When the Industrial Revolution began in England 200 years ago, rural populations migrated from the countryside to the industrial centers. Because of the rapid increase in urban population, acute housing shortages developed. The working class lived on narrow streets, crowded together in rooms with small and often shuttered windows. The air outside was heavy with the pollution pouring out of the factories.
Children growing up in the filth and squalor there developed what was commonly called the "English disease." The long bones, especially those of the legs, became bent or bowed. People generally considered rickets, as the disease was later called, to result from malnutrition and unhealthful conditions.
In 1890 a Doctor Palm traveled around the world, carefully tabulating the incidence of rickets. He noted the prevalence of this disease in both the large cities and the countryside. Yet he found that while rickets was common among the malnourished children reared in filth and poverty in the West, among those raised in similar circumstances in Japan, China, and India, it was a rare disease.
Dr. Palrri also noted that upper-class children in England who suffered no malnutrition or uncleanliness but who were confined indoors for various reasons occasionally developed rickets. He made the astute observation that rickets occurred in children who had not been ex posed to sunlight. This and many other studies led eventually to the discovery of vitamin D, of its synthesis in the skin under the action of sunlight, of its role in the absorption of calcium, and in the deposit of calcium in the bones for strength and rigidity.
Although it has long been known that the sun is man's ultimate source of food and energy, only recently have we begun to realize that sunlight influences chemicals within our bodies, drives our biological clocks, and even determines how soon we mature.
Sunlight, or solar radiation, is electromagnetic energy emitted by the sun. It travels in waves of varying length measured in nanometers, or billionths of a meter. The spectrum of radiation that falls on us is made up of three main types: ultraviolet (5 percent), visible light (40 percent), and infrared (54 percent). The balance consists of cosmic rays, gamma rays, X-rays, radio waves, and electric waves. Most of the spectrum is not visible to the human eye. The emissions that do reach the earth are filtered through the atmosphere and an outer layer of ozone.
Except at the equator, the number of daylight hours varies according to the season of the year--longer in the summer and shorter in the winter. Ultraviolet radiations also have a seasonal variation. In the northern United States and similar latitudes in Europe and Japan, only one fifteenth as much is present in December as in June--virtually none before nine in the morning or after three in the afternoon.
Solar energy penetrates the skin, affecting the various tissues either directly or indirectly. Light energizes specialized cells and certain light-receptive chemicals in the tissues. It triggers nerves, stimulating them to transmit messages, and forms hormones that are transported to distant parts of the body.
The ultraviolet band, between 290 and 315 nanometers, is responsible for the formation of vitamin D (or D3 ). The high-energy photons that make up light penetrate the superficial layers of the skin, or epidermis, and convert a chemical, provitamin D, to previtamin D. Then the warmth of the skin gradually changes previtamin D to vitamin D.
In large amounts, vitamin D is toxic. Prolonged exposure to the sun, however, causes provitamin D to change photochemically to harmless substances that do not act like vitamin D. When the bloodstream absorbs these harmless sub stances, sunlight changes them back to vitamin D. Otherwise, as the skin sloughs off they are lost to the body.
On a clear summer day in the Temper ate Zone, the average fair-skinned person whose face and hands are exposed to the sun for 10 to 15 minutes will get all the vitamin D needed. Because their skin contains more pigmentation, Black and Asian people require six times the exposure to sunlight to develop the same amount of vitamin D as do fair-skinned people.
The elderly, especially if confined in doors, should exercise care to obtain sufficient amounts of vitamin D. The skin thins with age. As a result, not only can it burn more readily, but it may produce only half as much vitamin D as it did when it was younger. Since plant foods do not contain vitamin D, the elderly should eat foods fortified with the vita min, or use milk, eggs, or a supplement. Otherwise osteomalacia (a defect in bone mineralization) or osteoporosis (a loss of bone minerals) may become a problem.
Sunlight provides us with other benefits. Ultraviolet rays kill most bacteria exposed to full sunlight within two hours. Even reflected winter sunlight passing through the glass of north windows will destroy germs in the dust on windowsills and floors. These same germs could survive for months in the dark, neglected corners of a room.
The light cycles of day and night appear to influence human biologic rhythms. Cortisone levels reach their peak in the morning and gradually fall during the day, being lowest in the evening. Should you begin to work nights, it will take you 5 to 10 days to reverse the cycle.
Visible light entering the eye also affects the production of melatonin, a hormone that is produced by a tiny gland in the back and center of the brain. Melatonin encourages sleep. Light blocks the production of this hormone, so its concentration is lowest at the end of the day. When darkness comes, the amount of melatonin in the body rises.
Light also has its effects on the process of maturing. Children born blind mature sooner than do sighted children, and those who lose their sight in childhood begin adolescence earlier. In response to certain wavelengths of visible light, the retina of the eye signals the pituitary gland, which then transmits the signal to the ovaries or testicles. These glands respond by causing the maturing process to proceed at the appropriate pace.
As red blood cells wear out, they release hemoglobin, the protein pigment that carries oxygen and carbon dioxide. Hemoglobin in turn breaks down to form bilirubin, a substance that becomes toxic when it reaches too high a concentration. The mature livers of full-term infants can readily handle this substance, but the immature livers of premature infants cannot. In many cases jaundice occurs, and if the level of bilirubin rises too high in certain areas of the brain, it will damage the neurons. Exposing infants to sunlight or a sunlamp prevents brain damage by changing the bilirubin molecule into water-soluble compounds that are easily excreted by the kidneys.
