Re: Do sunscreens protect equally against UV, regardless of SPF


Jamie

Jamie: I found this by googling. Interesting info:

The sun emits a number of different types of radiation, including
ultraviolet radiation (UVR). UVR is divided into three types: UVA, UVB
and UVC, each of which reflect different wavelengths. Radiation exists
as waves--much the same as ocean waves, in fact. The wavelength is
simply the distance between the tops of two consecutive waves.

UVC rays are completely filtered out by the earth's atmosphere so they
don't reach us. UVB rays, however, are only partially blocked by the
atmosphere, so some does get to us. These are the rays that cause
sunburn and some types of skin cancer. Since sunscreens have
traditionally been designed to stop sunburn, it is the UVB rays that
they normally block. The SPF number of a sunscreen is an indication of
how much UVB a sunscreen will block. An SPF 2 will block 50% of the UVB
rays so you can stay in the sun twice as long without burning as you
would have been able to without the sunscreen. With an SPF 15, 93% of
the UVB rays are blocked and you can stay out 15 times as long.

There are a couple of very important things to keep in mind: (1) The
SPF number assumes the sunscreen was applied before exposure. If you
apply the sunscreen after being exposed to the sun, the number no
longer applies. (2) Also, if you stay out longer than the SPF claims to
protect for, you will still be burned. As an example, if you would
normally burn in 10 minutes and you put on an SPF 2 before you go out,
it will take 20 minutes before to burn. Sunscreens do not provide total
protection; they simply lower the amount of UVB that gets to you. It's
analogous to standing behind a screen. The screen blocks only some of
the light but some continues to get through, so if you stand there long
enough eventually you will get burned.

UVA is divided into two parts, long UVA and short UVA. For all intents
and purposes, short UVA behaves very much like UVB. It can cause
sunburn and is at least partially blocked by most sunscreens. Long UVA,
however, does not cause sunburn. In fact it does not cause any sort of
immediate reaction, even in pretty large doses. The SPF number provides
no information about a sunscreen's UVA blocking capabilities.
Unfortunately, even the words "broad spectrum" don't tell you much
about the amount, if any, of long wave UVA protection. Current law
allows the labeling of sunscreens with the words "broad spectrum"
and "protects against UVA" when they only block short UVA. There is
no legal requirement to block long UVA and, in fact, the vast majority
of sunscreens on the market do not.

Why bother to block UVA

It turns out that long UVA can cause the skin to age and is likely
involved in the more serious skin cancers such as melanoma. Wrinkles,
uneven pigmentation, sagging and age spots associated with old age are
mostly due to sun exposure and could be prevented with sun avoidance,
proper clothing and a good sunscreen

.

Relevant Pages

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