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 Zinc
and zinc compounds play an important role in human health. Zinc
deficiency can manifest itself in many ways including growth
retardation, decreased immune function, skin disturbances, gastrointestinal
dysfunction, and various blood disorders.
Although zinc has
been recognized as an indispensable element for animals since
1934, the essentiality of zinc to humans was documented much
later in 1963. Since then, it has become well known that zinc
deficiency is common throughout the world, especially in developing
countries. Currently, zinc is known to be required by approximately
200 enzymes in the human body.
Zinc containing compounds
are also of great benefit to humans. For example, some zinc
containing compounds are well known antimicrobials. Zinc oxide,
too, has many benefits including its use as a broad-spectrum
sunblock.
History
Zinc oxide has been
used as a dermatotherapeutic agent for over 300 years, first
as a component of calamine and then on its own in various preparations.
In the "Pharmacopea
Londinensis" published in 1618 for the London College of
Physicians, Lapis Calaminaris is mentioned as a component in
three of 42 therapeutic ointments. In the U.S. Dispensary of
1883, calamine is mentioned and recommended for use as "a
mild astringent and exsiccant in excoriations and ulcerations".
Calamine was originally
a naturally occurring mixture of zinc containing compounds which
was used as is or heated and pulverized into a powder which
was then applied to the skin. What we know as calamine today
is actually "neo-calamine", a mixture of zinc oxide
and iron oxides. This mixture was standardized in 1947 with
the publication of the recipe in that year's National Formulary
in the United States.
Today, zinc oxide
is still widely used as a topical therapeutic medication. In
fact, it may well be the most commonly used topically applied
drug of all time. One of the more important uses for zinc oxide
is as a sunscreen. Why is zinc oxide ideally suited for this
purpose ?
Ultraviolet
Radiation (UVR)
The earth is continuously
showered with solar radiation: the electromagnetic energy emitted
by the sun. This energy is the source of all that we know and
yet, like many other good things, too much can be harmful. Among
the spectrum of radiation that the sun emits, Infrared, Visible
and Ultraviolet radiation command our attention.
The solar spectrum
is divided into various portions by wavelength, which is measured
in nanometers (nm). UVR covers from 200 to 400 nanometers. UVR
is divided into UVC (200 - 290 nm), UVB (290 - 320 nm) and UVA
(320 - 400 nm). UVA is further categorized as UVA I (340 - 400
nm) and UVA II (320 - 340 nm), also called long and short UVA
respectively. UVR is generally credited with most of the biologically
significant sequela of sun exposure like sunburn, skin cancer
and visible aging.
UVC, also known as
Germicidal UV, is very toxic. As its name implies, it is lethal
to many microorganisms as well as to most plant life. In addition,
it is carcinogenic to humans. Fortunately, virtually all UVC
is filtered out by the ozone layer.
UVB makes up about
18% of the solar UV spectrum (prior to attenuation by the earths
atmosphere) but only about 1% of the UVR that reaches the earths
surface, because it is largely filtered out by the ozone layer.
Despite its relatively low presence however, UVB is associated
with much of the damage caused to humans by sun exposure. Traditionally,
UVB was credited as being the sole cause of sunburn and various
skin cancers. Although still considered a major cause of sunburn,
UVB is no longer thought to be acting alone with respect to
skin cancer. It also seems likely that other wavelengths (UVA)
will be found to be involved in tumor formation, and perhaps,
in some cases, even as the primary agent.
UVA makes up about
75% of the solar UV spectrum but about 99% of the earthly spectrum.
This is because UVA is largely unaffected by the ozone layer.
Much more abundant, UVA is also much less energetic than UVB
and thus is thought to be biologically less significant. UVA
is the major cause of skin darkening (tanning) and aging.
In addition to the
sun, there are some man-made sources of UV exposure. They include
welding arcs, germicidal lamps, some laboratory equipment and
tanning lamps.
