SITNFlash

Greetings SITN members!

Happy July to all! We hope that you are having a wonderful summer. This month’s article covers a topic that you probably already know a lot about: the importance of protecting your skin from too much sun exposure. “Fun in the sun” defines summertime for many of us, but too much sun can be damaging to our bodies, causing premature aging, skin damage, and of course, cancer. We hope that this article describing the science behind UV radiation, sunscreen, and tanning, will help you to feel a bit more informed and prepared next time you step outside this summer.

Enjoy!

It is commonly known that ultraviolet rays from the sun can lead to cancer, but did you know that in the United States, skin cancer affects more than 1.3 million people annually? In fact, it is the most common of all cancers, accounting for one out of every three new cancer diagnoses. Many of these skin cancers could be prevented by taking simple precautions, such as applying adequate sunblock. Although it doesn’t make you invincible, just a thin layer of sunblock can provide substantial protection from the sun's damaging rays. Clearly, sunblock is important, but how does it work? And why does ultraviolet exposure increase one's risk of skin cancer? Read on to find out!

Ultraviolet (UV) radiation is a type of electromagnetic radiation, just like visible light, but with a shorter wavelength. UV rays are classified based on wavelength into three categories: ultraviolet A (UV-A, 315-400 nanometers), ultraviolet B (UV-B, 280-315 nanometers), and ultraviolet C (UV-C, 100-280 nanometers). Most of the ultraviolet radiation from the sun that reaches the earth's surface are UV-A rays. UV-B rays are less abundant at the earth's surface, because a significant portion of UV-B rays is absorbed by the ozone layer. UV-B rays penetrate less deeply into the skin than do UV-A rays, but can be damaging nevertheless. UV-C radiation is extremely hazardous, but luckily it is completely absorbed by the stratospheric ozone layer and does not reach the surface of the earth.

Different wavelengths of UV radiation have different effects on the body. UV-A, plays an essential role in the formation of vitamin D by the skin. However, UV-A is also harmful because it can damage the connective tissue and thereby induce premature aging in the skin, as well as increase a person's risk for developing skin cancer and possibly cataracts. UV-B, causes damage at the molecular level to the fundamental building block of life— deoxyribonucleic acid (DNA). DNA readily absorbs UV-B radiation, and when it does, the DNA molecule changes shape. Specifically, UV-B radiation causes covalent bonds to form between adjacent parts of DNA molecules, producing DNA lesions called thymidine dimers. These changes to DNA often result in mutations and can ultimately lead to the development of cancer. While UV-B radiation damages DNA, causing the formation of moles and some types of skin cancer, it can be almost completely blocked by sunscreen. UV-A radiation, on the other hand, causes melanoma, a very dangerous type of cancer and is often not adequately blocked by sunscreens.

How do sunblocks work?

Most sunblocks work by absorbing, reflecting, or scattering the sun's rays. These products contain either a chemical block which absorbs UV light (such as oxybenzone) or a physical block which reflects UV light (such as titanium dioxide or zinc oxide). The chemical sunscreens are generally capable of absorbing high energy UV photons (particles of energy) and releasing the energy as lower energy rays. Since minimal chemical change occurs to the compounds themselves, the sunscreen can retain its potency. Physical sunblocks provide broad spectrum UV-A and UV-B protection and are gentle enough for everyday use. Physical sunblocks are especially good for individuals with sensitive skin. The majority of active ingredients used in sunblock work against UV-B rays, whereas only a few work against UV-A rays, so reading labels is key when choosing a sunblock formulation.

What is a sun tan?

When your skin is exposed to UV rays, a hormone called alpha-melanocyte stimulating hormone is produced. This hormone stimulates the development of a brown pigment called melanin. Melanin, which is produced by cells called melanocytes, helps to block UV penetration and prevent damage to the more vulnerable deeper skin tissues. The released melanin moves toward the outer layers of the skin and becomes visible as a tan. The tanning effect then protects the body from absorbing more solar radiation. So, while a tan provides some protection from UV radiation, by the time a tan appears, damage has already occurred.

What is SPF?

Sunblocks are assigned a sun protection factor (SPF) according to their measured effectiveness in offering protection from UV rays. To determine SPF, a clinical test is conducted which incorporates the full solar spectrum (90% UV-A and 10% UV-B energy). Protection from SPF 30 sunblock is not double that of SPF 15. Rather, SPF is a laboratory measure of the effectiveness of the sunscreen. It is determined by the inverse of the proportion of UV-B that penetrates the sunscreen. Although sunblock use helps minimize sun damage, it is important to realize that no sunblock completely blocks all UV radiation. The degree of protection from a particular sunblock depends on factors such as skin type, amount applied, and frequency of re-application. The activities in which one engages also matters. For example, although many sunblocks are advertised as “waterproof”, swimming may lead to loss of sunscreen from the skin – either from prolonged exposure to the water, or from towel-drying, which will physically rub off the sunblock. In addition, the time of day and the percentage of UV reflected or scattered by the environment (such as water) can all influence the effectiveness of sunblock.

Recommendations

Protection from sun exposure is important all year round. UV rays are present even on cloudy and hazy days and can reflect off surfaces such as water, cement, sand, and snow. UV rays can cause skin damage during any season or temperature. The hours between 10 a.m. and 4 p.m. during daylight savings time (9 a.m. - 3 p.m. during standard time) are the most hazardous for UV exposure in the continental United States. Also, check the sunblock's expiration date. Sunblock without an expiration date has a shelf life of no more than three years. Exposure to extreme temperatures can shorten the shelf life of sunblocks. Sunblocks that block both UV-A and UV-B rays and are rated SPF 15 or higher are generally recommended. All sunblocks should be applied 15-20 minutes before sun exposure in order to allow a protective film to develop, then re-applied after water contact and sweating. Some sunblocks can lose effectiveness after two hours, so reapply frequently. Your skin (and your DNA) will thank you!

-- Abigail Bracha, Harvard Medical School

For More Information:

Food and Drug Administration (PDF File): <http://www.fda.gov/ohrms/dockets/dailys/00/Sep00/090600/c000573_10_attachment_F.pdf>
Skin Cancer Foundation: <http://www.skincancer.org>
Wikipedia: Ultraviolet: <http://en.wikipedia.org/wiki/Ultraviolet>
Wikipedia: SPF: <http://en.wikipedia.org/wiki/Sun_protection_factor>

Here's the schedule for the 2006 lecture series! The series kicks off on September 21st with an introductory lecture entitiled "Making Sense of Science in the News," with a new lecture every week thereafter on a wide range of topics in the headlines, from mental health to global warming. Lastly, this year we will be continuing to offer or lectures on two night at two locations, so feel free to attend the seminar that is more convenient for you! Here is the full schedule:

Of course, this information may change as we get closer to the start of the series, so check http://sitn.hms.harvard.edu/ for the latest updates!