Ultraviolet rays

UV-A and UV-B rays are defined as wavelengths between 320—400 nm and 290-320 nm, respectively. Rays with wavelengths longer than 270 nm and an intensity distribution similar to UV-A and UV-B rays were produced by a 660 watt xenon-arc lamp (Suntester XF-180, Shimadzu, Japan). The UV-A and UV-B were prepared by excluding wavelengths shorter than 320 nm and 290 nm, respectively, from the xenon-arc lamp using special filters. The UV-B contains both UV-B and UV-A rays while UV-A contains only UV-A rays. The intensity of UV-B between 290—320 nm was approximately double the intensity of actual UV-B rays and the intensity of UV-A between 320—400 nm was identical to that of actual UV-A. 

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Ozone s presence in the atmosphere (amounting to the equivalent of a layer 3 mm thick under ordinary pressures and temperatures) helps prevent harmful ultraviolet rays of the sun from reaching the earth s surface. Pollutants in the atmosphere may have a detrimental effect on this ozone layer. Ozone is toxic and exposure should not exceed 0.2 mg/m (8-hour time-weighted average - 40-hour work week). Undiluted ozone has a bluish color. Liquid ozone is bluish black and solid ozone is violet-black. 

When writing a Lewis structure we restrict a molecule s electrons to certain well defined locations either linking two atoms by a covalent bond or as unshared electrons on a sm gle atom Sometimes more than one Lewis structure can be written for a molecule espe cially those that contain multiple bonds An example often cited m introductory chem istry courses is ozone (O3) Ozone occurs naturally m large quantities m the upper atmosphere where it screens the surface of the earth from much of the sun s ultraviolet rays Were it not for this ozone layer most forms of surface life on earth would be dam aged or even destroyed by the rays of the sun The following Lewis structure for ozone satisfies fhe ocfef rule all fhree oxygens have eighf elecfrons m fheir valence shell... 

Difference in optical properties can be used as the basis to separate solids in a mixture. Optic properties include color, light reflectance, opacity, and fluorescence excited by ultraviolet rays or x-rays. Differences in elec trical conductance can also be used for separation. With appropriate sensing, the particles in a moving stream can be sorted by using an air jet or other means to deflect certain particles away from the mainstream (Fig. 19-10). The lower limit of particle size is about... 

The more common adverse effects seen with the administration of these dm include nausea, diarrhea, headache, abdominal pain or discomfort, and dizziness. A more serious adverse reaction seen with the administration of the fluoroquinolones, especially lomefloxacin and sparfloxacin, is a photosensitivity reaction. This is manifested by an exaggerated sunburn reaction when the skin is exposed to the ultraviolet rays of sunlight or sunlamps. 

Gentamicin may cause photosensitivity. Take measures to protect the skin from ultraviolet rays (eg, wear protective clothing and use a sunscreen when out in the sun). 

We love the sun. We complain about rainy days, and we look forward to summers at the beach. But we hate sunburns, wrinkled skin, and melanomas. All of those nasty things are the result of the ultraviolet rays that come with the warmth and light of sunshine. 

The invisible ultraviolet rays of the sun can cause immediate and longterm skin damage in the form of sunburn, rashes, premature wrinkling, and skin cancer. To avoid overexposure, we are encouraged to apply sunscreens and sunblocks to protect the health of our skin. Chemistry clearly distinguishes between these two formulations, and the chemical structure of these products dictates how well these materials perform. 

Phytochemicals or phytonutrients are bioactive substances that can be found in foods derived from plants and are not essential for life the human body is not able to produce them. Recently, some of their characteristics, mainly their antioxidant capacity, have given rise to research related to their protective properties on health and the mechanisms of action involved. Flavonoids are a diverse group of phenolic phytochemicals (Fig. 6.1) that are natural pigments. One function of flavonoids is to protect plants from oxidative stress, such as ultraviolet rays, environmental pollution, and chemical substances. Other relevant biological roles of these pigments are discussed in other chapters of this book. 

In a different arena, a new sunscreen is under development from chemical compounds produced by reef-building corals. Corals living in a reef cannot move about, and in shallow water they are continually exposed to the sun s ultraviolet rays. This radiation can be as harmful to corals as to humans, so it seemed likely that sedentary corals must somehow limit radiation damage to their bodies. Pursuing this idea in the laboratory, investigators found that corals synthesize a family of ultraviolet-absorbing compounds. 

Xiao L, Takada H, Maeda K, Haramoto M, Miwa N (2005) Antioxidant effects of water-soluble fullerene derivatives against ultraviolet ray or peroxylipid through their action of scavenging the reactive oxygen species in human skin keratinocytes. Biomed. Pharmacother. 59 351-358. 

USP acid test analy chem A United States Pharmacopoeia test to determine the carbonizable substances present In petroleum white oils. yti es pe as ad. test) UV stabilizer chem Any chemical compound that, admixed with a thermoplastic resin, selectively absorbs ultraviolet rays used to prevent ultraviolet degradation of polymers. Also known as ultraviolet stabilizer. yu ve sta ba.iTz ar)... 

Some of its compounds, particularly the oxides, are used in chemical industries as catalysts to speed up organic chemical reactions. The yellow-brown vanadium pentoxide (V O ) is used as a catalyst to facilitate the production of sulfuric acid by the contact process. Vanadium pent-oxide is also used as a photographic developer, to dye textiles, and in the production of artificial rubber. When combined with glass, it acts as a filter against ultraviolet rays from sunlight. 

CFCs. This is adequately discussed in Chapter 12, Section 4, but certainly it deserves to be listed here. Chlorine atoms from photodissociation of CFCs in the stratosphere have led to depletion of the ozone layer protecting us from ultraviolet rays. These substances are being phased out. Substitutes have been hard to find. Do we change our way of living by giving up certain products, or do we increase R D spending to find substitutes more quickly These questions must be answered. 

World Health Organization (WHO). Online. Available HTTP (accessed 9 April 2003). Site contains information on the global efforts to reduce UV radiation exposure. 

Krypton dilluoride may be prepared by the reaction of krypton with fluorine in an electric discharge at low pressure and hquid oxygen temperature. Also it may be made by irradiating krypton with ultraviolet rays in a fluorine—argon gas mixture at liquid helium temperature (-196°F). 

Protect against exposure to sunlight and ultraviolet rays by using sunscreen and wearing protective clothing... 

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