A sun-tan may look good but is not healthy. Sun exposure to fair skin is important in skin aging and skin malignancy. The wavelengths of the solar radiation involved are in the ultraviolet (UV) spectrum - from 200-400 nm. The  increased incidence of cutaneous malignancy from sun exposure and increased UV radiation (UVR) caused by thinning of the stratospheric ozone  is now a major health concern. Ozone is one of the natural sunscreens the upper atmosphere and used to be a more effective filter against solar ultra-violet radiation.  UV exposure causes sunburn, skin aging, photodermatoses and skin cancer. Ultaviolet light is divided into three bands; A, B & C. UVA and UVB are both responsible for photoageing.

Ultraviolet sub-bands:

UVA : at wavelengths of 320-400 nm accounts for  90% of UVR reaching the Earth's surface.

UVB : 290-320 nm  is 10% of UVR reaching the earth surface and  is largely responsible for skin cancer.  UV-B is absorbed in the upper stratosphere (about 25 miles above the earth) at the level of the ozone layer. uVB is 1000 times more potent in causing sunburn (erythema) than UVA.

UVC : 200-290 nm  is absorbed in the upper atmosphere, but would cause severe cellular damage if it reached living organisms.

UV effects on Gene Expression

UV light changes the behavior of skin cells by changing the expression of genes and/or damaging DNA. UV radiation causes cyclobutane phyrimidine dimers, 6-4 photo products and single strand breaks. UV increases synthesis of transcription factor proteins that enter the nucleus, bind to genes and increases production of the protein transcribed by the gene.  Protective substances in cells such as p53 can repair DNA damage and stop cells from proliferating. Imperfect repair leaves permanent mutations with changes in the growth characteristics of the skin and risk of cancer.

Protective Measures

Avoid  sunburns

Avoid tanning parlors

Use  protective clothing and sunscreens

Checkout any supicious skin lesion

Sunscreens are no substitute for avoidance of mid-day sunlight and protective clothing. Recent studies suggest that people are using sunscreens more frequently and exposing themselves more often to sun, increasing the total dose of UVR  they receive.

Sunscreens can be divided into 2 main types :

Physical:  These are opaque pastes and creams that reflect or scatter incident UVR. Examples are zinc oxide, titanium dioxide and magnesium silicate. They protect against both the UVB and UVA. The white paste, zinc oxide is the most effective sunblock. A transparent, micronized form of zinc oxide has been marketed as Z-Cote and is be incorporated into a number of other skin products.

Chemical: These chemicals act by absorbing UVB: para-aminobenzoic acid (PABA), PABA esters, salicylates, cinnamates, anthranilates and the benzophenones. Benzophenone compounds  absorb UVB and  wavelengths from 250-365 nm,  However, it is less effective than PABA in the UVB spectrum. Cinnamates can also absorb UVA. Sunscreen   products often contain more than more than one active ingredient.

Examples of commonly used preparations are :

Sunsense : Titanium dioxide Co 3%, Oxybenzone 5%, Ethylhexyl-p-methoxycinnamate 7.5%, Butylmethoxy-dibenzoylmethane 1.5 %

(ii) Coppertone : Ethylhexyl p-methoxylcinnamate, 2-ethylhexyl salicylate,   Octocrylene, Oxybenzone

A Sun Protection Factor (SPF *) >15 is required; more than 15 times the sun exposure is required to produce the same reddening of skin by comparison with unprotected skin. The greater the SPF, the greater the protection. Sunscreen agents such as PABA esters are potential sensitizers, causing allergic contact dermatitis.

OZONE DEPLETION

SPEAKERS: Dr. Daniel Albritton, NOAA Aeronomy Lab, Boulder, CO, Dr. Margaret Kripke, University of Texas MD Anderson Cancer Center, Houston, TX

More than 25 years ago, scientists first hypothesized that human activities could harm the stratospheric ozone layer, which is our shield against solar UV-B radiation. Subsequently, research has focused on understanding the nature and make-up of the stratospheric ozone layer and its relationship with humankind. For example, the ozone-destroying role of several industrially-produced chemicals has been determined, the Antarctic "ozone hole" has been observed and explained, and the relation between ozone loss and increased surface UV-B radiation has been characterized. World governments began to formulate international agreements to protect the ozone layer, with scientific understanding providing major support for these decisions. This seminar will summarize key points of our present scientific understanding of ozone depletion, what research results they are based upon, and the outlook for the future of our ozone layer.

In the early 1970's chemists Paul Crutzen and Harold Johnston described the effects of nitrogen oxides on stratospheric ozone chemistry. In 1974 chemists Mario Molina and Sherwood Rowland realized that human-produced chlorine compounds, particularly CFC's, could deplete the Earth's ozone layer. These scientists, together with Paul Crutzen, were recently awarded Nobel Prizes for their research.

Large seasonal depletion of ozone (up to 100% at some altitudes) is observed each year over Antarctica, where the meteorology and extremely old wintertime temperatures are enhancing the ozone-depleting chemistry of CFC's and other human-produced chemicals. Downward trends of about 4-5% per decade have been observed at mid-latitudes in both hemispheres. Although the phenomenon is not yet fully understood, the weight of evidence indicates that these losses are due in large part to human-produced chemicals. Ozone depletion is observed to cause an increase in UV-B radiation at the Earth's surface. Monitoring data show that the growth in concentrations of ozone-depleting chemicals in the atmosphere is slowing, consistent with the declining production required by international agreements. The maximum ozone depletion (and increase in UV-B radiation) is likely to occur within the next 10 years; thereafter, the ozone layer is expected to slowly recover over the next several decades.

Ozone and Health Effects of Ultraviolet Radiation

The amount of UV-B radiation in natural sunlight is dependent upon the concentration of ozone molecules in the atmosphere. Any reduction in stratospheric ozone concentration will result in increased amounts of UV-B radiation reaching the surface. Even a small increase in UV-B radiation is likely to have important consequences for plant and animal life, and will almost certainly jeopardize human health. The best understood harmful effects of UV-B radiation on human health are basal and squamous cell cancers of the skin and eye damage, including cataracts, which can lead to blindness.

UV-B radiation also contributes to the development of melanoma skin cancer and perturbs the body's immune system in ways that can reduce immunity to infectious agents, although magnitude of the impacts cannot yet be estimated. UV-B radiation may also affect human health indirectly by interfering with the food chain. On a global scale, UV-B radiation may increase the infectious disease burden, cause blindness, and reduce the world's food supply.

The current pattern of ozone depletion will cause the incidence of skin cancer to continue to rise at least until the year 2050 and probably beyond. For each 1% reduction in ozone, the incidence of non-melanoma skin cancer will increase by 2%. This means that a sustained 10% decrease in the average ozone concentration would lead to about 250,000 additional non-melanoma skin cancers each year. Each 1% decrease in ozone concentration is estimated to increase the incidence of cataracts by about 0.5%. Increased UV-B radiation could increase the severity of some infections in human populations. Furthermore, skin pigmentation does not seem to provide much protection against the immunosuppressive effects of UV irradiation in humans. Any lowering of immune defenses is likely to have a devastating impact on human health.

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