Ultraviolet light: blocking

Verticillate epithelial keratopathy due to amiodarone, in which there is whorl-shaped pigmentation of the cornea, has been proposed to be worsened by soft contact lenses (SEDA-15, 171). Two patients with hard contact lenses and amiodarone-associated keratopathy both complained of increased sensitivity to sunlight and were fitted with ultraviolet light-blocking lenses instead, as a precaution against further corneal damage however, the authors did not think that the contact lenses had contributed to the damage (102). 

Gamma Rays, X-Rays and Ultraviolet Light blocked by the upper atmosphere (best observed from space). 

PABA is taken orally in vitamin supplements. However, its widest use is as a sunscreen. Taking it orally will not protect a person from the sun as a sunscreen PABA acts as a topical dye that absorbs ultraviolet light. To block UV rays from the sun, a person needs to paint it directly onto the skin. 

Figure 9.4 Repair of DNA inactivated by ultraviolet light. Light causes die dimerization of adjacent thymine residues that block DNA replication. The four enzymes shown are involved in removal and replacement of a portion of the DNA that contains the dimer.

The sunblocks zinc oxide, titanium dioxide, and iron oxide are inorganic chemicals that are not absorbed into the skin. These substances consist of opaque particles that reflect both visible and ultraviolet light. In addition, zinc oxide blocks virtually the entire UVA and UVB spectrum and thus offers overall protection. The particulate nature of these sunblocks enhances their effectiveness at reflecting sunlight. The smaller the particle size, the greater the surface area available for reflection, and the more effective the sun protection offered by the formulation. 

The correlation between definite photoproducts and biological action of ultraviolet light discussed in this section is confined to a particular action of ultraviolet light, namely an inactivating or lethal action. Microorganisms are killed, DNA synthesis in cells is inhibited, transforming DNA is inactivated, template activity of DNA is reduced, and phage and viruses are inactivated because the pyrimidine dimer which is formed in-strand is apparently able to provide a block which hinders DNA or RNA replication. 

Nitric oxide is again catalytic but here its effect is to change 03 to 02. This is a potential problem because 03, which absorbs ultraviolet light, is necessary to protect us from the harmful effects of this high-energy radiation. That is, we want 03 in the upper atmosphere to block ultraviolet radiation from the sun. However, we do not want it in the lower atmosphere where we have to breathe it and its oxidation products. 

However, it should be imderstood that many recently developed contact lens materials contain polymers that block the transmission of light in the ultraviolet region. Therefore when an ultraviolet light source, such as a... 

Even after DNA has been initially replicated, the genome is still not safe. Although DNA is remarkably robust, ultraviolet light as well as a range of chemical species can damage DNA, introducing changes in the DNA sequence (mutations) or lesions that can block further DNA replication... 

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