It’s the end of summer and if you’re like my family, we are spending Labor Day weekend out in the sun – swimming, boating, and playing bocce and cornhole. Even though the days are getting shorter and the sun’s rays are less direct, the danger of sunburn is still painfully real. So you pull out the sunscreen and lather up to keep those nasty ultraviolet (or UV) light rays from damaging your skin. In fact, continued sun exposure without protection can even lead to cancer. Not wanting to take any risk you reach for the highest SPF sunscreen you can find.
Is there really a difference between 30 SPF and 80 SPF? What does SPF mean anyway?
The Sun Protection Factor (SPF) rating is a measure of the fraction of sunburn-producing UV rays that reach the skin. For example, “SPF 15” means that 1/15th of the burning radiation will reach your skin; assuming sunscreen is applied evenly at 2 milligrams per square centimeter (mg/cm2) – a pretty thick covering. You can determine the effectiveness of a sunscreen by multiplying the SPF factor by the length of time it takes for you to burn without sunscreen. So if you sunburn in 10 minutes without sunscreen, SPF 15 sunscreen will protect you for about 150 minutes.
But what if your sunscreen is from last year (or even the year before)? Does it still work?
The dermatologist will tell you to always buy new sunscreen each summer but there is a fairly simple experiment you can do to test the viability of your sunscreen. You can compare different brands with similar SPF ratings or to see how last year’s sunscreen compares to a fresh bottle.
All you need is:
- Sunscreens to test
- Photo-reactive paper such as Sunprint
- Blank overhead transparency sheets
- 1/8 tsp. measuring spoon
- Permanent marker
- Ruler
- Scissors
- Cookie sheet
- Bowl of water
To test your sunscreen:
- Cut your overhead transparency paper into 10 cm squares. Use a sharpie to label each with the type or strength of sunscreen you will use.
- Apply the appropriate sunscreen to each transparency square. For lotion type sunscreen use 1/8 teaspoon of sunscreen for each square to get the recommended coverage. For sprays or sticks, apply as direction on the label. Make sure you keep one square as a control with no sunscreen on it (SPF 0).
- Quickly and carefully remove the photoreactive paper from its protective envelope and place the sheet(s) on the cookie sheet. Immediately cover the sheets with the transparencies sunscreen side up.
- Place the cookie sheet with the papers and transparencies out in the sunlight. Be sure no shadows cover the sheet. When the paper labeled SPF 0 is almost white (1-4 minutes depending on the intensity of the sunlight) bring the cookies sheet indoors.
- Quickly label each piece of paper with the appropriate SPF and then rinse them in the bowl of water for 1 minute.
- Lay the papers flat to dry.
Now you can compare the effectiveness of the different sunscreens. As more UV light gets through the sunscreen, the photoreactive paper will turn whiter.
What other materials can you test?
These ideas are from the SunPrint web site:
- Put varying amounts of one type of sunscreen on the acrylic to see if the thickness affects the amount of light that can get through.
- Compare other sunscreen products such as hand lotion, face makeup, lip balm, hair gel, or zinc oxide ointment
- Test the sun-screening capabilities of some greasy things you might find around the house: baby oil, shortening, cocoa butter, salad oil, dairy butter, etc.
- Try placing a pair of U.V. protective sunglasses on your Sunprint paper and compare the print left by clear lenses or inexpensive non-U.V. protective sunglasses.
