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3. The physiology and pathology of sunburns
and suntans
3a. Sunburns and suntans
The skin is the largest organ in the body covering 20 square
feet (1.8 m2) in the average sized adult (AAD, 2006). It varies
in thickness from 1 to 3 mm and provides a waterproof protective
coating for the internal organs. The skin contains a large blood
supply and nerve network (Dorlands, 1981). It offers protection
from invasion of bacteria, viruses, and parasites, and it is a
living armor, because it is self repairing (heals). The skin
through the sweat glands help regulate body temperature with
evaporation of sweat for cooling. Blood vessels in the skin
contract to shift blood flow to deep areas of the body to
conserve heat, or they dilate to allow superficial blood flow to
radiate heat from the body.
The skin is composed of three layers – the epidermis, the
dermis, and the hypodermis or subcutaneous tissue (fatty tissue)
(figure 6). Below the skin are muscles or bone. The top layer of
the epidermis consists of dead cells that are constantly being
shed. The epidermis has no blood vessels or nerves. One of the
specialized cells in the epidermis is the melanocyte which
contains melanin a dark pigment. Melanin is a yellow-black
pigment derived through a series of oxidation steps from the
amino acid tyrosine. The dermis and lower layers of the
epidermis are the active portions of the skin with a multitude
of cell types and functions. As the epidermis cells age, they
are gradually transported to the exterior where they eventually
die. There is a rich blood and nerve supply in the dermis
(Gardner, 1963).
The skin is subject to three types of burns as illustrated by figure 6. These most commonly result form thermal injury in a
fire, but they can also be obtained by chemical and radiation
injuries, such as a sunburn. First degree burns represent a
reddening or erythema that mainly affects the epidermis. They
heal rapidly. Second degree burns are deeper and more painful.
They include destruction of a portion of the dermis and often
included severe blistering which is a healing protective effect.
Second degree burns take longer to heal, and they are subject to
infection. They typically do not produce scars. Third degree
burns are severe injuries which destroy the entire thickness of
a portion of the skin. If they are widespread over the body,
they are life-threatening with a high mortality rate. Third
degree burns require extensive prolonged treatment and are
subject to severe infections and severe scarring. Curiously,
third degree burns often initially do not cause much pain as all
the nerve fibers have been destroyed by the burn. Most sunburns
represent first and second degree burns, but third degree burns
are possible, and prolonged sun exposure can even lead to death
from a sunburn.
Figure 6. The composition of the skin and the three types of
burns. From: Medline (2004).
The erythema or skin redness produced by a sunburn occurs as
fast as 30 minutes after exposure. Its cause is unknown, but it
involves a long sequence of inflammatory5 reactions with
microscopic and macroscopic changes in the skin. Less intense
exposure to ultraviolet radiation results in an increase in skin
pigmentation (suntan). This takes place in two stages. First,
there is an immediate pigment darkening in the skin, perhaps
from oxidation and redistribution of the skin pigment melanin.
Next, there is a delayed response of at least 24 to 72 hours in
which there is increased synthesis of melanin in the epidermis.
Repeated exposure to UVR results in further skin thickening and
melanin production (Foster, 2005). The melanin absorbs the UVR
and provides a protective effect. Darker skinned individuals
have more melanin in their skin and are thus more resistant to
the ill effects of ultraviolet exposure. This is presumably a
long term genetic response to exposure of their ancestors to
increased sunlight.
The principle cause of sunburn is UVB radiation which accounts
for 85% of the effect. However, UVA comprises the majority of
the ultraviolet radiation reaching the Earth’s surface at
midday. It therefore accounts for a significant amount of the
immediate and long term effects of ultraviolet radiation. The
minimal single dose of UVR measured as energy per unit area
needed to produce redness at an exposed place is known as the
minimal erythema dose or MED. Moderate to severe sunburns are
produced by 3 to 8 MEDs (Foster, 2005). When compared with white
skinned individuals, moderately pigmented races require 3 to 5
time more UVR exposure to cause erythema, while blacks can be up
to 30 times mores resistant to the effects of UVR. Tanning
increases the resistance to burning by a factor of only 2 to 3.
Moist skin allows more penetration of UVR, and the UVR exposure
increases 4% for every 300 m increase in altitude (Foster,
2005).
3b. Consequences of chronic Sun exposure – wrinkles,
cancer, cataracts
A light skinned individual may not take more than a 15 minute
exposure prior to suffering a sunburn (Medline Plus, 2004). The
consequence of exceeding a MED is a sunburn. Long term exposure
to the Sun at levels not producing a sunburn leads to protective
tanning and further darkening of the skin. Tans project an
active, healthy appearance. However, long term exposure to the
Sun has a variety of unpleasant consequences, including
wrinkles, skin aging, skin cancer, and cataracts.
