On 8/10/2018 4:52 AM, Sepp Ruf wrote:
John B. Slocomb wrote:
On Fri, 10 Aug 2018 00:36:32 +0000 (UTC), Ralph Barone wrote:
Joerg wrote:

A very fair-skinned cycling friend uses SPF100 and puts it on
generously. Even after a 5h ride in the glistening sun his skin remains
very white so the sun blocker must be protecting because it prevents
even a minimal tan.


If memory serves me properly (and it sometimes does), early sunscreens
blocked only one of UVA and UVB (I can't remember which), with the result
that you didn't tan, but your skin got damaged anyway. If modern sunscreen
has a similar deficiency in protection, you could find that increased
sunscreen use doesn't correlate with lower skin cancer rates.
Alternatively, what if sunscreen itself is a carcinogen?

I don't remember UVA or UVB but when I was going to school in Miami a
friend had some sort of sun block that "prevented sun damage but
allowed tanning". The idea of tanning was, I suppose, because
everyone else on the beach had a nice tan and I guess the "white
skins" didn't want to stand out.

It allowed the existing cells to tan by passing UVA, but blocked UVB.

According to the London Telegraph's reporting, some modern sunscreen norms
require that a UVA protection claiming label must be backed up by a UVA PF
of at least one-third of the stated SPF (UVB) rating. Boris Johnson was
wrong indeed: Letter-boxes (taped shut) are best!
https://www.huffingtonpost.co.uk/entry/boris-johnson-defended-by-leading-imam-over-burka-comments_uk_5b6bf966e4b0530743c71456

But as rbt's readers exclusively die from the effects of wrong gear
selection, lightweight component failure, bike route muggings, closed
brewpubs, wildlife attacking, styrofoam lids or missing styrofoam lids, the
only rbt-relevant sunscreen quality is whether it dissolves expensive grips
and other plastic materials (including black lycra matters) and, maybe, if
it can be used as an emergency chain lube.

Hey, don't go bringing chain lube into this discussion! That's going to
_really_ make it get ugly!

--
- Frank Krygowski