Ultraviolet (UV) radiation is well known for contributing to skin aging and skin cancer risk. People use UV-blocking creams, lotions and sprays known as sunscreen to protect themselves when outdoors. These are rated with an SPF number (sun protection factor) that specifies a time limit for safe sun exposure. There is evidence that manufacturers overstate the SPF numbers for their products. You can test them yourself in a science fair experiment.
The two important classes of UV light relating to skin are UVA (400-320 nanometers) and UVB (320-290 nanometers). The atmosphere (especially the ozone layer) reflects or absorbs most UVB. Because absorption increases with path length through the atmosphere, UVB exposure changes with latitude, time of day and season, being strongest closer to the equator, in midday and in summer. UVA radiation, on the other hand, is fairly constant. Most glass and plastic blocks UVB, but less so UVA.
Sunscreen and SPF
The SPF rating is used to indicate safe exposure time. For example, SPF 30 allows you to stay in the sun twice as long as SPF 15 before burning. Roughly, the transmission percent T is related to the SPF by T = 100/SPF. Both UVA and UVB penetrate into the skin layer (dermis). UVA penetrates deeper, but is less powerful. UVA is responsible for aging skin and may contribute to certain kinds of skin cancer. UVB is responsible for suntan (and burn!) and more strongly associated with skin cancer. All sunscreens block UVB. The ones that also block UVA are called broad spectrum.
You want to show whether different sunscreens with the same SPF block the same amount of UV light. To do this you will smear sunscreen on a plastic “window” and use an inexpensive UV meter. These measure both the UVA and UVB spectrums.
You will establish two controls -- one with no sunscreen, and one with total blockage. The first control is necessary because the material you are using will probably block some UV itself and you want to establish a transmission baseline. The second control establishes that the meter gives a negative reading when not exposed. Use a thick piece of cardboard or other opaque material for the second control.
Polyethylene has good transmission in the UV range. Use sheets as thin as possible that will still lay flat. You can obtain polyethylene sheets from a supplier, in the form of clear sheet protectors, or you can recycle type 1 plastic from food containers or other packaging.
Conducting the Tests
To create the samples, use a 1-inch foam brush to smear an even 6-inch long stripe of each sunscreen you want to test on the plastic sheet. Using a permanent marker, label the stripe with the product name. Place a sample strip over the meter’s lens as closely as possible. Use the edge of a stripe so you can still read the screen. Take multiple readings all along the stripe and average the results. This will constitute a reading for that sample. Label the sample with the result. Remember to do the two controls.
Interpreting the Results
Your meter will read the UV irradiance, a measure of how bright the UV light is. The clear control (no sunscreen) represents 100% transmission. So, to obtain a relative transmittance, divide your average reading for each sample by the average reading for the clear control. Use the formula to assess whether your measured SPF value matches the manufacturer’s claim.
- Skin Cancer Foundation: Understanding UVA and UVB
- Skin Cancer Foundation: Shining Light on Ultraviolet Radiation
- Consumer Reports: What to Know about Sunscreen Before Buying It
- Alberta Health Services: Sunscreen Protection is about Time, not Strength
- Sci ELO Brazil: Sun Protection Factor: Meaning and Controversies
- University of Southern California, San Francisco School of Medicine: UV Radiation
- Photo Credit travnikovstudio/iStock/Getty Images
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