We also know that a bright, sunny day buoys our feelings. The long ultraviolet waves release endorphins the body's own mood elevator. Researchers are now using sunlight experimentally to treat depression.
As research advances, we are finding that sunlight plays a role in the growth and development of bodies and minds, in the functioning of kidneys, in the regulation of blood cells, and in many metabolic activities. But herein lies a problem. In industrial nations it is becoming difficult to get enough sunlight. Dust, smoke, and other pollutants in the atmosphere block out ultraviolet rays and reduce the quality of visible light.
The solar spectrum that reaches the earth includes ideal proportions of ultra violet, visible, and infrared rays. Although incandescent light is similar to sunlight, the largest portion of its radiation is infrared, providing heat rather than light. Visible light from fluorescent bulbs differs from sunlight and from incandescent light, which are the result of heating a dark object to a very high temperature. Fluorescent light is not the product of heat, but of the activation of certain chemical phosphors.
To meet the demands of shop, office, and home, engineers have attempted to produce an adequate amount of light as economically as possible. Fluorescent light excels at this. However, the spectra of that light, the intensity of that spectra, and the hours of exposure to that light have been given little if any consideration.
Because light does much more than allow us to see, there is a growing concern that malillumination, like malnutrition, may prove to be a problem in developed countries.
What, then, can a person do? First, become light-conscious. For too long we have taken light and sunlight for granted. During lunchtime or break-time, go outside, weather permitting. Even as short a time as 10 to 15 minutes a day, three or four times a week, will improve your well-being and general health. If your work confines you indoors, expose yourself to moderate amounts of sunlamp rays. Studies in northern Europe and north Russia, lands of weak sunshine and long periods of indoor activity, show that health can be improved by the artificial radiation from a sunlamp.
Don't overdo it
As with every other good thing, how ever, it is possible to go overboard in one's concern to get enough light. Overexposure damages the skin and the eyes.
The same band of sunlight that helps produce vitamin D is also responsible for sunburn, aging of the skin, and skin cancer. Anyone who remains outdoors in the bright sun more than 15 minutes per day should protect himself or herself with an appropriate sunscreen. Containing chemicals that absorb ultraviolet light, sunscreens protect against skin damage--but they also greatly limit or completely block the formation of vitamin D. Sunscreens are rated by their sun protection factor (SPF). SPFs range from 2, providing minimal protection, to as high as 50--the higher the SPF number, the longer the period of protection. If you would sunburn in an hour of exposure without a sunscreen, an SPF-2 sunscreen would protect you against burning for two hours.
For the best protection, you should apply the sunscreen 30 to 60 minutes before exposure to the sun. Since the degree of protection depends to a large extent on the thickness of the application, skimping is self-defeating. Reapplying the sun screen will not lengthen the time of protection it offers. Swimming and toweling off sweat, however, remove some of the sunscreen (despite water-resistant and waterproof claims), and so reduces, the time of protection. In these circumstances, you should reapply the sun screen.
In 1988 in the United States, more than 500,000 people developed skin cancer, and this disease caused 8,000 deaths. Early detection of skin cancer is the key to controlling it. Any new growth on the skin or a sore that does not heal should be checked. Other warning signs are existing warts or moles that show signs of changing color or increasing in size, or a reddish patch that often itches.
Excessive sunlight may also damage the eyes. Three areas are particularly vulnerable. The cornea can become sun burnt or inflamed (as occurs in snow blindness). Light reflected from water or sand increase exposure, as does, to an even greater degree, increasing altitude.
Ultraviolet rays passing through the cornea can injure the lens, too. The in jury occurs slowly, insidiously, and painlessly. Over years the lens discolors, becoming yellow. When the discoloration impairs vision, it is called a cataract. There are diseases, such as diabetes, that may also trigger cataract formation.
Intense visible light passing through the cornea and lens may damage the retina. Looking directly into the sun at mid day or watching a solar eclipse without appropriate glasses may "sunburn" the retina.
Just as sunscreens can protect the skin, sunglasses can safeguard the eyes. How ever, there is need for care in selecting sunglasses. Plain dark glass decreases the visible light entering the eyes, resulting in the pupils dilating. Unless the glass also blocks the ultraviolet rays, the dilated pupils will permit more of these damaging rays to enter the eyes.
Glasses should reduce more than just visible light. Select glasses that absorb or block ultraviolet rays, or that are approved by the American National Standards Institute. The Sunglass Association of America divides sunglasses among three categories: special purpose--which block at least 99 percent of the sun's eye-damaging ultraviolet rays; general purpose--which block 95 percent; and cosmetic--which block 70 percent.
Morris Wayler, chairman of the Food and Drug Administration's committee on sunglass regulations, recommends sunglasses with 99 percent protection. Some of the more expensive sunglasses actually use poorer quality lenses that provide little protection. You can purchase sunglasses with special-purpose lenses for $12 to $17.
Sunglasses are essential when you are boating, fishing, skiing, or driving--especially through snowy landscapes, in desert areas, or in higher elevations. Every 1,000-foot rise in elevation brings a 4 percent increase in ultraviolet radiation exposure. Hikers on a 10,000- foot mountain receive 40 percent more ultraviolet light than do those at sea level.
Whether in winter or summer, remember that although sunlight is essential to life and health, overexposure will prematurely age your skin and place you at high risk for cancer or cataracts later in life. When it comes to sunlight, also re member the old adage--enough is enough.
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