SUNSCREEN RANT: WHY IS MY FACE TAN?
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free skin care · Sun May 10, 2009 5:20 pm

basically, in a (misshapen) nutshell:
physical ss makes use of titanium dioxide, zinc oxide, and other iron oxides (if tinted) to deflect the rays of the sun off the area to which it is applied. Since pure zinc oxide is opaque, there are all kinds of issues about how small the particles can be and/or how dilute the compounds are in order to attain a sheer cream without compromising coverage.
Physical ss is literally a "physical" block against the sun's rays. However, since there are several types of sun rays, physical blocks protect mostly against UVB rays and do not do such a good job filtering out UVA rays. UVB rays are those responsible for color, and UVA rays are supposedly responsible for aging issues.
Now, any ss that claims to have "broad spectrum" protection is supposed to protect across the UVA-UVB ray spectrum, but many physical ss do not, since they rely less on ZnO and more on Ti20 (which is less of an effective physical block).
As for chemical sunscreens, they make use of highly unstable chemicals that, when exposed to the sun, "react" with the radiation and neutralize it right there on your skin. This is why sometimes when you wear them your skin will feel very "hot" in direct exposure--because your skin is absorbing (rather than deflecting) radiation.
Because most chemical ss compounds degrade almost immediately upon exposure to sunlight (in fact, there is a class action suit against american ss makers because of their misleading spf claims for the last 30 years), the europeans have innovated this area by forming new chemical compounds such as tinosorb that will effectively stabilize those unstable compounds, so they last longer on the skin and are more effective at their job. However, research on tinosorbs is slim and funded by huge pan-euro cosmetics companies, so I'm just not sure.
Also, chemical sunscreens (even the ones with tinosorbs) degrade if you put any physical sunscreens or other inorganic compounds anywhere near them--such as mineral makeup, powder, or foundation. So they are inherently unstable and it is difficult for the consumer to monitor when and how to apply them. Chemical ss are popular with people who are concerned about anti-aging, since even though they tend toward instability, they still offer more of a broad spectrum of protection against the sun's radiation.
in the US, sun protection only ever took into account UVB radiation, which is responsible for tanning and for burning only. The US measures "SPF," or "sun protection factor," as the amount of time one can stay in direct exposure to the sun without burning. So if you normally can stay out for 10 minutes without burning and you wear spf 30, you can stay out for 300 minutes before you would need to re-apply.
as for PPD, this measures, I think, the level of protection against UVA rays that photosensitize the skin. If someone else can enlighten, that would be great. I know, obv., that a high ppd and spf are the best!

i hope that helps!

--avalange

Loumomofthree wrote:

O.K. I am officially confused.

Can someone direct to where I can read about the difference and benefits of physical sunscreens vrs chemical sunscreen...and how to tell the difference between the two...?
the more I read the more I seem to be confused..I need the basic facts...Sunscreens for Dummies.. Laughing

thanks so much Lou

free skin care · Sun Dec 27, 2009 12:53 pm

avalange wrote:

oh no! hey, no problem!
somehow, it is not a problem with all chemical sunscreens--not so sure why, but i am more comfortable knowing how physical ss works as opposed to the chemical kind. if you search my name and bioderma you will find many posts, but there is one in particular in which i first realized that its lack of effectiveness ws due to my dusting powder on my skin after applying it--to control the shine, just like you!

--avalange

fat_swan wrote:

avalange wrote:

also, bioderma, when i wasn't putting any powder over the top, also kept my skin untanned all summer long.

avalange wrote:

chemical sunscreens (even the ones with tinosorbs) degrade if you put any physical sunscreens or other inorganic compounds anywhere near them--such as mineral makeup, powder, or foundation.

Oh my gosh, I had no idea! Shock

Well, that explains why I keep getting freckles and moles-- I always cover the SS with a layer of Jurlique Silk Dust to control the stickiness... Boy, that is SO depressing Crying or Very sad

Thank you for the clarification Avalange!

avalange: could you tell me more about which chemical sunscreens are okay to be used with physical ones? I was thinking of using a chemical one with a physical one over the top but had vaguely heard that they did not react well with each other either. I would really like to find out.

Nanci's All Natural suncreen is great. It feels light on me but then I am used to very thick sunscreens. An added bonus with her sunscreen is that it is more like a cream (a very thick one - not a solid but still thick) I think it gets even thicker/ less liquid in colder weather and I got a big bottle of this through in hand luggage on my flights recently. Yeah!

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