Sunscreen

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, if it is 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 Sunscreen

Sunscreens can be broadly classified into 2 groups: physical blockers and chemical absorbers.

Physical Blockers

Physical blockers are effective at protecting against both UVA and UVB radiation. They work by reflecting or scattering UV radiation.  The two most common physical blockers are titanium dioxide and zinc oxide.

AdvantagesDisadvantages

Are chemically inert

Poor cosmetic appearance

Extremely safe

Bright fluorescent colors added to improve the cosmetic appearance

Protect against the full UV spectrum

Chemical Absorbers

Chemical absorbing sunscreens often contain a combination of ingredients to allow 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.

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.

The table below is a list of some of the common chemical absorbers available and the protection they provide against the UV range.

Aminobenzoic acid derivatives

ChemicalUVB (290-320nm)UVA II (320-340nm)UVA I (340-400nm)

PABA

Partial

None

None

Glyceryl PABA

Partial

None

None

Padimate O

Partial

None

None

Roxadimate

Complete

Partial

None

Benzotriazoles

ChemicalUVB (290-320nm)UVA II (320-340nm)UVA I (340-400nm)

Medroxyl

Partial

None

Partial

Benzophenones

ChemicalUVB (290-320nm)UVA II (320-340nm)UVA I (340-400nm)

Dioxybenzone

Complete

Complete

Partial

Avobenzone

Complete

Complete

Partial

Oxybenzone

Complete

Complete

Partial

Sulisonbenzone

Complete

Complete

Partial

Cinnamates

ChemicalUVB (290-320nm)UVA II (320-340nm)UVA I (340-400nm)

Octocrylene

Complete

Complete

Partial

Octyl methoxycinnamate

Complete

None

None

Salicylates

ChemicalUVB (290-320nm)UVA II (320-340nm)UVA I (340-400nm)

Homosalate

Partial

None

None

Ethylhexyl salicylate

Complete

None

None

Trolamine salicylate

Complete

None

None

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