Sunlight can cause premature skin aging. It is estimated
exposure to UVA or UVB may account for up to 90% of the symptoms
of premature aging, such as easy bruising and excessive wrinkles
(Brannon, 2006). This is greatly accelerated by exposure to the
Sun in fair skinned individuals. A tanned youth may turn into a
very wrinkled old man or woman. UVA does leak through the
atmosphere and is considered a major cause of skin damage. UVA
apparently penetrates deeper into the skin and is more constant
than UVB during the day. UVA is not filtered by glass (Brannon,
2006). UVC would even be more harmful to the skin, but is not a
factor for sunburn, suntan, or the long term effects of the Sun,
because it is completely absorbed by the atmosphere and does not
cause skin damage. Both UVA and UVB are responsible for long
term damage to the skin and probably for the long term systemic
effects of exposure to excessive sunlight (ADD, 2006; Brannon,
2006).
Long term exposure to the Sun with repeated burns increase one's
likelihood of getting a skin cancer. In the United States over
one-half of all new cancers are skin cancers with more than one
million cases diagnosed per year (AAD, 2006). There are three
main types of skin cancers: basal cell carcinoma, squamous cell
carcinoma, and melanoma. Basal cell and squamous cell
carcinomas account for 79% and 15%, respectively, of these new
cases. Basal cell carcinoma and squamous cell carcinoma have
better than a 95% cure rate if found early. They are locally
aggressive but only rarely metastasize (spread to distant
organs) late in the course of the disease. The risk factors for
these cancers are fair skin, advancing age, and male sex (Ferrini,
1998). Five or more sunburns doubles one’s risk of a developing
a skin cancer (ADD, 2006).
Five per cent of new skin cancers are melanomas, and 1% are
miscellaneous rare malignancies. Melanoma is an aggressive
malignancy with 7770 deaths in the US attributed to it in 2005
(ADD, 2006). Older white males have the highest mortality rate
from melanoma. The highest incidence of melanoma is found in
Australia, and the second highest incidence is in the U.S. in
Arizona, where there are large numbers of fair skinned
individuals receiving too much exposure to the Sun. The American
Academy of Dermatology (AAD) considers tanning salon exposure as
harmful as outdoor Sun exposure (ADD, 2006). Some dermatologists
would go so far as to say there is no such thing as a safe tan,
because a tan is the skin’s response to an injury, and every
time you tan, you accumulate more damage to the skin.
It is well known that intense ultraviolet light can produce
temporary damage to the retina of the eyes from the unprotected
viewing of welding or from prolonged exposure to sunlight
reflecting off snow. Acute exposure to excessive ultraviolet
light can also produce photokeratitis, an inflammation of the
cornea and iris, and photoconjunctivitis, an inflammation of the
membranes that line the eyelids and the white surfaces of the
eye (WHO, 2006).
Cataracts represent the clouding of the lens with crystalline
formation and impairment of vision. They are caused by many
factors, including trauma, “aging”, and congenital diseases.
There is an association between long term UVB exposure and
cataract formation (AgingEye, 2006). The World Health
Organization (WHO) estimates as many as 20% of cataracts may be
due to UV radiation (WHO, 2006). The most effective protection
of the eyes from UVB radiation is to limit one’s exposure to the
midday Sun and to use sunglasses with UV radiation protection.
Lens color or tint is not indicative of the UV blocking ability
of a pair of sunglasses (AgingEye, 2006). Fortunately, most
modern spectacles made of plastics or polycarbonate absorb much
of the UVA radiation and almost all of the UVB radiation. Skiers
should be especially cautious in selecting their sunglasses due
to the highly reflective nature of snow and ice. Also, skiers
should use fully wraparound sunglasses to prevent UVR from
entering the eye at oblique angles.
The immune system may be harmed through prolonged exposure to
UVR by changing the activity and distribution of the cells
participating in the immune response (WHO, 2006). This has not
been proven in humans, but experimental studies in rodents show
UVR can suppress immune responses and make rodents more
susceptible to infections. Further research needs to be done in
this regard.
The mechanisms for the damaging effects of UVR are unknown. It
is known UVA is not energetic enough to modify the cellular DNA
bases, but it may play an important role in the formation of
cellular carcinogenic oxygen radicals. However, wavelengths in
the mid UVB region can directly attack cellular DNA. The UVR is
absorbed by the double bond in pyrimidine bases in DNA opening
up the bond and allowing it to react with other molecules. Such
reactions are common, and each cell in the skin might experience
50-100 such reactions during a second of sunlight exposure (Goodsell,
2001). Most of these are corrected within seconds after they
happen using a cellular process known as “nucleotide excision
repair.” Interestingly, rodents show a weakened nucleotide
excision repair mechanism (Goodsell, 2001). This might be a
factor in their seeming sensitivity to immune deficiency after
exposure to UVR.
