From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: talk.environment,sci.environment,sci.energy
Subject: Re: UV radiation and skin cancer
Date: Sun, 24 Aug 1997 09:49:41 GMT

"Steven Hales" <shales@pipeline.com> wrote:

>Will Stewart wrote in article <33FF5977.28CD@patriot.net>...
..
All gone. As usual, Will selects sites that support his
position, rather than generally informative ones like
Robert Parson's FAQ. Don't be so frustrated Steven, it's
just his style - others will be more appreciative of the
content of your posts.

One minor nit...

> UVA is the main culprit in malignant melanoma and is irrelevant to
>the discussion.  No sunscreen in the world will protect Will Stewart
>or anyone against the harmful effects of UVA exposure.

Many peoples have been exposed to low-level UV radiation, and
researchers have found that the melanocytes within their skin
have activated on exposure to UV to synthesise melanin which is
then delivered to adjacent keratinocytes. " This melanin pigment
is nature's best photo-protective shield " Cutaneous malignant
Melanoma is much rarer in dark-skinned populations that exist
side-by-side with susceptible white-skinned populations.

However, we are no longer even waiting for that evolutionary
process, as there is now a new generation of sunscreens available
using Roche's UV-A sunscreen, Parsol 1789. These new formulations
can provide both UV-B ( 280-290 to 315-320 nm ) and UV-A
(315-400nm) protection.

It is now realised that the criteria used to produce the various
sunscreen protection factors omits the important UV-A, and that the
action spectra tail into UV-A is also very significant. Especially
in light of recent ressearch that shows oxidative damage to RNA
and DNA can be induced (in vitro) by UV-A. Thus the planned
development SPF 50-100 sunscreens has been replaced by development
of broad spectrum sunscreens, often also with additives such as
vitamin E derivatives ( as antioxidants to scavenge free radicals ),
combined with suncreens for both UV-A and UV-B.

Whilst people have historically been concerned about the skin-aging
effects of UV-A, and it's now accepted that skin cancer and erythema
can also be adverse effects of exposure to UV-A. As UV-A can be
affected differently (than UV-B) by atmospheric effects like clouds,
it's becoming more common to discuss surface " solar UV radiation "
( rather than separate UV into A,B or C ) when considering impacts
on organisms.

         Bruce Hamilton

From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: sci.med.nutrition
Subject: Re: Free radicals
Date: 27 Jul 1998 06:55:31 GMT

In <35bb83e9.17511010@news.mindspring.com> Stuart writes:

>Related follow-up question -
>UVB photons appear to damage DNA.  Could these photons be raising the
>level of excitation of electrons at the skin surface making the DNA
>more willing to enter into a reaction and thus be more susceptible to
>oxidative stress?


   No, the photons damage DNA directly, by exciting an electron in a
double bond in a thymine base, and causing it to form a new set of
bonds with a neighboring thymine.  Such a dimer puts a kink in the DNA
and causes problems in pairing when it wants to unzip.  There are
repair systems that cut such dimers out, but they aren't perfect.
People who have a disease called xeroderma pigmentosa have defective
repair systems for this sort of thing, and get skin cancer from the
least amount of UV exposure.

                                    Steve Harris, M.D.

From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: sci.med
Subject: Re: Sunscreen and UVA radiation
Date: 25 May 1999 01:43:01 GMT

In <timleeFC9174.DIr@netcom.com> timlee@netcom.com.DELETE-THIS.BIT
(Timothy J. Lee) writes:

>A few years ago, it was widely publicized that most sunscreen
>chemicals were mainly effective against UVB radiation (which
>causes sunburn), but less effective against UVA radiation (which
>may do other damage.  It was noted that avobenzone or physical
>blocking stuff was most effective against UVA radiation.
>
>However, most sunscreens on the store shelves contain the same
>stuff that is not considered to be very effective against UVA
>radiation, though they say "UVA/UVB" on them.  Very few contain
>either avobenzone or physical blocking stuff.  Why?



   They now contain UV-A absorbers like oxybenzone, which we didn't see
in years past.  Generally you see they are a mix of chemicals for UVA
and UVB.

From: jrfox@no.spam.fastlane.net.no.spam (Jonathan R. Fox)
Newsgroups: sci.med
Subject: Re: Sunscreen and UVA radiation
Date: Tue, 25 May 1999 21:32:27 GMT

On Tue, 25 May 1999 04:15:27 GMT, timlee@netcom.com.DELETE-THIS.BIT
(Timothy J. Lee) wrote:

>sbharris@ix.netcom.com(Steven B. Harris) writes:
>|In <timleeFC9174.DIr@netcom.com> timlee@netcom.com.DELETE-THIS.BIT
>|(Timothy J. Lee) writes:
>|>
>|>A few years ago, it was widely publicized that most sunscreen
>|>chemicals were mainly effective against UVB radiation (which
>|>causes sunburn), but less effective against UVA radiation (which
>|>may do other damage.  It was noted that avobenzone or physical
>|>blocking stuff was most effective against UVA radiation.
>|>
>|>However, most sunscreens on the store shelves contain the same
>|>stuff that is not considered to be very effective against UVA
>|>radiation, though they say "UVA/UVB" on them.  Very few contain
>|>either avobenzone or physical blocking stuff.  Why?
>|
>|   They now contain UV-A absorbers like oxybenzone, which we didn't see
>|in years past.  Generally you see they are a mix of chemicals for UVA
>|and UVB.
>
>But oxybenzone was in common use before the UVA radiation concern
>was commonly publicized.  So was oxybenzone really effective against
>UVA radiation, or was it just another one of the ingredients that
>was not considered very effective against UVA radiation?

Oxybenzone and the benzophenones in general are more complete UVB
blockers with partial UVA protection.

Perhaps the labelling says "UVA/UVB" because the benzophenones do have
some UVA protection, although not as much as UVB.  This is consistent
with the publicity you mentioned.

The anthranilates and dibenzoyl-methanes are more complete UVA
blockers.

--
Jonathan R. Fox, M.D.