Invisible rays that are part of the energy that comes from the sun. Ultraviolet radiation can burn the skin and cause skin cancer.
Ultraviolet radiation is made up of three types of rays — ultraviolet A, ultraviolet B, and ultraviolet C. Although ultraviolet C is the most dangerous type of ultraviolet light in terms of its potential to harm life on earth, it cannot penetrate earth’s protective ozone layer. Therefore, it poses no threat to human, animal or plant life on earth.
Ultraviolet A and ultraviolet B, on the other hand, do penetrate the ozone layer in attenuated form and reach the surface of the planet. Because ultraviolet A is weaker than ultraviolet B, scientists long blamed ultraviolet B as the sole culprit in causing skin cancer in persons with a history of sunburn and repeated overexposure to ultraviolet radiation. Recent research, however, has also implicated ultraviolet A as a possible cause of skin cancer.
Ultraviolet B rays are more likely than ultraviolet A rays to cause sunburn, but ultraviolet A passes further into the skin. Scientists have long thought that ultraviolet B can cause melanoma and other types of skin cancer. They now think that ultraviolet A may add to skin damage that can lead to cancer. For this reason, skin specialists recommend that people use sunscreens that block both kinds of radiation.
In addition to natural light from the sun, artificial light from tanning lamps contains ultraviolet A and ultraviolet B. Electric arc lamps can also generate ultraviolet light to heat furnaces for melting and to enable motion-picture projectors to show movies.
Though ultraviolet light can damage health, it can also maintain or improve health. When ultraviolet light strikes human skin, it triggers the production of vitamin D, which promotes the growth of bones and teeth.
The taste of substances such as glutamate and monosodium glutamate (MSG) that are found in foods like bouillon and other stocks. Umami is the fifth primary element of taste. The other four are sweet, sour, salty, and bitter. It was discovered in 1907 by Kikunae Ikeda. Umami is the Japanese word for delicious flavor.
- Umbilical cord
The cord that connects the developing fetus with the placenta while the fetus is in the uterus. The umbilical arteries and vein run within this cord. The umbilical cord is clamped and cut at birth, and its residual tip forms the bellybutton.
The vestige left behind on a newborn’s belly when the umbilical cord is cut. Also called the navel or belly button.
- Unassisted childbirth
Birth of a baby without the assistance of an obstetrician, midwife, or other health professional. This may be by accident or by design. Unassisted childbirth is fraught with risks since about a fifth of apparently normal pregnancies develop unexpected complications during labor and turn into high-risk, sometimes life-threatening, situations.
By Atul Gawande. Metropolitan Books/Henry Holt & Co., New York, 2002.