Ultraviolet

Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of visible light, but longer than that of soft X-rays. The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelength of visible light. It is colloquially called black light, as it is invisible to the human eye.

UV itself can be subdivided into near UV (380-200 nm wavelength) and extreme or vacuum UV (200-10 nm). When considering the effects of UV radiation on human health, the range of UV wavelengths is often subdivided into UV-A (380-315 nm), UV-B (315-280 nm), and UV-C (280-10 nm). See 1 E-7 m for a list of objects of comparable sizes.

The sun emits ultraviolet light in both the UV-A and UV-B bands, but because of absorption in the atmosphere's ozone layer, 99% of the ultraviolet light that reaches the Earth's surface is UV-A.

Health effects

In general, UV-A is the least harmful, but causes sunburns at high exposures.

High intensities of UV-B light are hazardous to the eyes, and exposure can cause welder's flash (photokeratitis or arc eye).

Both UV-B and UV-C damage collagen fibers and thereby accelerate aging of the skin.

UV-B light has been linked to skin cancers such as melanoma. The radiation ionizes DNA molecules in skin cells, causing covalent bonds to form between adjacent thymine bases, producing thymidine dimers. Thymidine dimers do not base pair normally, which can cause distortion of the DNA helix, stalled replication, gaps, and misincorporation. These can lead to mutations, which can result in cancerous growths. The mutagenicity of UV radiation can be easily observed in bacteria cultures.

This cancer connection is the reason for concern about ozone depletion and the ozone hole.

As a defense against UV light, the body tans when exposed to moderate (depending on skin type) levels of radiation by releasing the brown pigment melanin. This helps to block UV penetration and prevent damage to the vulnerable skin tissues deeper down. Suntan lotion that partly blocks UV is widely available (often referred to as "sun block"). Nevertheless, most dermatologists recommend against prolonged sunbathing.

A positive effect of UV light is that it induces the production of vitamin D in the skin. Grant (2002) claims tens of thousands of premature deaths occur in the U.S. annually from cancer due to insufficient UV-B exposures (apparently via vitamin D deficiency).

Astronomy

In astronomy, very hot objects preferentially emit UV light (see Wien's law). However, the same ozone layer that protects us causes difficulties for astronomers observing from the earth, so most UV observations are made from space. (See UV astronomy, space observatory).

Uses

Fluorescent lamps produce UV light by the emission of low-pressure mercury gas. A phosphorescent coating on the inside of the tubes absorbs the UV and turns it into visible light.

Ultraviolet light is increasingly being used to disinfect drinking water and in waste water treatment plants. Dr. James R. Bolton (Ultraviolet Consulting) while working on contract with (Calgon Carbon) discovered that ultraviolet light could treat Cryptosporidium, previously unknown. The findings resulted in two US patents and the use of UV light as a viable method to treat drinking water.

Ultraviolet lamps are also used in analyzing minerals, gems, and in other detective work including authentication of various collectibles. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light; or may fluoresce differently under short wave ultraviolet versus long wave ultra violet. UV fluorescent dyes are used in many applications (for example, biochemistry and forensics). The fluorescent protein Green Fluorescent Protein (GFP) is often used in genetics as a marker.

Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. Since microorganisms can be shielded from ultraviolet light in small cracks and other shaded areas, however, these lamps are used only as a supplement to other sterilization techniques.

Ultraviolet light is used for very fine resolution photolithography, as required for manufacture of semiconductors.

It is advisable to use protective eyewear when working with ultraviolet light, especially short wave ultraviolet. Ordinary eyeglasses give some protection.

Some insects, such as bees, can see into the near ultraviolet, and flowers often have markings visible to such pollinators.

References

• Grant, William B. (2002). An estimate of premature cancer mortality in the U.S. due to inadequate doses of solar ultraviolet-B radiation. Cancer 94 (6), 1867–1875.