Ultraviolet radiation (UV) accounts for just a small proportion of the sun’s radiation that makes its way through the atmosphere and reaches the earth’s surface. Despite the fact that it is a relatively small percentage, however, UV radiation can have serious effects on the environment and on people, especially if they are exposed to it for long periods of time. The most common, and mildest, effect of over-exposure is the reddening of the skin, or a slight sunburn. But over-exposure over many years, especially if it has begun in childhood, can lead to much more serious symptoms, such as premature skin aging, an increased susceptibility to cataracts and other eye problems, a weakened immune system and in some circumstances, certain forms of skin cancer. The length of time in which exposure to UV rays can become harmful to one’s health varies from one individual to another, according to skin type and general state of health, as well as other factors, such as location (whether the ozone layer in one given area is heavily damaged or not), the time of day, cloud cover, altitude and the manner in which people are exposed. On a cloudless day, the sun’s UV rays are strongest during the early afternoon hours. The higher the sun is in the sky, the stronger the radiation as rays enter the atmosphere at a smaller angle. This is also why radiation exposure is more dangerous during the summer. Ultraviolet radiation is more intense when there are no clouds. Clouds help temper the strength of radiation to varying degrees, depending on their type and density. Light, scattered clouds have very little shielding effect (blocking just 10 percent of radiation), while low, dark clouds provide far more protection (up to 80 percent). In very specific circumstances, when clouds are scattered and very bright, they may even increase radiation exposure. When the sun’s disc is visible, clouds offer little, if any, protection from ultraviolet rays. Ultraviolet radiation exposure increases the further from sea level, because the atmospheric elements that absorb radiation decrease the higher we go. Studies have shown that ultraviolet radiation increases by about 10 percent every 1,000 meters from sea level. A person can be exposed to radiation either directly, when under the sun, or indirectly, when the rays bounce off another surface. The amount of radiation reflected depends on the type of surface. Trees, grass, earth and water reflect less than 10 percent of ultraviolet radiation, in contrast to freshly fallen snow, which reflects 80 percent, or dry sand, which reflects approximately 20 percent. People in snowy mountains or on sandy beaches are thus more exposed to radiation. Approximately 95 percent of ultraviolet radiation makes its way through water and 50 percent of that can actually penetrate up to 3 meters in depth. When we swim, therefore, and our bodies are just a few inches below the water’s surface, we are almost fully exposed to UV rays. The UV index The ultraviolet index has been established worldwide as a means of representing the potential dangers from prolonged exposure to the sun. It also includes temperature and the wind-chill factor. The real indices for ultraviolet radiation, as well as forecasts for the next day, are announced in almost all countries in the daily weather report or are posted on the Internet. In Greece, the address for the UV index is http://lap.physics.auth.gr/uvindex. Under normal circumstances, the UV index in Greece can reach up to 10 or 11, figures that reveal high levels of ultraviolet radiation and, as a result, the need to be well protected from the sun. Because Greece is a small country, the UV index between north and south does not vary greatly – it is just half to one point higher in the south than in the north. Our eyes can be protected from harmful sunrays with lenses fitted with special UV-B and UV-A filters. Wearing sunglasses is especially important for children, as their eyes are considerably more susceptible to harmful radiation rays than adults. The protection factor indicated on sunscreen lotion packaging in fact tells us how long we can stay in the sun while using lotion without suffering a sunburn. It is worth noting that reapplying lotion during the day does not increase the amount of time we can safely stay in the sun; only a higher sun protection factor cream can do that. We must also be careful to apply lotion liberally as the protection indicator is only valid if the cream is well rubbed into the skin. Furthermore, the use of some products on the skin, such as psoralens, porohyrens, coal tar, antibiotics and a variety of other skin products may adversely affect the effectiveness of sunscreen lotion. Skin types Sunburn occurs not just because of the strength of the sun, but also because of our skin’s susceptibility to it. There are four main skin types: At one end of the spectrum are people who rarely get a tan, have red hair and light-colored eyes and who are extremely sensitive to the sun. At the other end are dark-skinned people who never get sunburns and who have dark hair and eyes. The effectiveness of sunscreen does not just depend on the quality of the lotion, but also by the manner in which it is applied. Most people use too little; the suggested amount for adults is a full 30-40 grams, which amounts to a proper handful. Sunscreen lotion should also be applied before going out in the sun and then again after swimming. If sunscreen is used properly it can offer very effective protection against sunburn, skin cancer and premature aging. Clothes Clothes are the best protection for the skin, though thin fabrics do not shield the skin from harmful sunrays. Any parts of the body not covered by clothes should be protected with sunscreen lotion. It is recommended that during the first days they venture out into the hot sun, adults should use lotion with a minimum 15 sun protection factor and children, 20. Small children and babies should be especially well protected. The skin and eyes are most affected by ultraviolet radiation. Extensive exposure over many years can cause serious damage to the skin, eyes and immune system. The early symptoms of over-exposure are sunburn and photokeratitis (also known as snow blindness). More advanced symptoms are skin cancer, premature skin aging and eye cataract. While UV-B rays can cause sunburn and various forms of skin cancer, UV-A rays can change the skin’s collagen, the protein in the skin’s connective tissue, thus speeding up premature aging. It is worth noting that while the skin automatically releases melanin, which helps the skin protect itself from harmful rays and gives it the tanned look, the eyes have no such protective mechanism. (1) Christos Zerefos is a professor at the University of Athens in climatology and the atmospheric environment.