About Sunscreens
What Is Sunscreen SPF?
SPF is the abbreviation for Sun Protection Factor and is the system used worldwide to determine how much protection a sunscreen provides, applied to the skin at a thickness of 2 mg/cm2. This system calculates how much UV radiation (mostly UVB) it takes to cause a barely detectable sunburn on a given person with and without sunscreen applied. For example, if it takes 10 minutes to burn without a sunscreen and 100 minutes to burn with a sunscreen, then the SPF of that sunscreen is 10 (100/10).
Currently there is no internationally agreed-upon test for measuring UVA protection in human skin. An estimate is made by a laboratory test in which the proportion of radiation passing through a measured amount of sunscreen is determined. To ensure some protection against UVA, products with physical blocking agents making up some of the active ingredients are recommended.
Two Classes of Suncreen
Sunscreens can be broadly classified into 2 groups: chemical absorbers and physical blockers.
Chemical absorbers work by absorbing ultraviolet (UV) radiation and can be further differentiated by the type of radiation they absorb, UVA or UVB, or both UVA and UVB.
Physical blockers work by reflecting or scattering the UV radiation.
Chemical Absorbers
The table below is a list of some of the common chemical absorbers available and the protection they provide against the UV range.
| Chemical | UVB (290-320nm) | UVA II (320-340nm) | UVA I (340-400nm) |
|---|---|---|---|
| Aminobenzoic acid derivatives | |||
| PABA |
Partial | None |
None |
| Glyceryl PABA |
Partial | None | None |
| Padimate O |
Partial | None | None |
| Roxadimate |
Complete |
Partial | None |
| Benzotriazoles | |||
| Medroxyl | Partial |
None | Partial |
| Benzophenones | |||
| Dioxybenzone | Complete | Complete | Partial |
| Avobenzone | Complete | Complete | Partial |
| Oxybenzone | Complete | Complete | Partial |
| Sulisonbenzone | Complete | Complete | Partial |
| Cinnamates | |||
| Octocrylene | Complete | Complete | Partial |
| Octyl methoxycinnamate | Complete | None | None |
| Salicylates | |||
| Homosalate | Partial | None | None |
| Ethylhexyl salicylate | Complete | None | None |
| Trolamine salicylate | Complete | None | None |
Chemical absorbing sunscreens often contain a combination of ingredients to get coverage against both UVB and UVA radiation. Some are also combined with physical blockers.
Some organic formulations may degrade when exposed to sunlight; they may therefore not perform as well as expected.
Physical Blockers
Physical blockers are effective at protecting against both UVA and UVB radiation. The 2 most common physical blockers are titanium dioxide and zinc oxide.
| Advantages | Disadvantages |
|---|---|
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Important links
THINGS TO THINK ABOUT WHEN USING SUNSCREEN
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How much: The effectiveness of a sunscreen depends on how much of it is applied. Most people apply their sunscreen at about one third of the thickness recommended.
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How often: The effectiveness of a sunscreen is affected by how often it is applied. Most people forget to reapply it every couple of hours.
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What type: Alcohol-based lotions, sprays, or gels are better for oily or hairy skin. Creams are suitable for dry skin, and milky lotions are the easiest to apply. Special sticks are suitable for noses, lips, and around the eyes.
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Broad spectrum: Use a good broad-spectrum sunscreen of at least SPF 15.
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Take time: It takes 20 to 30 minutes for a chemical absorber sunscreen to be absorbed by the skin, and it can be rubbed off very easily; so apply it at least half an hour before going out in the sun. This does not apply to physical blockers which take effect immediately.
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Activities: Reapply immediately after swimming, excessive sweating, or if rubbed off by clothing or toweling. This should be the case even if the product claims to be "water resistant."