Children are particularly sensitive to the acute effects of
ultraviolet exposure. Most of the non-melanoma skin cancers are
in fair skinned individuals who tend to burn. However, dark
skinned people do develop skin cancers, and they are also
susceptible to the long term effects of UV radiation on the eye
and the immune system (WHO, 2006).
3c. Sun protection
The best Sun protection is to stay out of the Sun, but that is
not always possible or desirable. Swimming, skiing, walking,
tennis, soccer, and lying in a hammock often include exposure to
the Sun. Keeping the exposure to proper intervals, wearing as
much protective clothing as is practical, and using sunscreens
will prevent sunburns and may obviate or lessen the long term
detrimental effects of Sun exposure.
Physical inorganic sunscreens, such as opaque zinc oxide and
titanium oxide, reflect both UVA and UVB. The organic or
chemical sunscreens are non-opaque and absorb UVA, UVB, or both
by directly absorbing the UVR and dissipating it as heat. The Sun Protection Factor (SPF) is an experimentally determined
number that relates how well a protective agent works relative
to no protection in the midday Sun. An agent with SPF of 15
will delay a burn by a factor of 15 over no protection at all.
It filters 92% of the UVB (Ferrini, 1998). The SPF mainly
measures UVB protection. There is no good way to estimate UVA
protection. Because of their differing wavelengths, UVA and UVB
change their relative proportions reaching the ground as the day
progresses. UVA is less absorbed by a thicker atmosphere and is
less affected by the atmosphere in the early morning and late
afternoon, while the absorption and scattering of UVB is more
intense due to its shorter wavelength. UVB more easily
penetrates the thinner atmosphere in the midday, and it
increases its proportion in the UV spectrum relative to UVA in
the midday (Ferrini, 1998).
A dark tan on a white skin only offers a SPF of 4. Sunscreens
are not intended to increase one’s exposure to the Sun. They are
intended to increase protection during unavoidable exposures
(WHO, 2006). Sunscreens at levels of SPF of 15 and above are
quite effective against sunburn if applied properly and
regularly (AAD, 2006). Sunscreens come in a large variety of
ointments, creams, gels, lotions, sprays, and wax sticks. They
are all labeled with SPF numbers, and they have a variable
resistance to being washed off by water.
While sunscreens are protective against sunburns, it is a hotly
contested issue as to whether they protect against the long term
effects of chronic UVR exposure, mainly skin cancer (Ferrini,
1998). The majority of the data seems to favor them offering
some protection against basal cell and squamous cell carcinoma.
The data is quite mixed regarding any protection against
melanoma. In fact, there is some evidence to suggest the
incidence of melanoma increases with sunscreen usage, possibly
from carcinogenic effects of the sunscreens themselves or
possibly from the sunscreens permitting one to have a longer
exposure to sunlight without suffering a sunburn. Until the role
of sunscreens can be better assessed, it is best they be used to
protect one against excess Sun exposure for activities in the
Sun that can not be avoided, but they should not be used to
extend one’s Sun exposure.
In the United States the Ultraviolet (UV) Index is reported
daily. It offers an analysis of UV light levels on a scale of 1
to 11+ (worse). The UV Index is a forecast of the amount of
ultraviolet radiation expected to reach the Earth's surface when
the Sun is high in the sky. It depends on the Sun's elevation,
the cloud cover, and the amount of ozone in the atmosphere.
Special care is supposed to be taken when the index is 5 or
higher. For US cities, the UV Index information is available
daily from a number of web sites (NOAA, 2006). The UV Index
conforms with the international guidelines for UV reporting
established by WHO.
WHO defines a Global Solar UV Index (UVI).
The UVI is a forecast of the maximum amount of UV radiation
expected to reach the Earth’s surface at Solar noon. For the
midday Sun at noon close to the equator, the UVI can reach 20,
while summertime values in mid Northern latitudes rarely exceed
8 (WHO, 2006). The Australian Government through the Bureau of
Meteorology maintains a SunSmart UV Alert web page for a daily
guide to Sun protection (SunSmart, 2006). It is based on the WHO
Global Solar UV Index. According to SunSmart, when the UV Index
reaches 3, sensible Sun protection is warranted; this level of
Sun exposure also makes it unlikely one would suffer from
Vitamin D deficiency6.
Notes
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