Photochemistry
Photochemistry is
simply chemistry that takes place as the result of light's interaction
with molecules. Virtually any wavelength of light can induce
some sort of photochemistry. In biology however, UVR wavelengths
seem to cause the most significant changes in humans.
The body has many
molecules that can absorb UVR and hence participate in photochemical
reactions. For instance, nucleic acids (DNA, RNA), melanin,
various proteins, hormones, and many drug metabolites absorb
UVR. All of these molecules are potentially changed and damaged
by sun exposure.
It is known that
UVR consistently and specifically damages DNA and one gene in
particular, known as p53, is a marker for such damage. Although
the precise pathway is not yet mapped, this is most certainly
one avenue that can lead to skin cancer. It is also well documented
that UVR induces the formation of free radicals in the skin.
These free radicals go onto produce many deleterious reactions
that can also lead to damage, including cancer.
Skin Cancer
Skin cancer occurs
more frequently than all other cancers combined. In the US alone,
there will be about 1,000,000 new cases of skin cancer this
year. Importantly, the rate of skin cancer is increasing rapidly
for reasons that are not entirely clear but surely have something
to do with sun exposure.
There are two types
of skin cancer, Melanoma and Non-Melanoma. Melanoma is a cancer
(malignant neoplasm) that forms from melanocytes. Melanocytes
are cells that form melanin (the brown pigment we all make).
Melanomas usually occur in the skin but can form anywhere there
are melanocytes such as the eyes, nails, central nervous system
and mucosal surfaces.
Of the types of skin
cancer, melanoma is by far the most fatal accounting for the
vast majority of skin cancer related deaths. The sun's role
in melanoma is not yet definitively established although it
seems likely that UVR exposure is critical.
Non-Melanoma Skin
Cancer (NMSC) is a term that collectively refers to the two
less lethal forms of sun related skin cancer, Basal Cell Carcinoma
(BCC) and Squamous Cell Carcinoma (SCC). Together they occur
far more often than Melanoma but are also, fortunately, less
likely to cause death.
Other sun-related
skin changes
Aside from causing
skin cancer, the sun is responsible for many other changes in
our skin. Sun exposure will cause wrinkling, yellowing, and
thinning of the skin. In addition, sun exposed skin will lose
its elasticity. It is estimated that 90% of the skin changes
that we associate with aging are actually due to sun exposure.
These changes are collectively referred to as photoaging. In
other words, if we were constantly protected from the sun, our
skin would barely change from the time we were about 20 years
old. Most of photoaging is the result of damage to collagen
and elastin, two important structural components of the skin.
Between them, they provide the skin with strength and elasticity.
Action
spectrum
All of the photochemical
reactions described above are the result of specific wavelengths
interacting with specific molecules. The collection of wavelengths
that can cause a particular reaction is called the Action Spectrum
for that reaction. For example, the Action Spectrum for sunburn
is predominately in the UVB range. In other words, UVB causes
sunburn.
A long suspected,
but only recently proven fact is that the Action Spectrum for
photoaging is in the UVA region, specifically in the UVA I portion
of the ultraviolet spectrum. This is important because zinc
oxide effectively blocks this portion of the UVR spectrum.
Sunscreens
Since the late 1920s,
sunscreens have been used by people to protect themselves from
the harmful effects
of the sun. The original sunscreens were organic molecules that
worked by absorbing the sun's radiation. Para-Amino Benzoic
Acid (PABA), perhaps the best known sunscreen, was patented
in 1943 and enjoyed a long period of common usage. It has since
fallen out of favor but other organic sunscreens still make
up the vast majority of the chemicals used for this purpose.
In addition to the
organic sunscreens, there are also several inorganic chemicals
used to block the sun. Some common ones are zinc oxide, titanium
dioxide and iron oxide. The inorganic chemicals offer good protection
and, unlike their organic counterparts, are not absorbed into
the skin. Because of this, they do not cause any adverse reactions
such as allergies and are thought to be generally safer. Among
these, zinc oxide has an unmatched history of safe and effective
long-term use.
