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Sun avoidance and regular sunscreen use are
widely promoted by organizations and individuals interested in cancer
prevention. However, 70% of those who participated in a beach survey
were on the beach to get or to maintain a suntan. Although they
stayed on the beach for an average of 4 hours, only half were using
sunscreen.
On the mountains, and despite past sunburn experience,
skiers often do not use sunblock. A springtime survey of skiers
in Alberta showed that only two-thirds were using sunscreen, and
of those using a sunscreen, one third were sunburned at the time
of survey.
There appears to be some confusion about the
messages people are getting about sunscreens and why they're so
important. Some use sunscreens to prevent sunburn, whereas others
use sunscreens to improve sun tanning. Some wish to avoid the wrinkling
associated with sun exposure, and others believe that sunscreens
prevent all types of skin cancer. Yet others seem to believe that
they can use sunscreens in order to prolong their time in the sun.
- Prevents sun burns
- Prevents photodamaged skin (aged look - brown
spots, wrinkles)
- Prevents actinic keratosis and perhaps squamous
cell cancers
- May prevent other skin cancers
A sunburn is caused by exposing your skin
to too much ultraviolet (UV) light from the sun, that is, lightwaves
that measure approximately 200-400nm.
UV light can be broken down into 4 component parts that include:
- ultraviolet A1 (UVA1), 340-400nm
- ultraviolet A2 (UVA2), 320-340nm
- ultraviolet B (UVB), 290-320nm
- ultraviolet C (UVC), 200-290nm
There are two major types of UV sun damage:
UVA-type damage, with light wavelengths of 340-400nm, and UVC-type
damage, which includes light wavelengths of up to 340nm. UVA-type
damage can cause the skin to tan and may cause very weak burns,
while UVC-type damage causes skin to burn.
The average user of sunscreen tends to
use significantly less sunscreen than the amount required to achieve
the SPF listed on the container.
Many experts recommend that frequent application
during sun exposure is required. However, a group of children were
tested by spreading on 1 application of sunscreen to one side of
their bodies, and four applications to the other.
They then spent 6 hours in the sun. One application
provided the same level of protection as four applications, confirming
the adequacy of a single daily application of a sunscreen in that
situation.
As molecules of sunscreen are present in
their active state in the sunscreen, sunscreens work immediately
upon application. The only reason for application early is to allow
absorption into the skin so that the sunscreen is less likely to
be washed off, should the person be entering the water. Even so,
modern sunscreens are quite resistant to removal from the skin.
SPF is the ratio of the minimal ultraviolet
dose required to produce redness with and without a sunscreen. For
example, if it took ½ hour for your skin to become sunburned
without any sunscreen, then for a sunscreen that has a 15 SPF rating,
you could stay in the sun for 15 times longer (or 7.5 hours) before
you get sunburned. This is provided, of course, that you've applied
the sunscreen properly so that you're getting the prescribed protection.
Sunscreens can be both an irritant and
an allergen, though allergic reactions are rare. Irritant reactions,
however, abound. One classic error in sunscreen application is to
put a large amount of sunscreen on the forehead. Perspiration and
gravity can cause the sunscreen to migrate down your forehead into
your eyes, causing a stinging sensation. Some people attribute this
to an allergic reaction and discontinue use. It's also important
to wash your hands after applying sunscreen, since rubbing your
sunscreen covered finger near your eyes can induce an irritant reaction.
It appears that a great deal of time can
elapse between actual sun damage and the development of skin cancer
or other skin problems like photoaging. Therefore, it is important
to protect your skin from an early age when you are out in the sun.
Many elderly people can become quite obsessed
by sun avoidance, and their quality of life can suffer. Sometimes,
if they are diagnosed with an actinic keratosis or basal cell carcinoma,
they can become anxious and almost leap from shadow to shadow. However,
few of them are likely to develop new skin cancers from present
sun exposure. As long as they are prudent about avoiding excessive
sun exposure and protecting their skin to prevent sunburn, they
can continue to enjoy time outdoors.
There is clear evidence that sunscreens
are helpful in preventing actinic keratoses, which are warty lesions
that can occur on sun-exposed skin of the face or hands. Research
has shown that these lesions can develop into a cancer called squamous
cell carcinoma, and that this is linked to a cumulative exposure
to the sun.
However, there is surprisingly little evidence
that sunscreens have much effect in preventing another kind of skin
cancer called basal cell carcinoma, or for malignant melanoma. For
malignant melanoma and for basal cell carcinoma, the character and
timing, that is, the type of sunlight and your age at the time of
the exposure to the sun appears to be more important than the cumulative
dose.
Adapted from an article by David I. McLean,
MD, and Richard Gallagher, MA
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