Ultraviolet Light and Skin
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 in 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.
UVA : at wavelengths of 320-400 nm accounts for 90% of UVR reaching the
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.
Avoid tanning parlors
Use protective clothing and sunscreens
Checkout suspicious skin lesions
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 sunscreens are
Sunsense : Titanium dioxide Co 3%, Oxybenzone 5%,
Ethylhexyl-p-methoxycinnamate 7.5%, Butylmethoxy-dibenzoylmethane 1.5 %
Coppertone : Ethylhexyl p-methoxylcinnamate, 2-ethylhexyl salicylate,
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
Adapted from presentations by Dr. Daniel Albritton, NOAA Aeronomy Lab,
Boulder, CO, Dr. Margaret Kripke, University of Texas MD Anderson Cancer Center,
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
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
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 can reduce immunity to infectious agents. 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.