sure to point the UV meter directly at the sun for each measurement. When is it most dangerous for your skin to be outside in the sun?
- You have learned in the Introduction section that some active ingredients in sunscreens can be toxic to marine life and even humans. Can you design an experiment to investigate the toxicity of sunscreen chemicals on water organisms? The Science Buddies project Looking Downstream: Could Nanosilver in Consumer Products Affect Pond Life? might give you an idea of how to tackle this question.
- This project focused on a small subset of the green chemistry principles, such as pollution prevention and using less hazardous and safer chemicals. How do different sunscreens compare when considering additional sustainability factors? Review the 12 principles of green chemistry and do an analysis of different sunscreens based on other sustainability criteria.
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