In the past, sun
damage was synonymous with sunburn. It was assumed that the
same rays that caused sunburn would also cause skin cancer and
other sun related problems. Accordingly, sunscreen development
was aimed at the invention of sunscreens that stopped sunburn
which is a UVB phenomenon.
We now know that
the entire UV spectrum, and not just UVB, is important. Photoaging
and perhaps even some forms of skin cancer can be caused by
UVA. There is a lack of ingredients that effectively block UVA,
especially UVA I. Fortunately, zinc oxide does.
Sun Protection
Factor: SPF
How much protection
a sunscreen provides is described by the SPF number on the container.
This system is used virtually worldwide.
The test consists
of determining how much UVR (mostly UVB) it takes to cause a
barely detectable sunburn on a given person. As an example,
if it takes 10 minutes to "burn" without the sunscreen
and 100 minutes with the sunscreen then that product has an
SPF of 10 (100/10). An SPF of 15 blocks about 94% of the UVR
and is generally considered adequate for most people most of
the time.
Since the SPF system
is based on sunburn (a UVB phenomenon), it tells you very little
about UVA protection. Again, this stems from the fact that,
at the time the SPF system was created (the 1970's), UVB was
considered the only important part of the spectrum. A meaningful
test for UVA is currently being devised but, for the time being,
consumers have to rely of the sunscreen manufacturers to use
the right ingredients, like zinc oxide.
One should note that
a claim of "Broad Spectrum" on the label is no guarantee
of adequate UVA coverage. This is because a "Broad Spectrum"
claim can be made simply by including one of several ingredients
that block only a portion of the UVA spectrum. As above, this
is based on the old assumption that UVA was not very important.
It is expected that the various regulatory agencies around the
world will correct this problem in the near future.
Zinc oxide
as a true broad-spectrum sunblock
Zinc oxide has been
used for centuries to protect and heal the skin. When it was
first intentionally used as a sunblock is not clear but, at
least during this century, it has been considered common knowledge
that zinc oxide is the most effective sunblock available. Everyone
can recall lifeguards and tennis players with zinc oxide on
their lips and noses. The military even issued zinc oxide paste
to pilots during both World Wars to use in case they were downed
and excessively subjected to the elements.
UVB causes sunburn
and UVA causes skin aging and skin darkening (tanning). Both
UVA and UVB are involved in skin cancer. Given this knowledge,
any consumer product claiming sun protection needs to block
both UVA and UVB. To do anything else is doing a disservice
to the public. Recall that a sunscreen that blocks only UVB
will still prevent a sunburn.
Sunburn is the body's
built in alarm system that tells us when we have had too much
UVR. When this response is "bypassed" people stay
out longer than they normally would and, if using only a UVB
block, expose themselves to unnaturally high doses of UVA radiation.
This is one of several proposed explanations for the rising
incidence of skin cancer.
Fortunately, zinc
oxide blocks virtually the entire UVA and UVB spectrum. This
makes it the most complete block known. Why then, is it not
used in all sunscreen products?
There are a couple
of answers to this. First, the recognition of zinc oxide's potential
as the ultimate sunblock is a relatively recent event. Second,
the traditional organic sunscreens have a tremendous amount
of commercial inertia that needs to be overcome. Third, even
in a microfine form, zinc oxide will appear white when used
in large amounts. To deal with this, the cosmetic formulators
must be educated on the proper use of particulates.
In the past couple
of years, the market has seen the first elegant sunscreens based
on zinc oxide become available. Sometimes zinc oxide is used
alone but usually it is used in combination with one of the
organic sunscreens. More and more products based on zinc oxide
are expected to appear in the market in future. This is especially
true given the trend towards using sunscreens in daily wear
products such as moisturizers. In this type of formulation,
it is particularly important to use ingredients like zinc oxide
that are not only effective, but also non-irritating.
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