sunblock & spf

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if you look at the list of "active ingredients" on any product boasting sunscreen protection, you'll notice a list of unfamiliar [and unpronounceable] chemicals. it is however possible to add sunscreen protection to your lotions, creams and balms using natural inorganic minerals that are not absorbed into the skin, are [thought to be] generally safer, and don't foster any adverse reactions such as allergies.
it is very important to note that a particular spf (sun protection factor) is not achieved just by adding a certain amount of some ingredient(s) that equates to a certain value. this value is determined by laboratory testing of the product and is necessary before any such claims can be made. also be warned that making such claims re-classifies the product as a drug and is thereby subject to the stringent regulations of the fda.
the highest spf values are achieved by combinations of ingredients which may include zinc oxide and / or titanium dioxide but always include other chemical sunscreen agents. for the purpose of this page, i am only considering zinc oxide and titanium dioxide.
understanding ultraviolet rays
there are three type of light rays in the ultraviolet spectrum. uva, uvb, and uvc. ultraviolet c is the most dangerous but it cannot penetrate earth's protective ozone layer. it therefore poses no threat.
because ultraviolet b is stronger [than uva], it has long been considered the sole culprit in causing skin cancer [in persons with a history of sunburn and repeated overexposure]. recent research, however, has also implicated ultraviolet a as a possible cause of skin cancer. in addition to natural light, artificial light from tanning lamps contains uba and uvb. electric arc lamps can also generate ultraviolet light.
the sun protection factor
the spf or sun protection factors used to describe sunscreens are defined according to international standards. they correspond to the relation between the amount of [uvb] radiation that will cause sunburn on unprotected skin and the degree of radiation that will cause sunburn on protected skin. so by using a product with an spf of 6, for example, the skin's ability to protect itself from sunburn is multiplied by 6. these factors are established under laboratory conditions with a given amount of sunscreen. in real life, the sunscreen should be reapplied frequently.
the method [used by l'oréal] to calculate the uva protection factor is based on the amount of immediate pigmentation remaining after two hours of exposure for unprotected skin compared to skin protected by a sunscreen. it is known as the ppd (persistent pigment darkening) or uva factor. it is only uvas which cause this immediate darkening of the skin.
zinc oxide

titanium dioxide
like zinc oxide, titanium dioxide is an inert earth mineral. in addition to being used as a thickening, whitening, lubricating ingredient it is also used as a sunscreen. as with zinc oxide, it comes in a white powdered form and is water soluble. the micronized (small particles) form is dispersible in oil. there is also a micronized form of zinc oxide.
titanium dioxide is very similar to zinc oxide in that they both are physical sun blockers. there are both inert and are therefore not absorbs by the skin. consider the commercial sunblocks that afford higher spf values. whereas they may give superior protection, these organic chemicals are being absorbed by the skin. and since sunblock needs to be re-applied repeatedly, this increases your exposure.
so how much and of what do i use to give my product an spf of 15? in addition to those natural substances listed below, there are the myriad chemical sunscreen agents (e.g. avobenzone, ethylhexyl methoxycinnamate, ethylhexyl salicylate, octinoxate, octisalate, oxybenzone, etc.), most of which are not available to the general public. regardless of the substance, the amount used, i would think, would be determined by the properties of the ingredient and how it affected the end product ...and it's effect on the skin. the desired spf can only be determined by laboratory testing ...and those results submitted to the fda for the right to make the sunblocking claim and include such information on the product label.
one such product "Paula's Choice Pure Mineral Sunscreen SPF 15", which uses zinc oxide and titanium dioxide as the only sunscreen agents, boasts an spf of 15. if we were to assume that since this is a commercial product, it has been tested and fda approved. from that assumption we could conclude that adding 6.84% titanium dioxide and 2.23% zinc oxide to your lotion would give it an spf of 15. But that assumption may not be (and probably isn't) true. the spf is also determined by the medium in which the sunscreen agents are suspended as well as the other ingredients in the lotion. also, in the case of zinc and titanium, the particle size has an important influence on their efficacy ...the smaller the particle, the more surface area to deflect the UV rays.
so, what and how much of it you use is a trial and error process and finally left up to your own personal discerment. in the end, you will have a product that has some sunblocking properties, but -- and I can't stress this enough -- unless you're having the product lab tested and verified to provide a specific degree of sunblock protection, that claim cannot and should not be made.
all that having been said, i'm listing here some addidional information that i ran across offering suggestions for adding spf. these are not the results of lab testing and this information is to be taken with many grains of salt.
this [excerpt from] the ingredients list for a lotion bar actually quotes an spf value.
to 1.75 oz of melted product:
add 1 tsp. titanium dioxide for an spf of 15
add an additional 1/2 tsp. for spf 30
this ingredients list for lip balm with spf will give an indication of the proportions used -- no actual spf value was quoted.
1-1/2 oz beeswax pearls
1 oz cocoa butter
1-1/2 oz shea butter (natural)
2 ozs. avocado oil
1-2 teaspoons vanilla and orange flavor oil
1/4 - 1/2 teaspoon titanium dioxide white
1 teaspoon vitamin E liquid
and finally, the chart below was supplied by "ponte vedre shop shoppe" (stating that it ws supplied to them by their supplier) and attributes a particular sfp value to a percentage of concentration.
(spf 2-5)
(spf 6-11)
(spf 12-19)
ultra high
(spf >20)
titanium dioxide<4%8%12% 20%
titanium dioxide, micronized2%4%6%10%
zinc oxide5%10% 15%25%
zinc oxide, micronized3%7.5%12%20%

natural spf substances
in addition to zinc and titanium, there are some other natural substance that have their own spf properties. since i cannot identify the "authoritative" source of this information, i'm passing it on as purely informative.
zinc oxide -- spf: 2 - 45
titanium dioxide -- spf: 2 - 45
jojoba oil -- spf: 4
red raspberry seed oil -- spf: 28-50
sesame seed oil -- spf: 4
shea butter -- spf: 4
there is also green tea [extract], and white camellia oil, which is extracted from the seed of the camellia flower (i.e. tea plant: camellia sinensis). green tea has been found to counteract the effects of both uvb & uva radiation and protects against skin cancer. green tea does not absorb uv rays like a sunblock. it works with the skin on a cellular level. it inhibits uvb-induced erythema (redness of the skin caused by increased blood flow to the capillaries). it also blocks the cancer-causing changes to the skin cells caused by sun exposure. it does this by causing abnormal cells to self-terminate thereby preventing the development of abnormal growths. lastly, it supports melanin production, the skin's own natural sunburn protection.
final note
the purpose of this page is not to promote sunscreens derived solely from zinc oxide and/or titanium dioxide over commercial products with organic chemical sunblockers. only you can decide the amount of protection you need depending on your activity. if you are going to hawaii to lie about in the sun, an inert sunblock, albeit natural, may prove to be completely inadequate.
if you are using only zinc oxide and/or titanium dioxide as a sunscreen, and your product is not being tested by a laboratory, you should make no claims regarding sunblocking. you should not even allude to sunscreen protection on the label. at best you could simple state [verbally, not in writing] to the end user that the product contained substances that are known to have sunblocking properties.
advisory: there's been a lot of interest in using [products containing] "micronized" titanium dioxide and "micronized" zinc oxide as natural sunscreens. be advised that there is also considerable discussion about these "micronized" particles as being in and of themselves carcinogenic ...thereby defeating their purpose. to draw your own conclusions, google "titanium dioxide nanoparticles sunscreen" (or similar keywords) to see the litany of documents exploring the possible deleterious effects of sunscreen with these nanoparticles.