Sunscreen
From Chempedia
Sunscreen: Absorbing the Facts
Introduction
As summer approaches each year, the days become longer, the hot summer sun gets stronger, and it seems we’re constantly being reminded to apply sunscreen to protect our skin. Still, many people ignore these recommendations, and sunburns are certainly not a rare occurrence. Perhaps this is because many people simply do not understand why sunscreen is necessary, and how it protects them from the sun. To clarify these uncertainties, people need to understand how the sun’s rays penetrate the skin, the chemical mechanisms behind sunscreen, and what the SPF of a sunscreen indicates.
The Sun & UV Radiation
The sun emits what is known as electromagnetic radiation, which is divided into two major types, infrared and ultraviolet radiation. Most infrared radiation is absorbed by our ozone, but some of the ultraviolet radiation is not, so sunscreen was designed to protect our skin from harmful UV rays.3 UV radiation is composed of three major parts. The most abundant type of UV radiation on Earth is known as UV-A, and it is minimally absorbed by our protective ozone layer. UV-A has the largest wavelength of the three types falling between 320-400 nm on the electromagnetic spectrum. UV-A radiation penetrates the skin to the deepest layer, known as the dermis, and is the cause of the pre-mature aging look of the skin which is characterized by a leathery look and wrinkles. The second type of radiation is known as UV-B (280-320nm), and this type could be classified as the most serious. UV-B is partially filtered by the ozone, so it is less abundant then A, but it is still capable of causing damage to the skin of exposed individuals. UV-B radiation penetrates under every layer of the epidermis, to where cells are produced, disrupting the dividing of the cells and causing complications with DNA replication. This is why UV-B radiation is responsible for the majority of sunburns, and is a large contributor into the development of skin cancer. UV-C (190-280nm) is the final type of UV radiation. UV-C is the least abundant of the three types since most of it is filtered by the ozone, but this type has the highest energy or intensity. In fact, UV-C radiation is so strong that it is capable of killing bacteria and sterilizing hospital equipment.2 Time of day, distance from the equator, and other environmental factors can further affect the levels of UV radiation.4
Chemical Mechanisms of Sunscreen
Topical sunscreens work by combining the active ingredients of both organic and inorganic compounds.
Two of the most common inorganic ingredients are zinc oxide (ZnO), and titanium oxide (TiO2).8 These inorganic compounds protect the skin from painful burns by reflecting the sun’s harmful UV radiation.8 The smaller the particle size of ZnO and TiO2, the better the product performs. Conversely, the organic ingredients in sunscreen essentially absorb the UV radiation rather than reflecting it. Some of the most common organic ingredients include octyl methoxycinnamate, oxybenzone, octyl salicylate, and octylcrylene.8 These compounds take in high energy waves, and through reactions in their functional groups, convert them to lower forms of energy, such as heat energy, which can be harmlessly dissipated from the body.1 These ingredients are not capable of protecting for an indefinite length of time; therefore sunscreen must be reapplied often to remain effective.
Defining SPF
Sun protection factors, simply termed SPF, is the amount of additional time a person can stay in the sun before they may burn, as apposed to a person with no sunscreen protection.6 For example, an SPF of 20 indicates that a person could remain outside twenty times longer than a person without sunscreen and sustain the same burn level.6 Sunscreens are available with SPF levels up to 50, but an SPF of 15 to 30 is typically sufficient, as an SPF of 10 will block 90% of UVB rays.7 To calculate SPF, scientists utilize what is called the MED or, Minimally Erythermal Dose. The MED value of a person wearing sunscreen is defined as how quickly a person would burn with 1 fluid oz. of sunscreen on the whole body.5 A MED value is also assigned for a person not protected by sunscreen. The value for MED with sunscreen is then divided by the value for MED without to obtain the SPF.5
Summary
The sun emits two main types of radiation that have harmful effects on the human skin, UV-A and UV-B. Sunscreens work to protect our skin from these types of ultraviolet radiation by either reflecting or absorbing them, depending on the chemical compounds involved. Sunscreens can be classified by their sun protection factor (SPF), which indicates how long an individual can stay in the sun as compared to a person without protection. By exploring the science behind sunscreen, one can easily see why applying the appropriate SPF sunscreen is important to the health and safety of human skin, since it protects us from certain types of skin cancer, premature aging, and painful epidermal burns that are causes by the sun’s UV radiation.
Footnotes
1.Bowen, Debra . The Fragrance, Toiletry and Cosmetic Association. Food and Drug Administration. Maryland, Rockford. September. 1998.p1-9.1.
2.Clarke, Diana. Melanin: Aging of the skin and skin cancer. 17 June 05. <http://www.yourskinandsun.com/article61068.html> 20 Sep. 05.
3. Clarke, Diana. Sunscreen: The basics. 27 April 05. <http://www.yourskinandsun.com/sun101.html> 20 Sep 05.
4. Clarke, Diana. UV radiation detection and forecasting. 18 June 05. <http://www.yourskinandsun.com/article1072.html> 20 Sep. 05
5. The Alchemist Corner. <www.groups.msn.com/TheAlchemistsCorner/sunscreenchemistycanbeabsorbing.msnw> 20 Sep 05.
6. SPF Ratings. Coolibar, Inc. 2005. <http://www.coolibar.com/spf-ratings.html> 22 Sep. 05.
7. Sunscreen Chemistry. MediaWiki 27 Aug. 05 <http://scienceofspectroscopy.info/wiki/index.php?title=Sunscreen_Chemistry>22 Sep. 05.
8. Sunscreen. NationMaster. <http://www.nationmaster.com/encyclopedia/Sunscreen> 22 Sep. 05.
Researched and Written by: Morayo Allibalogun, Sabre Beam, Andrew Birchem, and Deborah Anderson
