Q. There are lots of sunblocks out
there with SPF numbers on them. What exactly do these numbers mean?
A. Since sun exposure is an extensive and important
subject, I will devote two columns to it. This is the first.
Sunblocks — or
sunscreens — work to prevent the damage of ultraviolet (UV) rays, an
invisible component of sunlight. There are three types of UV rays: UVA, UVB and
UVC.
UVA is the most abundant of
the three ultraviolet rays at the earth’s surface. These rays penetrate through
the outer skin. Many of the UVB rays are absorbed by the stratospheric ozone
layer, so there aren’t as many of these at the earth’s surface as the UVA rays.
UVB rays don’t penetrate as
far as UVA rays but are still harmful.
UVC radiation is extremely
hazardous to skin, but it is completely absorbed by the ozone layer.
Sunburn and suntan are signs
of skin damage. Suntans appear after the sun’s rays have already killed some
cells and damaged others. UV rays do more harm than damage skin; they can also
cause cataracts, wrinkles, age spots and skin cancer.Sunscreens
are given SPF (Sun Protection Factor) ratings that tell you how well they
protect you from damaging rays from the sun. The SPF ratings can be as low as 2
and as high as 100-plus.
Here’s how the ratings work:
If you apply a sunscreen rated at SPF 2, you will double the time it takes for
your skin to burn. A sunscreen rated at SPF 15 will multiply the burning time
by 15. Dermatologists strongly recommend using a broad-spectrum (UVA and UVB
protection) sunscreen with an SPF of 15 or greater year-round for all skin
types.
The SPF number indicates the
screening ability for UVB rays only. Research is being to done to establish a
system to measure UVA protection.
There is a point of
diminishing returns with sunscreens. Here’s how it goes:
• A
sunscreen with an SPF of 2 screens 50 percent of UVB rays.
• A
sunscreen with an SPF of 15 screens 93 percent of UVB rays.
• A
sunscreen with an SPF of 30 screens 97 percent of UVB rays.
• A
sunscreen with an SPF of 50 blocks 98 percent of UVB rays.
• A
sunscreen with an SPF of 100+ blocks 99 percent of UVB rays.
Not applying enough
sunscreen can seriously reduce your protection. You should use an ounce —
about a palmful — on your whole body to gain the full protection
indicated by the SPF on the product. Also, dermatologists advise reapplication
every two hours or after swimming or sweating.
It seems logical that if you
use half the required sunscreen, you will get only half the protection, but
that doesn’t seem to be true. A study in the British Journal of Dermatology
found that you get the protection of only the square root of the SPF. So, in
theory, if you use a half ounce of sunscreen rated at
64, you won’t get the protection of an SPF 32, but only the protection of an
SPF 8.
In addition to applying a
sunscreen, you should protect yourself by avoiding the sun between 10 a.m. and 4 p.m., wear
protective clothing and wraparound sunglasses, avoid sunlamps and tanning beds,
and check your skin regularly for changes in the size, shape, color or feel of
birthmarks, moles and spots.
If you have a question,
please write to fred@healthygeezer.com
All Rights Reserved © 2009
by Fred Cicetti