Facts of the Matter Richard Brill

'Invisible' UV radiation spans broad spectrum

LIGHT is an oxymoron when used in conjunction with ultraviolet. In the physical sense, light refers to the sensation produced by stimulation of the organs of sight by energy from a small segment of the electromagnetic spectrum.

Ultraviolet (UV) radiation occupies a place on the electromagnetic spectrum next to visible light, but it is "invisible light."

The human eye is sensitive to electromagnetic radiation that has wavelengths in the range of around 400 to 700 nanometers (nm). A nanometer is one one-billionth of a meter.

The UV spectrum is much broader than the visible one and today is further subdivided because of its wide range of wavelengths and their different effects.

UVA (400 < 315 nm) is long wave or "blacklight;" uvb (315 < 280 nm), is medium wave; and uvc (280 - 10 nm), is short wave or "germicidal." radiation with wavelengths shorter than 10 nanometers are called x-rays.

Although invisible to the human eye, some birds, reptiles, and insects such as bees, can see the long UVA wavelengths. Many fruits and flowers are brighter in UVA wavelengths than in visible light. Many birds have patterns in their plumage that are undetectable in visible light but observable in UV.

Some naturally occurring minerals glow with fluorescence when illuminated with UVA. The most common of these is the mineral fluorite (calcium fluoride), from which the term originates. Some varieties of calcite (calcium carbonate) also fluoresce.

We usually visualize light as a wave, but it releases its energy in packets called photons. The energy of a photon is unique for a given wavelength and increases as wavelength decreases.

Fluorescence occurs when an atom in the mineral absorbs a photon, which then triggers the emission of another photon with a longer wavelength and less energy. The energy difference between the absorbed and emitted photons ends up as the molecular vibrations of heat.

THE SUN emits UV radiation across the entire UV visible, and infrared spectrum.

But the atmosphere absorbs strongly in the UVB and UVC range so that 99 percent of the UV radiation that reaches Earth's surface is UVA.

UVC generates ozone from oxygen atoms in the stratosphere, also known as the ozone layer, while UVB is absorbed as the molecular vibrations of heat.

It is UVA that is mostly responsible for the skin damage that leads to sunburn, but UVB adds to the effect although present in much smaller amounts.

Liquid water is virtually transparent to all UV, and is not scattered by clouds like visible light, which is why you might come home from a cloudy beach outing with a red glow.

UVB induces the production of vitamin D in the skin, which helps the body use calcium to build bone tissue. An estimated tens of thousands of deaths occur in the United States annually from a variety of cancers due to vitamin D deficiency from insufficient UVB exposure.

Vitamin D deficiency can cause rickets or osteomalacia that weaken the bones and can cause bone pain and lead to fractures when bones are stressed by weight.

UVB, and to a lesser extent UVA, damage DNA and initiate pathways for various types of skin cancer. UVA is less pernicious but penetrates deeper into the skin and causes wrinkling. Most sunblock chemicals do not block the more abundant UVA, which has also been implicated in causing cancer.

How ironic that too little sun and too much sun can both be carcinogenic.

On the other hand, both UVB and UVA are used to treat skin conditions such as psoriasis and leukoderma, and diagnose certain fungus infections.

It just happens that the energy of UV photons lies in the range of the bonding energy of life's chemicals, which are made mostly of hydrogen, carbon, and oxygen.

When a UV photon strikes an atom in a DNA molecule it can cause irreparable damage by breaking a chemical bond and causing another to form.

One common damage event is when adjacent thymine bases bond with each other, instead of with their complimentary adenine bases across the spiral DNA ladder. These can lead to mutations, some of which may become cancerous growths.

DESPITE the potentially harmful effects of overexposure to UV, it has found many uses in science and technology.

UV images of stars have added much to our knowledge and understanding of them and our own sun. There are currently two satellites positioning themselves near the sun to take stereo UV photos that will give astronomers close-up, 3D images of flares and other solar activity.

UV lamps can help to analyze minerals, gems and other substances since materials that look the same under visible light may look different or fluoresce differently in UVA versus UVB.

UV lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities, and to disinfect drinking water.

Some inks, coatings and adhesives are made with resins that polymerize when exposed to a specific wavelength and intensity of UV.

There are many applications that include glass and plastic bonding, optical fiber coatings, the coating of flooring, paper finishes in offset printing, and composite dental fillings.