Ultraviolet (UV) light from the sun is a major
health hazard. Not only is it responsible for the greatest number of cancers of
the skin, but it is a major cause of aging skin, as well as cataracts and
related conditions. An understanding of exactly how UV light interacts with the
skin’s components is critical to an appreciation of just how damaging UV rays
can be.
The Photon
The key concept in understanding light energy is
the photon. A photon is actually a very tiny particle that travels in a wave
pattern from the sun to the earth. Each photon has a specific amount of energy,
which scientists measure in several ways. The most common way of measuring
photon energy is in energy units known as joules. The other method commonly used
is in electron volts, generally abbreviated eV. There is a relationship between
the wavelength at which a photon travels and the amount of energy it carries. We
call this an inverse relationship, because the shorter the wavelength, the
greater is the amount of energy carried by the photon. For example, UV light
designated as UVA travels at a wavelength of 315 to 400 nm, and is 3.94 eV to
3.10 eV, whereas UVB is 285 to 315 nm, and ranges from 3.94 to 4.43 eV. How does
this energy relate to damage in the skin?
energy
All biological structures are composed of
molecules, which in turn are composed of atoms, which are held together by
energy bonds. It is characteristic of each molecular structure that the bond
energy is always specific for a particular type of bond. For example, a
carbon-to-carbon bond, written as C-C, will be of a different energy than a
carbon-to-oxygen bond, written as C-O. For light to have an effect on a chemical
structure, it must be able unite with that compound. As an example, the major
target in the skin for UV light is the DNA molecule, since it is so large and
has so many components. When a ray of UVB light strikes a particular molecular
structure in the epidermis (such as a purine, or pyrimidine base in the DNA) it
will combine with that particular molecular structure in such a way that it will
have one of three effects. Extra energy, when absorbed by a molecular structure,
can be dissipated as heat. It may also be irradiated as light at a lower energy,
a process known as fluorescence. Both methods will allow the molecule to return
to its natural state when the extra energy disappears. The third method is more
complex. The molecule will undergo a change in molecular structure to be able to
accommodate the extra energy. This is not a good thing, for the change in
structure may alter the molecule to where it becomes abnormal, and thus is
essentially a different compound. We call this new compound an adduct. The body
has a number of systems that are able to correct these abnormal molecules or
eliminate them. As an example, often times the DNA molecule, when irradiated
with UV light at certain energy levels, will chemically combine two adjacent
bases-to-bases such as thymine. These two bases will become chemically linked
together, side by side, thus forming a new compound known as a dimer. These
abnormal bases are nonfunctional and cannot be reproduced by the body, so they
have to be eliminated and replaced with a normal molecule of thymine.
As the sun’s energy enters the skin, it is
absorbed by a large number of chemical compounds in the skin. The depth of
penetration depends on the wavelength of the UV light striking the skin. UV
light in the UVB range will affect primarily the tissues and cells in the
epidermis, while the UVA range will interact more readily with the dermal
structures.
You can take advantage of this terminology, UVA
and UVB, to help you to remember the types of damage they cause. Associate UVB
with basal cell carcinoma and UVA with aging.
While there may be considerable overlap between
the effects of both UVA and UVB, as a rule, the energy in UVB is much stronger
than UVA, so it creates a different kind of damage. Much of the damage involves
alteration of DNA. Keep in mind, also, that the effect of the UV light will
always depend on the amount of energy in the light, as well as the length of
time an individual is exposed. There are a few practical things to remember. UV
light exposure is highest between the hours of 10 a.m. and 2 p.m. The angle at
which the sun strikes the earth is also critical, so that morning and evening
sunrays are less potent, since they contain mostly infrared light. Anything that
tends to scatter the sunlight will reduce its energy level, but does not
completely eliminate the danger from UV light. A cloudy day, therefore, will
offer you some protection, but not enough to allow you to stay out safely all
day. Many a fisherman has learned this to his or her regret.
Vitamin D
People often ask about exposure to UV light and
production of vitamin D. In the summer, UVB rays form a reaction in the skin
cells that will produce vitamin D. A fair-skinned person exposed to midday sun
for 10 minutes, with enough skin exposed (as in, wearing a swimsuit) will get
enough UV radiation to produce about 10,000 international units of the vitamin.
Unfortunately, if your skin type is a high Fitzpatrick number, you will not have
enough to supply your needs for vitamin D from exposure to the sun. And, UVB
rays cannot fully penetrate the atmosphere in the wintertime, due to the low
angle of the winter sun. At the present time, the current United States
government’s recommendation for quantities of vitamin D is believed, by many
experts, to be too low. In general, the recommendation of 2,000 international
units per day in the wintertime would be an effective dose, and also in the
summer, if you are not a sun lover. Vitamin D is known to protect individuals
against breast cancer, prostate and colon cancer, osteoporosis and depression.
There is also evidence that adequate doses of vitamin D may be protective
against heart disease.
broad-spectrum-sunscreenProtection
How do we protect ourselves from the harmful
effects of the sun? There are several ways to avoid sun damage. Obviously,
staying out of the sun and/or wearing sensible protective clothing are two ways.
For those individuals who want to enjoy the benefits of the sun, there are
products that can afford some protection. It is important to understand that no
product will give you 100 percent protection from UV light from the sun. There
are two ways to reduce the sun’s UV light using commercial products. The first
method is to employ physical blocks, such as afforded by zinc oxide or titanium
dioxide. These inorganic compounds are now ground to nanometer size, which makes
them invisible on the skin, eliminating the ugly white rub-out of the products.
The nanoparticles, however, absorb the UV light and chemically produce free
radicals. There are pulmonary studies which indicate that these free radicals
are capable of reacting with DNA in the skin. Initial studies show that
nanoparticles of zinc and titanium dioxide do not penetrate beyond the stratum
corneum. For all intents and purposes, we can say these products appear to be
safe, but the final word is not in. The many chemical sunscreens in use absorb
the energy of the UV light as mentioned above, and can undergo chemical changes
that produce potentially dangerous adducts. Recently, a product known as
SolastayTM (ethylhexyl methoxycrylene, Hallstar) was originally designed for use
in skin care and sun care products. It is a highly efficient singlet state
quencher for Butyl Methoxydibenzoylmethane (BMDBM; Avobenzone) and Ethylhexyl
Methoxycinnamate (OMC; Octinoxate). Singlet oxygen is the common name used for
an electronically excited state of molecular oxygen (O2), which is less stable
than the normal form but is capable of producing free radicals. When added to a
sunscreen formula, SolastayTM is capable of stabilizing the chemical sunscreens
to prevent them from becoming harmful adducts. The addition of this product
should make the use of chemical sunscreens considerably safer.
When UV light passes through the skin, it is very
important for the aesthetician to understand what is happening. Keep in mind
that high energy of different frequencies contained in the waves all of UV light
will interact with many of the complex proteins in the skin. Proteins, being
composed of amino acids, by nature have a tendency to coil and cross-link to
specific configurations that are critical to the functioning of a particular
protein. Anything that alters this functional configuration will, in essence,
denature the protein. Let us look at elastin, the very important protein that
contributes to skin appearance, yet it makes up only one to two percent of the
proteins in the skin. It is now known that UVA is capable of inducing changes in
the fibroblasts of the skin, to induce them to produce more elastin. Most of the
elastin produced in the body is produced in our youth, but the elastin produced
in mature years tends to aggregate and form a less functional elastic material.
Dermatologists have designated this condition as solar elastosis. It is one of
the prime reasons that skin assumes a sagging, thin appearance with age. We can
help to prevent these changes by the addition of both topical and systemic
antioxidants.
