What is SPF?
The ability of a sunscreen to protect the user from UVB rays is defined as its Sun Protection Factor (SPF). The SPF is the ratio of the amount of ultraviolet radiation required to produce minimal pinkness (erythema) in skin covered by a sunscreen, assessed 24 hours after exposure, to the amount of UV radiation required to produce a similar level of pinkness in unprotected skin. If a sunscreen reduces the effect of sunlight on skin by 50% it would have an SPF of 2, if by 75%, the SPF would be 4, and so on. Most sunscreens have SPF ratings from four to as high as 40 or 50. The level of protection from ultraviolet radiation provided by the product increases as the value of the SPF rating increases.
The SPF applies for UVB rays only. The protection provided against UVA rays in chemical sunscreens is about 10% of the UVB rating.
An SPF of 15 will provide 15 times the amount of protection you'd get without using anything. Thus, if 10 minutes in the sun is enough to turn you red, your sunscreen would allow you to stay out for 150 minutes before burning. An SPF of 15 filters approximately 92% of the sun burning rays. For UVA the degree of protection is defined as the Phototoxic Protective Factor (PPF). These values are much lower, ranging from 1.5 - 4.8, because the energy contained in UVA is so much less.
Sunscreens are designed to protect against sunburn (UVB rays) and generally provide little protection against UVA rays. They come in two forms:
CHEMICAL SUNSCREENS prevent sunburn by absorbing the ultraviolet (UVB) rays but may increase your risk of cancers of the breast, ovaries, prostate, and colon. Chemicals such as avobenzone, benzophenone, ethylhexyl p-methoxycinnimate, 2-ethylhexyl salicylate, homosalate, octyl methoxycinnamate, oxybenzone (benzophenone-3) are used as the active ingredients.
PHYSICAL SUNSCREENS contain inert minerals such as titanium dioxide, zinc oxide, or talc and work by reflecting the ultraviolet (UVA and UVB) and visible rays away from the skin.
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Use Non-Encapsulated Reflective Physical Sunscreens
Physical reflective sunblockers contain inert minerals such as titanium dioxide, zinc oxide, red petrolatum, or talc and work by reflecting the ultraviolet (UVA and UVB) rays away from the skin. They come in two forms - pure sunblocker and various micronized or encapsulated versions.
The best overall reflective sunblocker, at least in theory, is pure titanium dioxide. Zinc oxide has somewhat better UVA and UVB blocking but also can generate free ionic zinc ions on the skin. Some women think zinc oxide increases facial pore size. Since copper complexes are used by the body for skin regeneration and remodeling, the generation of zinc ions could displace copper ions and reduce the rate of skin renewal.
One problem with titanium dioxide is that it is whiter than zinc oxide and more difficult to formulate as a transparent products. Thus you may prefer zinc oxide products. Pure sunblockers tend to give a pasty look to the skin. This is the reason for the popularity of micronized sunblockers which give a better cosmetic appearance on the skin.
Avoid Micronized or Siliconized Physical Sunscreens.
Prof. Nicholas Lowe (Dermatology, UCLA) has reported that micronized or encapsulated physical sunblockers such as titanium dioxide penetrate into the skin while pure titanium dioxide remains on the skin's surface - where you really want it to stay.
Micronized minerals are usually used as 'micronized' particles in the size range of 20-50 microns. But these particles are small in comparison with the wavelength of the ultraviolet and visible light and are virtually invisible and do not effectively scatter of reflect light. Pure, non-micronized, and often pasty, minerals are better reflectors of ultraviolet light.
Free Radical Generators and Estrogenic Chemicals
Chemical sunscreens act by strongly absorbing ultraviolet light in the UVB range (290 to 320 nm). They cannot reflect light. The UVB range is the range that is primarily responsible for sunburning and causing skin cancer. The UVA range (320 to 400 nm) is responsible for suntanning and photosensitivity reactions (increased sensitivity to sunlight as the result of certain medications, cosmetics, soaps, or plants). UVA is also responsible for serious skin damage. Such types of chemical sunscreens are potent generators of free radicals and many have strong estrogenic, "gender bending", activities and may increase your cancer risk for cancers of the breast, ovaries, prostate, and colon (see more below). PABA and PABA esters are rarely used today because of allergic problems.
Chemical Sunscreens Include:
Benzophenones (dixoybenzone, oxybenzone)
PABA and PABA esters (ethyl dihydroxy propyl PAB, glyceryl PABA, p-aminobenzoic acid, padimate-O or octyl dimethyl PABA)
Cinnamates (cinoxate, ethylhexyl p-methoxycinnamate, octocrylene, octyl methoxycinnamate)
Salicylates (ethylhexyl salicylate, homosalate, octyl salicylate)
Avobenzone [butyl-methyoxydibenzoylmethane; Parsol 1789] - This is the only chemical sunscreen currently allowed by the European Community. However, its safety is still questionable since it easily penetrate the skin and is a strong free radical generator.
Do Physical Sunblockers Generate Free-Radicals?
There have been reports that physical sunblockers can also generate free radicals after exposure to ultraviolet light. However, even bare skin will generate free radicals when exposed to UV radiation. The key is whether the sunblocker form will penetrate the skin and be near the skin's sensitive proteins and DNA. Pure sunblockers such as zinc oxide and titanium dioxide do not pass into the skin and remain far from the skin's sensitive areas. However, sunblockers that are micronized and coated with plastics or silicone more easily pass the skin barrier and can reach the skin's sensitive areas. For this reason, pure physical sunblockers are the safest.
Do Chemical Sunscreens in Cosmetics Increase Cancer?
Worldwide, the greatest rise in melanoma has been experienced in countries where chemical sunscreens have been heavily promoted. The rise in melanoma has been exceptionally high in Queensland, Australia where the medical establishment has vigorously promoted the use of sunscreens. Queensland now has more incidences of melanoma per capita than any other place.
Drs. Cedric and Frank Garland of the University of California have pointed out that while sunscreens do protect against sunburn, there is no scientific proof that they protect against melanoma or basal cell carcinoma in humans1. The Garlands believe that the increased use of chemical sunscreens is the primary cause of the skin cancer epidemic. There is, however, some evidence that regular use of sunscreens helps prevent the formation of actinic keratoses, the precursors of squamous cell carcinoma.